(DRAFT) - Taxonomy
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
TAXONOMY
NAME - BLACKBIRD, YELLOW-SHOULDERED
OTHER COMMON NAMES - BLACKBIRD, YELLOW-SHOULDERED; BLACKBIRD, YELLOW-SHOULDERED and MONA ISLAND; CAPITAN;MARIQUITA DE PUERTO RICO
ELEMENT CODE -
CATEGORY - Birds
PHYLUM AND SUBPHYLUM - CHORDATA,
CLASS AND SUBCLASS - AVES,
ORDER AND SUBORDER - PASSERIFORMES,
FAMILY AND SUBFAMILY - EMBERIZIDAE,
GENUS AND SUBGENUS - AGELAIUS,
SPECIES AND SSP - XANTHOMUS,
SCIENTIFIC NAME - AGELAIUS XANTHOMUS
AUTHORITY -
TAXONOMY REFERENCES -
COMMENTS ON TAXONOMY -
Yellow-shouldered Blackbird
Agelaius xanthomus (Sclater, 1862)
KINGDOM: Animal GROUP: Bird
PHYLUM: Chordata CLASS: Aves
ORDER: Passeriformes FAMILY: Emberizidae
The yellow-shouldered blackbird is similar in size and shape to
the red-winged blackbird (A. phoeniceus) of North America (01). Male
and female yellow-shouldered blackbirds are similar, with adults
predominantly blackish neutral gray, and in sunlight, showing a faint
bluish-green reflection (02). The humeral patch in subspecies
A. x. xanthomus, is close to spectrum yellow and is usually edged with
a narrow white margin. In subspecies A. x. monensis the yellow lesser
wing coverts are paler and the middle wing coverts are extensively or
entirely white, or very pale yellowish white (03). Humeral feathers
under the wing are occasionally tinged with orange. Nestlings and
juveniles are dark grayish brown, with the humeral tract feathers in
nestlings (if present) and juveniles buff-yellow to spectrum yellow
and tipped with dark grayish brown. Most juveniles retain the
dark-tipped humeral feathers until the first post-nuptial molt (02).
Though similar in plummage, sexes differ in size, and most birds
Taxonomy - 1 (DRAFT) - Taxonomy
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
can be sexed using wing measurements. Males' wings are 1.1 times as
long as the females' (mean= 102.80 mm, SD= 1.95 for males and
mean= 93.3 mm, SD= 2.0 for females) (04). There is no difference in
measurements between the two sub-species (03).
Agelaius xanthomus was originally described by Sclater in 1862 as
Icterus xanthomus. The nominate form is from puerto Rico and Vieques
Island.
The second recognized subspecies of A. xanthomus is A. x.
monensis Barnes, 1945 from Mona and Monito Islands (03). A. xanthomus
is probably derived from the red-winged blackbird, A. phoeniceus, of
North America (03). The tawny-shouldered blackbird, A. humeralis,
which occurs in Cuba and Haiti, is morphologically intermediate
between A. phoeniceus and A. xanthomus (05). The yellow-shouldered
blackbird appears to be a superspecies with A. humeralis (06). Barnes
(03) reported that some authors considered A. xanthomus a subspecies
of A. humeralis.
The yellow-shouldered blackbird may also be known by the common
name Mona Island yellow-shouldered blackbird or by the Spanish names
capitan and mariquita de Puerto Rico.
Taxonomy - 2 (DRAFT) - Status
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
STATUS
Coded Status
Puerto Rico; Federal Endangered
Puerto Rico; State Listed
E: Federal Endangered
Federal Migratory
Non-consumptive recreational
COMMENTS ON STATUS -
U.S. STATUSES AND LAWS:
The yellow-shouldered blackbird (Agelaius xanthomus) has been
designated an Endangered species pursuant to the Endangered Species
Act of 1973 (50 CFR 17.11; P.L. 93-205, 87 Stat. 884; 16 U.S.C.
1531-1540), as amended. The species has this status wherever found
including the Commonwealth of Puerto Rico. This species' range is
restricted to Puerto Rico, Mona and Monito Islands. Critical Habitat
has been designated in Puerto Rico including all of Mona Island,
portions of the southwest area of the main island of Puerto Rico, and
Roosevelt Roads Naval Station (50 CFR 17.95(b)).
This species is protected by the Lacey Act (P.L. 97-79, as
amended; 16 U.S.C. 3371 et seq.) which makes it unlawful to import,
export, transport, sell, receive, acquire, or purchase any wild
animal (alive or dead including parts, products, eggs, or offspring):
(1) in interstate or foreign commerce if taken, possessed,
transported or sold in violation of any State law or
regulation; or
(2) if taken or possessed in violation of any U.S. law,
treaty, or regulation or in violation of Indian tribal law.
It is also unlawful to possess any wild animal (alive or dead
including parts, products, eggs, and offspring) within the U.S.
territorial or special maritime jurisdiction (as defined in
18 U.S.C. 7) that is taken, possessed, transported, or sold in
violation of any State law or regulation, foreign law, or Indian
tribal law.
The species is protected by the Migratory Bird Treaty Act
(16 U.S.C. 703-711 et seq.; 50 CFR 10.13).
RESPONSIBLE FEDERAL AGENCIES:
USFWS -Responsible for the management/recovery, listing, and
law enforcement/protection of this species.
DOD -Responsible for the law enforcement/protection of this
species with applicable State and Federal laws on
public land under their control. Also responsible for
management/recovery on Department of Defense lands.
All Federal agencies have responsibility to ensure that any
action authorized, funded, or carried out by that agency is not likely
to jeopardize the continued existence of the species or result in the
Status - 1 (DRAFT) - Status
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
destruction or adverse modification of Critical Habitat (50 CFR 402),
and to utilize their authorities to carry out programs for the
conservation of the species.
STATE STATUSES AND LAWS:
STATE: Commonwealth of Puerto Rico
DESIGNATED STATUS: Endangered
ADMINISTRATIVE AGENCY: Department of Natural Resources
STATE STATUTE: Organic Act, Sec. 155; and Wildlife Act, Sec. 85;
"Regulation to Govern the Management of
Threatened and Endangered Species in the
Commonweath of Puerto Rico." 1985.
INTERNATIONAL STATUSES, TREATIES, AND AGREEMENTS:
A. x. monensis and A. x. xanthomus are listed as "vulnerable" in
the IUCN Red Data Book, 1977.
ECONOMIC STATUSES:
No commercial utilization. The species is of interest to
naturalists and birding enthusiasts in non-consumptive recreational
pursuits.
76/11/19:41 FR 51021/ - Listing and Critical Habitat designation
81/02/27:46 FR 14652/14658 - Five year review
87/07/07:52 FR 25523/25528 - Notice of review
Status - 2 HABITAT ASSOCIATIONS
HABITAT - TERRESTRIAL
TERRESTRIAL
COASTAL
LAND USE -
Residential
Transportation, communications, and Util
Cropland and Pasture
Orchards, Groves, Vineyards, Nurseries,
Confined Feeding Operations
Deciduous Forest Land
Forested Wetland
Bare Exposed Rock
NATIONAL WETLAND INVENTORY CODES
NWI NWICLS NWIMOD NWISPEC
Estuarine, intertidal FO3
COMMENTS ON HABITAT ASSOCIATIONS -
A detailed evaluation of breeding, roosting, and feeding habitats
is needed for all three yellow-shouldered blackbird populations.
These three populations are distributed as follows: The subspecies
Agelaius xanthomus xanthomus is concentrated in eastern Puerto Rico on
Roosevelt Roads Naval Station (RRNS), near the town of Ceiba, and in
coastal southwestern Puerto Rico. Subspecies A. x. monensis occurs on
Mona and Monito Islands, approximately 65 km west of Puerto Rico. All
populations occur in areas classified as subtropical dry forest, but
individuals or small groups may extend into the subtropical moist
forest (i.e., San German) and subtropical wet forest (19), especially
during the non-breeding season (07).
Detailed studies are planned for this species on the Mona
population and on upland nests in southwest Puerto Rico. Most of the
work on blackbird behavior and habitat was conducted in southwest
Puerto Rico during the 1970's when monkey feeding stations were
established on two off-shore islands (05). These stations attracted
large numbers of blackbirds and cowbirds and a large percentage of
blackbird studies were concentrated in these two areas. About 250
pairs of blackbirds nested in adjacent mangroves (05). The monkey
stations were closed in 1981 but studies have not been conducted to
document the resulting changes, if any, in blackbird distribution and
behavior. Detailed studies of the biology of this species are
ongoing on Mona Island. Upland nesting areas in southwestern Puerto
Rico were studied during 1987 and many more nests were discovered
than during previous investigations.
Though once common throughout lowland Puerto Rico, the blackbird
is presently concentrated in the 3 major population centers mentioned
above. Within these areas, blackbirds nest in a variety of habitats,
including the following (01,02,04 05,07,08):
1) Mangrove pannes and salinas. The most important nesting habitats
in Puerto Rico are the revegetated pannes and salinas in the coastal
mangrove zone. The nest trees (primarily black mangrove, Avicennia
germinans; red mangrove, Rhizophora mangle; and white mangrove,
Laguncularia racemosa) are usually small or dead mangrove trees with
cavities or stumps with depressions.
Habitat Associations - 1 2) Offshore mangrove cays. In southwest Puerto Rico, blackbirds
nest on small cays (100-1000 sq. meters) approximately 250-500 meters
offshore. Most of the nests found have been at or near the edge of
these islands.
3) Black mangrove forests. In eastern Puerto Rico, blackbirds nest
in dense black mangrove forests along the fringes of small pools or
clearings.
4) Lowland pastures. In southwest Puerto Rico, blackbirds nest in
scattered ucar (Bucida buceras) growing in savannah-like pastures up
to the mangrove edge.
5) Suburban. Nests were found regularly in San German (in southwest
Puerto Rico) until 1982 in 16 to 18-meter tall royal palms (Roystonea
boriquena) surrounding a university campus (05). This area is
classified as subtropical moist forest (19). The campus has large
ornamental and fruiting trees, including mango, avocado and royal
palms. In the past, nests were reported in Carolina, Hato Rey, and La
Parguera and blackbirds were reported in Aguadilla, Isabela, Catano,
and Humacao (01,11).
6) Coconut and royal palm plantations. Blackbird nests were
reported in plantations in Boqueron, La Parguera, Boca Prieta, and
Mona Island.
7) Cactus-scrub. Blackbirds nest in cactus within cactus-thorn
scrub habitat in the central plateau of Mona Island. Mona Island is
classified as subtropical dry forest.
8) Coastal cliffs. Blackbirds nest on ledges or in crevices of
coastal cliffs on Mona Island. Perez-Rivera (14) also reported nests
on rocks surrounded by water.
9) On Mona Island two monospecific blackbird roosts have been
located in caves within the island's coastal cliffs (20,24).
During the breeding season in southwest Puerto Rico, blackbirds
appear to stay in the mangrove zone or arid coastal fringe (07).
This area is classified as subtropical dry forest (08). Most mangrove
nests (75% of Agelaius xanthomus nests found in southwestern Puerto
Rico in 1983 (12)) are over water, thus reducing rat access. Other
predator-free sites include rocks surrounded by water, steep cliffs,
palm fronds, small islands, and cacti. Mangrove nests are: 1) open,
cup-shaped nests placed near the mud or water in small mangrove trees;
or 2) cup nests placed in a cavity in the tops or sides of stumps or
dead trees (02,11). During Post and Wiley's studies, approximately
one-half of the population remained in the vicinity of the monkey
feeding stations during the non-breeding season, while the other half
dispersed daily to feed inland, returning at night to their mangrove
roosts. Some blackbirds formed flocks and followed available food
sources through the mountains of central Puerto Rico.
Presently, the majority of the southwest population appears to
return nightly to their mangrove roosts, but during the non-breeding
season, isolated individuals or small flocks have been seen at shiny
cowbird (Molothrus bonariensis)/Antillean grackle (Quiscalus niger)
roosts inland at local electrical power substations.
In the southwest, blackbirds forage among trees in pastures,
and within or adjacent to mangroves. Some may forage in nearby
dairies, cattle barns, croplands supporting corn or sorghum, and in
vineyards. Little is known about blackbird foraging behavior and
Habitat Associations - 2 movements in the eastern Puerto Rico and Mona Island populations.
Habitat Associations - 3 (DRAFT) - Food Habits
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
FOOD HABITS
TROPHIC LEVEL -
OMNIVORE
LIFESTAGE FOOD FOOD PART
General Deciduous Shrubs-Flowers/Fruit/Seed
General Cactus-Flowers/Fruit/Seed
General Deciduous Trees-Flowers/Fruit/Seed
General Molluscs
General Arthropods
Food Habits - 1 (DRAFT) - Environment Associations
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
ENVIRONMENTAL ASSOCIATIONS
G = General A = Adult
LIM = Limiting RA = Resting Adult
J = Juvenile FA = Feeding Adult
RJ = Resting Juvenile BA = Breeding Adult
FJ = Feeding Juvenile P = Pupae
L = Larvae E = Egg
RL = Resting Larvae
FL = Feeding Larvae
LIFESTAGE ENVIRONMENTAL ASSOCIATIONS
G Coastal Features: Vegetated offshore islands
G Terrestrial Features: Caves, dry
G Terrestrial Features: Cliffs/ledges
G Terrestrial Features: Rock outcrops
G Snags: Unknown
G Tree Cavities: Unknown
Environment Associations - 1 (DRAFT) - Life History
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
LIFE HISTORY
FOOD HABITS:
Available data on blackbird feeding habits include incidental
observations and data collected during 2 specific studies: 1) Wetmore
(10) analyzed the stomach contents of 55 blackbirds collected in
Puerto Rico from January through June (localities not available) and
2) Post (02) analyzed 25 food samples taken from nestlings in La
Parguera.
Post's study was conducted when monkey feeding stations were
established on two small offshore islands. These stations were
year-round communal feeding areas for the southwest blackbird
population until the stations were closed in 1981. Therefore, the
results of this study may be biased by these artificial feeding areas.
The following table summarizes the results of these studies:
----------------------------------------------------------------------
Class, Order or Group % samples % number % by
Family & Stage in which found* found* weight **
----------------------------------------------------------------------
Lepidoptera 56.4 28.32
Olethreutidae larvae 56 28.2
pupae 16 2.5
Noctuidae larvae 16 6.1 X
pupae 12 19.0
Unidentified 4 0.6
Orthoptera 10.4 1.95
Gryllidae 28 9.8
Blattidae 4 0.6 X
Acrididae X
Homoptera 11.0 9.06
Fulgaridae 8 10.4
Tropiduchidae 4 .6
Cicadidae X
Membracidae X
Coleoptera 3.0 35.21
Buprestidae 8 1.8
Elateridae 4 .6
Cerambycidae .81
Bruchidae 15.92
Unidentified 4 .6 18.48
Diptera 3.65
Dermaptera
Forficulidae 1.37
Hymemoptera 4 .6 1.03
Arachnida 18.4 7.83
Life History - 1 (DRAFT) - Life History
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
Unidentified 4 .6
Araneae
Anyphaenidae 40 9.8
Salticidae 20 3.7
Araneidae 8 1.2
Unidentified 20 3.1
Mollusca 1.56
Miscellaneous .43
Plant Material 60*** 9.9****
----------------------------------------------------------------------
* analysis of food brought to nestlings in La Parguera (02)
** content analysis of 55 stomachs taken from Jan. through July;
locations in PR unknown (11); X = present in samples
*** bread, rice and monkey chow
**** 5.73% was corn; only taken in May and June
----------------------------------------------------------------------
----------------------------------------------------------------------
Additionally, in Puerto Rico, blackbirds eat plant material such
as commercial animal food, grain, and cooked rice and sugar from bird
feeding stations (08). During the non-breeding season, especially in
January and February, blackbirds probe for nectar in flowers of Aloe
vulgaris (02) and bucare (Erythrina sp.) (11).
Only incidental data are available for blackbird food habits on
Mona Island. There, blackbirds have been observed feeding on insects
and spiders, fruits from Pithecellobium sp. (20) and the cacti
Selenicereus sp., Cephalocereus royenii (21), Harrisia portoricensis,
and Opuntia sp. (22), and seeds from gumbo limbo (Bursera simaruba)
and Ficus sp. (20). They also probe for nectar in flowers of Croton
discolor (23) and guama (Inga laurina) (21).
Based on the above data, the blackbird may be characterized as
an arboreal insectivore. However, they may also feed on a variety of
plant material.
HOME RANGE/TERRITORY:
There are no data available on home range/territory for the Mona
Island population of the yellow-shouldered blackbird. Data on
territory were collected in both mainland Puerto Rico populations,
while data on home range were taken only from the southwest Puerto
Rico population (02,04).
A territory is defended only during the breeding season and it
usually includes an area within three to five meters of the nest (02,
04). Males defend their mates only within this territory.
Both sexes defend the nest from potential predators. Birds in
nesting aggregations mob predators, with non-nesters in the area
joining in. Mobbing intensity increases with nestling age.
Intraspecific defense is primarily the males' role, and its intensity
is greatest during nest building and egg-laying.
Most of the home range data were collected when monkey feeding
stations were established on two small offshore islands. These
Life History - 2 (DRAFT) - Life History
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
stations became year-round communal feeding areas for the southwest
blackbird population (05). It is not known to what extent the
blackbird home range changed after closure of the stations in 1981.
Individual home ranges vary widely with no apparent seasonal
difference in size or pattern of use (02). Home range includes a
communal roost site, communal feeding areas, and nest site, if
breeding. The round trip distance between a communal roost area and
a communal feed area during one study was 10-12 km (02).
During the non-breeding season, approximately one half of the
southwest population remains in the coastal areas to feed, while the
others may move inland to feed, returning to roost in the coastal
mangroves each night (08). Small flocks or individuals may move
further inland, remaining to roost in such locations as electrical
power substations.
PERIODICITY:
The yellow-shouldered blackbird is diurnal, gathering in communal
roosts for the night, mainly in mangroves. These roosts also often
contain shiny cowbirds (Molothrus bonariensis) and Antillian grackles
(Quiscalus niger). However, on Mona Island, two monospecific
blackbird roosts have been located in caves in the island's coastal
cliffs (20,24). Cowbirds, though occuring on Mona Island do not roost
at these sites and the grackle is not found on Mona.
During the breeding season, blackbirds continue to gather in
communal roosts. However, females remain on the nest at night during
incubation and brooding, returning to roost when the chicks are
approximately one week old (02).
During the non-breeding season in southwest Puerto Rico, small
flocks of individual birds forage along coastal areas, some going
inland to feed but returning to the mangrove roosts at night (08). A
few may go further inland, roosting with Antillian grackles and shiny
cowbirds, in such areas as electric power substations.
MIGRATION PATTERNS:
The yellow-shouldered blackbird is non-migratory (01,02,05,07,
11). However, it does wander away from the nesting habitat,
particularly in the non-breeding season, but how far it travels from
nesting habitat is not known.
COVER/SHELTER REQUIREMENTS:
No quantitative information has been collected on blackbird
cover/shelter requirements. Such information can only be derived from
detailed evaluation of roosting, breeding, and feeding habitats.
Blackbirds show behavioral plasticity in their use of breeding
habitat and nest sites (i.e., steep cliffs, palm fronds, small
islands, cacti). However, the coastal mangrove system is the major
nesting location of mainland black birds (01,02,04,07,08,12).
Ground predators appear to be the main selection pressure in
determining habitat and nest site choice (02).
Blackbirds also use a variety of sites for roosting, including
electric power substations, mangrove cays, roadside shrubs, and caves
(Mona Island) (20,24).
Life History - 3 (DRAFT) - Life History
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
REPRODUCTIVE SITE REQUIREMENTS:
No quantitative information has been collected on
yellow-shouldered blackbird reproductive site requirements. A
detailed evaluation of their breeding habitat is needed to classify
essential elements.
Blackbirds show behavioral plasticity in their use of breeding
habitats and nest sites. Many nest sites chosen by blackbirds are
selected to optimize avoidance of ground predators (02,04,11). Most
mangrove nests (75% of Agelaius xanthomus nests found in southwestern
Puerto Rico in 1983 (12)) are over water, thus reducing rat access.
Other predator-free sites include rocks surrounded by water, steep
cliffs, palm fronds, small islands, and cacti.
When nesting in coastal mangroves, blackbirds prefer early seral
stages with small, sparse stands of immature mangroves 0.9 - 4.5
meters tall. Mangrove nests are: 1) open, cup-shaped nests placed
near the mud or water in small mangrove trees; or 2) cup nests placed
in a cavity in the tops or sides of stumps or dead trees (02,11).
Mainland open nests consist of a nest platform composed of
leaves, grass, cotton, and occasionally paper, string, plastic bags,
and twine. The nest cup is composed of grass leaves and stems and
cotton, and is usually lined with fine grass leaves and stems (02).
Seven cavity nests were lined with grass with nest platforms
usually composed of material from previous nests (02).
Open nests on mangrove cays are more substantial. A sample of 5
nests contained Sargassum sp. (30-90%), turtle grass (Thalassia)
(5-30%), miscellaneous wrack (3-40%) (plastic bags, nylon twine,
bamboo roots, pelican feathers, and burlap) and fine grass (one nest
only, 20%) (02). Sargassum is often abundant on mangrove roots, but
when in short supply, females may rob Sargassum from other blackbird
nests.
Nests in mangrove cays average 2.1 +/- 1.2 meters above water
(n=15) and in palm trees, 15.3 meters (range= 10.7 to 19.8 meters,
n=4) above ground (02).
In an effort to provide additional nest sites, the U.S. Fish and
Wildlife Service and the Commonwealth of Puerto Rico's Department of
Natural Resources have placed wooden nest boxes, artificial stumps
with cavities, and PVC pipes in mangrove salinas in Boqueron State
Forest (southwest Puerto Rico). These structures are designed to
prevent rat access. Blackbirds have nested in all three artificial
nest types.
REPRODUCTIVE CHARACTERISTICS:
Pairing begins 6 - 10 weeks prior to breeding (01,02). Breeding
is triggered by the onset of spring rains (02), usually April in
eastern Puerto Rico and May in southwestern Puerto Rico (01).
Breeding may occur as early as February on Mona Island (22) and lasts
through September. Yellow-shouldered blackbirds usually nest in
colonies. Post (02) studied a colony of 15 nests that were spaced
5 - 35 meters apart.
Both males and females (01,02) can breed their first year.
Blackbirds are monogamous and may remate for several years (02,04).
Normally, nesting occurs once a year, but if renesting is attempted,
the pair usually remains together (24). There is no evidence that the
Life History - 4 (DRAFT) - Life History
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
pair remains together outside the breeding season (02,04).
Females are sexually receptive only in the vicinity of the nest
site and only in the early stages of breeding. Only females build the
nest, incubate eggs, and brood chicks. Both sexes feed the young and
clean the nest site (01,02,04).
Clutch sizes range from an average of 1.61 eggs/nest in eastern
Puerto Rico, to 2.6 eggs/nest in southwestern Puerto Rico (02), and
2.75 eggs/nest on Mona Island (14). Egg incubation lasts 12-13 days,
and young remain in the nest 13-16 days (02).
Young remain in the nest area for an extended time period; then
follow the parents and are fed by them. It is not known how long the
family group remains together, but Post observed parents feeding young
24 days after the young fledged (02).
PARENTAL CARE:
Post (02) studied the biology of the yellow-shouldered blackbird,
including parental care. Aside from incidental observations, the
information provided here is taken from this study.
Males guard a small area around the nest site throughout the
breeding cycle (02,04). Although males are primarily involved with
intraspecies defense, females will guard the nest area during nest
building, incubation, and brooding. Both sexes mob intruders of other
species, with the intensity increasing as the chicks become older.
Adults defend only a small area around the nest and spend long periods
feeding away from the nest.
Only females build nests, incubate eggs and brood young (01,02,
04). Incubation begins with the laying of the second egg. Young are
brooded until they are approximately one week old, thereafter the
female returns to the roost at night with the male. Both sexes feed
the young, clean the nest cup, remove fecal sacs, and preen the
young's feathers.
The nestling period (the young are altricial) averages 14.6 days
(range 13-16 days). After fledging, young remain in the nest area
for an extended period, then leave to follow their parents. No
information is available on the length of time the family remains
together, but Post observed two young following and being fed by their
parents 24 days after fledging.
POPULATION BIOLOGY:
Current limiting factors include: cowbird parasitism, loss of
nesting and feeding habitat, predation by introduced mammalian
species (Rattus rattus, R. norvegicus, and Herpestes auropunctatus),
and avian pox (01,02,04,05,07,08).
Although precise estimates were not made of the Mona population
until March 1986, it is believed that the population is stable with at
least 338 individuals (20). Eastern Puerto Rico's population has
steadily declined from an estimated 200 (04,07,08,25) in 1976 to a
sighting of only 12 individuals in 1985 (26). The southwestern
population declined rapidly from an estimated 2,000 in 1976 (04,07,08,
25) to an estimated 423 individuals in 1982 (01) and to a minimum of
343 individuals in 1985 (27).
The blackbird Recovery Plan (06) identifies recovery of
blackbirds when the following populations are reached:
Life History - 5 (DRAFT) - Life History
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
Roosevelt Roads Naval Station: 250 pairs,
southwestern Puerto Rico: 1,000 pairs,
Mona Island: 250 pairs.
No information is available documenting longevity for the blackbird,
but the estimated 82.4% adult survival rate in southwestern Puerto
Rico indicates that blackbirds are relatively long-lived (02).
Survival rates of juveniles to adulthood is 65.5% with no significant
differences between sexes (02).
During Post's study (02) of the southwestern population and
museum specimens, males significantly outnumbered the females, but he
believes the sampling may have been biased, and identified five
possible sources of error. During the breeding season, the sex ratio
was 1:1.
SPECIES INTERRELATIONSHIPS:
Post (15) conducted a long-term study on the biology of the
yellow-shouldered blackbird, including the prevalence of
ectoparasites. He found three species of biting lice (Mallophaga),
Philopterus agelaii, Machaerilaemus sp., and Myrsidea sp., in the head
area of 69% of the blackbirds (n=265) examined. Males had
significantly more lice than females, and adults had significantly
more lice than juveniles. He also identified 2 species of acarina
mite from three nests: Ornithonipsus bursa and Androlaelaps casalis.
This type of mite is too small to be observed by superficial
observation of individual birds.
Pearly-eyed thrashers (Margarops fuscatus) steal eggs and young
from nests of other species, take over cavity nest sites, and steal
nest material (08). Thrashers are known to have destroyed blackbird
nests on Roosevelt Roads Naval Station (01), and Perez-Rivera (14)
reported the destruction of a blackbird nest by thrashers on Mona
Island. Blackbirds nest in cavities only in those areas of southwest
Puerto Rico where thrashers do not occur (08).
Introduced mammalian predators, such as rats (Rattus rattus and
R. norvegicus) and mongoose (Herpestes auropunctatus), are now
widespread in lowland Puerto Rico (01,04,10,11). Blackbirds usually
nest in predator-free sites, such as small islands, mangroves and
rocks surrounded by water, steep cliffs and cacti (01,04,08).
The shiny cowbird, Molothrus bonariensis, is a relatively recent
arrival to the Greater Antilles from South America (28). Within the
blackbird nesting area, the blackbird is a primary host for this brood
parasite (28). The cowbird deposits its eggs in the host's nest,
sometimes puncturing the eggs of its host. The host will then
incubate and brood the cowbird's young. Cowbird parasitism reduces
the number of eggs laid, causes nest abandonment, and causes loss of
eggs due to punctures (07,18). The blackbird has evidently not
coevolved defense strategies to counter the shiny cowbird (29).
OTHER LIFE HISTORY DESCRIPTORS:
None.
Life History - 6 (DRAFT) - Management Practices
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
MANAGEMENT PRACTICES
RESULT MANAGEMENT PRACTICE
Beneficial Controlling/Restricting Off-Road Vehicles
Beneficial Restricting/regulating human disturbance of populations
Beneficial Maintaining undisturbed/undeveloped areas
Beneficial Controlling/Restricting Pesticide Use
Beneficial Controlling/Restricting Herbicide Use
Beneficial Creating Artificial Habitat/Nesting Structure
Beneficial Maintaining Early Stages of Succession
Beneficial Stocking captive-reared wild-strain animals
Beneficial Disease Control Measures
Beneficial Controlling/Removing Invertebrates
Beneficial Controlling/Removing Native Vertebrates
Beneficial Controlling/Removing Exotic Vertebrates
Adverse Disease
Existing Disease
Adverse Parasites
Existing Parasites
Adverse Predation
Existing Predation
Adverse Rural Residential/Industrial Areas
Existing Rural Residential/Industrial Areas
Adverse Draining wetlands, marshes, ponds, lakes
Existing Draining wetlands, marshes, ponds, lakes
Adverse Competition
Existing Competition
Adverse Exotic/Feral/Introducted Species
Existing Exotic/Feral/Introducted Species
Adverse
Existing
Adverse Vegetation Composition Changes
Existing Vegetation Composition Changes
Adverse Harvesting
Existing Harvesting
COMMENTS ON MANAGEMENT PRACTICES -
From the mid-1800's through the 1940's, the yellow-shouldered
blackbird was reported as abundant or common throughout lowland Puerto
Rico. Between 1940 and 1972 no data was gathered on the species
(01,02,04,05,07). In 1972, Post and Wiley showed the blackbird had
become restricted mainly to three population centers (02). Since that
time, the three populations have steadily decreased in numbers.
Factors contributing to this decline have been (01,04,05,08):
1) Reduction in feeding habitat. In 1931, when blackbirds were
common in Puerto Rico, less than 25,000 acres of virgin forest
remained on the island (09). From 1900 until the 1980's, increasing
amounts of acreage in Puerto Rico were devoted to sugar cane. This
monoculture effectively reduced the amount of available feeding
habitat. The subsequent shift from sugar cane production toward
Management Practices - 1 (DRAFT) - Management Practices
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
industrialization and urbanization caused a reversion of the lands to
housing and industrial developments (01,02,04,07,10).
2) Reduction in nesting habitat. Until the 1940's, blackbirds
used a wide variety of nesting habitats. Wetmore (11) reported that
blackbirds nested most often in palms and occasionally deciduous
trees. Human use of lowlands reduced available nesting habitat.
Today, coastal mangroves are the most important areas used for nesting
by blackbirds (01,02,04,07,08,12). Coastal habitat has been greatly
modified with large areas of mangrove destroyed (e.g., through wetland
filling). In the southwest, the major blackbird breeding habitat is
one of the last extensive undisturbed tracts of mangrove in Puerto
Rico.
3) Introduced mammalian predators include the rats (Rattus
rattus and R. norvegicus), and mongoose (Herpestes auropunctatus) (01,
04,10,11). Many of the blackbird nest sites (i.e., small islands,
cactus, palm fronds, and steep cliffs) suggest that ground predators
have influenced nest site selection (01,04,08). In 1983, nest
predation accounted for 48% of the nests that failed in southwest
Puerto Rico's Pitahaya mangrove system (12). In this area, rat
predation increases sharply during the dry months of July and August,
when lower water levels allow rats access to nest areas (12).
4) Pearly-eyed thrasher (Margarops fuscatus). This aggressive
thrasher nests mostly in cavities and will often out-compete other
birds for nest sites (01,08). Blackbirds nest in cavities only in
southwest Puerto Rico mangroves where thrashers are not found.
Thrashers are known to eat eggs and kill young of various other birds
(13). Thrashers destroyed blackbird nests at Roosevelt Roads Naval
Station (01) and Perez-Rivera (14) reported that a thrasher destroyed
a blackbird nest on Mona Island.
5) Avian pox. Of 305 birds examined for pox lesions by Post,
19% were infected (04,07,08,15). Survival of 26 infected blackbirds
over a 15 month period was significantly lower than 38 non-infected
blackbirds (15).
6) Shiny cowbird (Molothrus bonariensis) parasitism. The
crucial factor in blackbird decline has been the phenomenal spread and
increase of shiny cowbirds in Puerto Rico since they were first
reported on the island in 1955 (16). For the years 1972 through 1975,
surveys of blackbird mainland nests showed a 73.7% cowbird parasitism
rate (08). Between 1977 and 1980, parasitism rates reached 93% at
Roosevelt Roads and 90% in southwest Puerto Rico (01). The shiny
cowbird is considered one of Puerto Rico's most numerous lowland birds
(01).
Although the shiny cowbird was reported on Mona Island in 1971
(17), cowbird parasitism has never been reported there for blackbirds.
The first in-depth study of blackbirds on Mona Island is planned for
1986 and 1987. This study should provid more information on shiny
cowbird parasitism on the island.
Cowbird parasitism causes a decrease in the number of blackbird
Management Practices - 2 (DRAFT) - Management Practices
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
eggs laid, increases the rate of abandonment, and decreases fledgling
success and nest productivity (07,18). Parasitism was found to
decrease fledglings per nest by 0.39 (07).
All of the above factors continue to threaten the existence of
the yellow-shouldered blackbird. Particularly threatening is the
continued destruction and alteration of mangrove nesting habitat in
southwest Puerto Rico.
APPROVED PLAN:
U.S. Fish Wildlife Service. 1983. Yellow-shouldered Blackbird
Recovery Plan. U.S. Fish and Wildlife Service, Atlanta, Georgia.
23 pp.
Actions recommended for the recovery of the yellow-shouldered
blackbird include:
1) Trap and destroy cowbirds in blackbird nesting areas.
2) Investigate the design of artificial nest boxes to exclude
cowbirds and thrashers. Install nest boxes in appropriate
habitat.
3) Destroy cowbird eggs in blackbird nests.
4) Monitor blackbird and cowbird populations.
5) Evaluate impact of disease on yellow-shouldered blackbirds and
initiate an immunization program if needed.
6) Assess the impact of pearly-eyed thrashers on yellow-shouldered
blackbirds. Pearly-eyed thrashers are known to take over
cavities or nests of blackbirds and destroy blackbird eggs.
7) Evaluate impact of feral monkeys on yellow-shouldered blackbirds.
8) Evaluate the impact of public access to blackbird breeding areas
and establish, as needed, public use regulations. Restrictions
may be needed on hiking and off-road vehicle use.
9) Essential habitat should be identified, then measures taken to
protect and improve it.
10) Control herbicide/pesticide use on agricultural lands.
Herbicides and pesticides may kill blackbirds directly as the
blackbirds feed in croplands or these toxicants may contaminate
water flowing into mangrove swamps and destroy these nesting
sites.
11) Monitor, and treat as needed, parasite infestations (mites) in
blackbird nests.
12) Maintain the early successional stage of mangrove swamps.
13) Control or restrict development of mangrove swamps.
Since publication of the recovery plan, captive breeding of
yellow-shouldered blackbirds has been identified as a needed recovery
activity.
Ongoing recovery activities include cowbird trapping
projects/studies on Roosevelt Roads Naval Station, Boqueron State
Forest, and Cabo Rojo National Wildlife Refuge; destruction of cowbird
eggs above a set number in blackbird nests, and monitoring of all 3
blackbird populations including roost surveys and nest productivity.
Management Practices - 3 (DRAFT) - References
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
References
***** REFERENCES FOR ALL NARRATIVES EXCEPT N-OCCURRENCE *****
01 U.S. Fish and Wildlife Service. 1983. Yellow-shouldered Blackbird
Recovery Plan. U.S. Fish and Wildlife Service, Atlanta, GA.
23 pp.
02 Post, W. 1981. Biology of the yellow-shouldered
blackbird - Agelaius on a tropical island. Bull. Florida State
Mus., Biol. Sci. 26(3):125-202.
03 Barnes, V., Jr. 1946. The birds of Mona Island, Puerto Rico. Auk
63:318-327.
04 North Carolina Foundation for Mental Health Research, Inc. [n.d.]
Population ecology of the yellow-shouldered blackbird. Final
report to Department of Interior, U.S. Fish and Wildlife Service,
Office of Endangered Species.
05 Post, W., A. Cruz, and J.W. Wiley. 1983. Management of the
yellow-shouldered blackbird, an endangered West Indian species.
Unpublished report on file at: U.S. Fish and Wildlife Service,
Boqueron, PR. ii + 26 pp.
06 American Ornithologists' Union. 1983. Check-list of North
American birds, 6th edition.
07 Wiley, J. 1981. Yellow-shouldered blackbird management.
Unpublished report to U.S. Fish and Wildlife Service, Office of
Endangered Species. 36 pp.
08 Post, W. and J.W. Wiley. 1976. The yellow-shouldered
blackbird-present and future. Amer. Birds 30:13-20.
09 Little, E.S., Jr. and F.H. Wadsworth. 1964. Common trees of
Puerto Rico and the Virgin Islands. Agriculture Handbook No. 249.
U.S. Department of Agriculture, Forest Service, Washington, D.C.
10 Wiley, J., E. Litovitch, and F. Nunez-Garcia. 1983. Cowbird
control in Puerto Rico. Unpublished report on file at: U.S. Fish
and Wildlife Service. 8 pp.
11 Wetmore, A. 1916. Birds of Puerto Rico. U.S. Department
Agriculture. Bull. 326:1-140.
12 Cruz, A. and T. Nakamura. 1984. The 1983 nesting season
of the yellow-shouldered blackbird (Agelaius xanthomus) in
southwestern Puerto Rico. Unpublished report on file at: U.S. Fish
and Wildlife Service, Office of Endangered Species. 32 pp.
13 Raffaele, H.A. 1983. A guide to the birds of Puerto Rico and the
Virgin Islands. Fondo Educativo Interamericano, San Juan. 255 pp.
14 Perez-Rivera, R.A. July 20, 1982. [Letter to Regional Director,
Fish and Wildlife Service, Atlanta, Georgia.]
15 Post, W. 1981b. The prevalence of some ectoparasites, diseases
and abnormalities in the yellow-shouldered blackbird. J. Field
Ornithology 52:16-22.
16 Grayce, R.L. 1985. Range extensions in Puerto Rico. Auk 74:106.
17 Bond, J. 1973. Eighteenth supplement to the checklist of the
birds of the West Indies (1956). Acad. Nat. Sci. Philadelphia.
Pages 1-12.
18 Wiley, J. 1982. Ecology of avian brood parasitism at an early
interfacing of host and parasite populations. Ph.D. diss., Univ.
of Miami, Florida. 345 pp.
References - 1 (DRAFT) - References
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
19 Ewell, J.J. and J.L. Whitmore. 1973. The ecological life zones
of PR and the USVI. U.S. Forest Service Research paper ITF-18.
72 pp.
20 Silander, S., J. Pagan, and E. Vazquez. April 2, 1986. [Memo to
Director, Terrestrial Resources Section. Commonwealth of Puerto
Rico, Department of Natural Resources.]
21 Danforth, S.T. 1926. An ecological study of Cartagena Lagoon,
Porto Rico, with special reference to the birds. J. Dept. Agric.
Porto Rico 10:1-36.
22 Perez-Rivera, R.A. 1980. Algunas notas sobre la biologica y
"status" de la sub-especie de Mona. Memorias del sequndo coloquio
sobre la fauna de Puerto Rico. Univ. P.R. Humacao. Pages 54-63.
23 Lewis, A. August 10, 1982. [Letter to Refuge Manager, Caribbean
Islands National Wildlife Refuges, Boqueron, Puerto Rico.]
24 Lopez, F. January 18, 1986. [Memo to Field Supervisor, Caribbean
Field Office, Boqueron, Puerto Rico.]
25 Post, W. 1977. Management practices for the recovery of the
yellow-shouldered blackbird. Research proposal submitted to the
U.S. Fish and Wildlife Service. 13 pp.
26 Heisterberg, J.F., F. Nunez-Garcia and A. Diaz-Perez. 1985.
Evaluating shiny cowbird control techniques in eastern Puerto Rico:
A progress report on work conducted June-September 1985. Bird
Damage Research Report No. 354. Denver Wildlife Research Center.
29 pp.
27 Silander, S., J. Pagan and E. Vazquez. September 25, 1985. [Memo
to Director, Terrestrial Resources Section, Commonwealth of Puerto
Rico, Department of Natural Resources.]
28 Post, W. and J. Wiley. 1977. The shiny cowbird in the West
Indies. Condor 19:119-121.
29 Cruz, A. and I. Manolis. 1983. The shiny cowbird in the Caribbean
Region: A study on the ecology of an avian brood parasite
undergoing range expansion. Abstracts of presented Posters and
Papers, 101st Stated Meeting of the American Ornithologists' Union,
September 26-30, 1983. Pages 13-15.
***** REFERENCES FOR N-OCCURRENCE NARRATIVE ONLY *****
01 Barnes, V., Jr. 1946. The birds of Mona Island, Puerto Rico.
Auk 63:318-327.
02 Heisterberg, J.F., F. Nunez-Garcia, and A. Diaz-Perez. 1985.
Evaluating shiny cowbird control techniques in eastern Puerto Rico:
A progress report on work conducted June-September 1985. Bird
Damage Research Report No. 354. Denver Wildlife Research Center.
03 Mitchell, R. 1985. [Letter to U.S. Fish and Wildife Service,
Caribbean Field Office, Boqueron, PR.]
04 Mitchell, Reese. 1985. Personal communication. Union Carbide,
Yabucoa, PR.
05 Nunez-Garcia, Fernando. 1985. Personal communication. U.S. Dept.
of Agric., 334 Fifteenth St., Bowling Green KY 42101.
06 Post, W. 1981. Biology of the yellow-shouldered
blackbird - Agelaius on a tropical island. Bull. Florida State
Mus., Biol. Sci. 26(3):125-202.
References - 2 (DRAFT) - References
Species BLACKBIRD, YELLOW-SHOULDERED
Species Id ESIS104009
Date 14 MAR 96
07 U.S. Fish and Wildlife Service. 1983. Yellow-shouldered Blackbird
Recovery Plan. U.S. Fish and Wildlife Service, Atlanta, GA.
23 pp.
08 Post, W. and J.W. Wiley. 1976. The yellow-shouldered
blackbird - present and future. Amer. Birds 30:13-20.
09 North Carolina Foundation for Mental Health Research, Inc. [n.d.]
Population ecology of the yellow-shouldered blackbird. Unpublished
report on file at: Department of Interior, U.S. Fish and Wildlife
Service, Office of Endangered Species.
10 Wiley, J.W. 1981. Draft. Yellow-shouldered blackbird management.
Manuscript. 36 pp.
11 Cruz, A. and T. Nakamura. 1984. Unpublished report. The 1983
nesting season of the yellow-shouldered blackbird (Agelaius
xanthomus) in southwestern Puerto Rico.
12 Furniss, S. 1986. Personal communication. Cabo Rojo National
Wildlife Refuge, Puerto Rico.
13 Post, W. March 17, 1982. Personal communication. [Memo to Refuge
Manager, Caribbean Islands National Wildlife Refuges, Boqueron,
Puerto Rico.]
14 Silander, S., J. Pagan, and E. Vazquez. September 25, 1985. [Memo
to Director, Terrestrial Resources Section, Commonwealth of Puerto
Rico, Department of Natural Resources.]
15 Post, W. and J.W. Wiley. 1977a. The shiny cowbird in the West
Indies. Condor 19:119-121.
16 Taylor, E.C. 1864. Five months in the West Indies. Part II,
Martinique, Dominica, and Porto Rico. Ibis 6:157-173.
17 Wetmore, A. 1927. The birds of Porto Rico and the Virgin Islands.
New York Acad. Sci. Scientific Survey of Porto Rico and the Virgin
Islands. 9:409-571.
18 Wetmore, A. 1916. Birds of Porto Rico. U.S. Dept. Agric. Bull.
326:1-140.
19 Danforth, S.T. 1936. Los Pajaros de Puerto Rico. Rand, McNally
and Co., New York.
References - 3