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Potentially Misidentified Species:
Another common Florida egret, the snowy egret (Egretta thula), is
somewhat similar in appearance, but it is taller and the bill and legs are
black in adult specimens. The legs and the wingspan of snowy egrets are
also longer than those of B. ibis (Peterson 1980, Hilty and Brown 1986).
II. HABITAT AND DISTRIBUTION
Regional Occurrence:
Bubulcus ibis is originally from Africa and Europe (primarily southern Spain)
but it has become broadly established throughout much of the world. Much of
this species' range expansion is related to livestock ranching. Cattle egrets
now occur as naturalized exotics in South-, Central- and North America, and
Australia. It is considered to be the most terrestrial of the herons and is
capable of thriving in agricultural and urbanized areas (Hancock and Elliott
1978, Telfair 1994).
The non-breeding range of B. ibis in North America includes the
contiguous 48 states (although limited breeding probably occurs throughout) and
also extends as far north as Alaska and Newfoundland. The typical summer
breeding range extends north through Virginaia and west into eastern Texas,
Oklahoma, and Kansas. Year-round B. ibis populations occur in Florida
and throughout the states bordering the Gulf of Mexico , and extreme southern
California (Kaufman 1996, Cornell Lab of Ornithology 2003).
IRL Distribution:
The first reports of Bubulcus ibis in the U.S. came from southern Florida in
the 1940s and the first breeding populations were identified in the Lake
Okeechobee region in the early 1950s (Weber 1972). The species is now abundant
throughout the IRL region.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Cattle egrets generally attain a length of 46-56 cm, a wingspan of 88-96 cm, and a weight of around 360 g (Scott, 1987).
Abundance:
Cattle egrets are abundant throughout Florida. Bryan et al (2003) reports that
Bubulcus ibis was the most common nesting species in nesting bird colony
surveys conducted in the Upper St. Johns River basin in 1993-1995 and
1998-2000. Similarly, Dugger et al (2005) confirmed that that B. ibis
was the most abundant waterbird utilizing the channelized Kissimmee River in
the wet season during surveys conducted between 1996 and 1998. This
information confirms the ability of B. ibis to effectively utilize
urbanized or otherwise altered habitats.
Reproduction:
Cattle egret breeding season in central Florida extends from mid April through
July (Weber 1972). Individuals enter the breeding population at 2-3 years of
age (Kaufman 1996).
Cattle egrets are colonial breeders that are often found alongside other
species of herons and egrets in mixed-colony shoreline breeding assemblages.
Males establish breeding territories and courtship involves elaborate male
displays. Once pairs have been established females construct nests utilizing
material provided mainly by the males. Nest building and breeding is usually
completed in three days, after which the birds begin to lose their breeding
colors (Weber 1972, Kaufman 1996).
There is a degree of promiscuity in the species, with males frequently mating
with more than one female during the breeding season (McKilligan 1990).
Embryology:
Mean clutch size of Bubulcus ibis in Florida is approximately 3-4 eggs per nest, although clutch
sizes ranging from 1-9 eggs have been recorded (Weber 1972, Kaufman 1996). One
egg with a length of 4-5 cm is laid every other day during nesting, and the
eggs hatch sequentially, approximately 24 days after they are laid. Survival
retes of the earlier hatchlings is better than that of the later hatchlings,
and is related to food availability (Weber 1972, 1975). Fledglings begin to
fly 25-30 days after hatching, and they become independent around 45 days
post-hatch (Kaufman, 1996).
IV. PHYSICAL TOLERANCES
Temperature:
Breeding populations of Bubulcus ibis are more or less restricted to the
southern United States, although breeding is believed to occur sporadically
throughout all or most of the country. The species is reported to range as far
north as Alaska and Newfoundland.
V. COMMUNITY ECOLOGY
Trophic Mode:
The common name of the cattle egret comes from their familiar habit of foraging
in pasturelands in association with livestock animals whose movements and
grazing activities flush out insect and other potential prey items. Cattle
egrets will also follow tractors in order to feed on the organisms that are
scared up (Scott 1987, Kaufman 1996). Crickets, grasshoppers, beetles, moths,
flies, and other small invertebrates and vertebrates make up the bulk of the
diet. Bubulcus ibis is an opportunistic forager; it will eat parasites off of
the bodies of the livestock with which they coexist and can also consume
relatively large prey items like crayfish, fish, frogs, small snakes, and even
bird eggs and nestling birds. In urbanized settings cattle egrets will
scavenge trash heaps for edible material (Kaufman, 1996).
Associated Species:
In the African portion of their native range, cattle egrets co-occur with
elephants, rhinoceros, hippopotamus, and other large herbivores, and across
much of their non-native range they associate primarily with cattle (Telfair
1994, Ivory 2000).
VI. INVASION INFORMATION
Invasion History:
Cattle egrets are notable among exotic species in that they are one of the very
few cases in which initial expansion into historically non-native areas appears to be a
largely natural occurrence unassisted by humans (Weber, 1972). The species is
highly migratory and is capable of dispersing in seemingly random directions
across thousands of kilometers in a few days (Kaufman 1996, CAST 2002).
Although the initial entry of Bubulcus ibis into New World appears to have not
relied on the helping hand of man, current range expansion within the U.S. is
related to extensive landscape conversion to pasturelands (Telfair 1994).
The first appearance of B. ibis in the New World dates to the late 1870s
and 1880s from Suriname in northeastern South America. By 1917, they had
appeared in Colombia, although they were not reported from Panama until 1954.
B. ibis apparently arrived in Florida in the 1940s and had begun
establishing breeding populations in the state in the 1950s (Wetmore 1965,
Hilty and Brown 1986). The animal is now abundant year-round throughout
Florida.
Potential to Compete With Natives:
Bubulcus ibis is highly adaptable and capable of living in a number of
human-altered agricultural and urbanized environments. Although the potential
exists for them to out-compete co-occurring native species for nesting areas
and food resources, published findings suggest there has been little impact of
B. ibis on native species in the U.S. (Weber 1972, Kauffman 1996).
Cattle egrets often occur as part of mixed-species nesting assemblages, and
Maxwell and Kale (1977) note that peak nesting in cattle egrets occurs after
that of most native herons. Dietary niche overlap is probably also minimal as
the diet of cattle egrets consists primarily of insects and terrestrial
invertebrates whereas other herons consume mostly primarily fish and aquatic
invertebrates (Weber 1972).
Possible Economic Consequences of Invasion:
The negetive economic impacts of cattle egrets in the IRL region and in Florida in general is likely to be minimal.
VII.
REFERENCES
Bryan J.C., Miller S.J., Yates C.S. and M. Minno. 2003. Variation in size and
location of wading bird colonies in the Upper St. Johns River Basin, Florida,
USA. Waterbirds 26:239-251.
Council for Agricultural Science and Technology (CAST). 2002. Invasive pest
species impacts on agricultural production, natural resources, and the
environment. Issue Paper 20. 18 p.
Cornell Lab of Ornithology. 2003. All About Birds species page: Bubulcus
Ibis. Available online.
Dugger B.D., Melvin S.L., and R.S. Finger. 2005. Abundance and community
composition of waterbirds using the channelized Kissimmee River Floodplain, Fl.
Southeastern Naturalist 4:435-446.
Hancock, J. and H. Elliot.1978. The herons of the world. Harper and Row
Publishing, New York. 304 p.
Hilty S.L. and W.L. Brown. 1986. A guide to the girds of Colombia. Princeton
University Press, Princeton, NJ. 836 p.
Ivory, A. 2000. "Bubulcus ibis" (Online), Animal Diversity Web. Accessed
July 16, 2007. Available online.
Kaufman K. 1996. Lives of North American birds. Houghton Mifflin Company,
Boston. 675 pp.
Maxwell G.R., II and H.W. Kale, II. 1977. Breeding biology of five species of
herons in coastal Florida. Auk 94: 689-700.
McKilligan N.G. 1990. Promiscuity in the cattle egret (Bubulcus ibis).
Auk:107:134-341.
Peterson R.T. 1980. A field guide to the birds. Houghton Mifflin Company,
Boston. 384 p.
Scott S.L. 1987. Field guide to the birds of North America. National Geographic
Society. Washington, D.C. 464 p.
Telfair R.C. II. 1994. Cattle egret (Bubulcus ibis). In: The birds of
North America, No. 113 (A. Poole and F. Gill, Eds.). The Academy of Natural
Sciences, Washington, D.C.
Weber W.J. 1972. A new world for the cattle egret. Natural History 81:56-63.
Weber W.J. 1975. Notes on cattle egret breeding. Auk 92:111-117.
Wetmore A. 1965. The birds of the Republic of Panama. Part I. Smithsonian
Miscellaneous Collections vol. 150. Smithsonian Institution, Washington, D.C.
483 p.
Report by:
J. Masterson, Smithsonian Marine Station
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