Green Heron
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| Green Heron | ||||||||||||||||||||
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 In February, migrant B. v. virescens and resident birds sometimes separated as B. v. maculata occur at Tarpon Springs, Florida.
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| Butorides virescens (Linnaeus, 1758) |
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The Green Heron (Butorides virescens) is a small heron of North and Central America. It was long considered conspecific with its sister species the Striated Heron (Butorides striata), and together they were called "Green-backed Heron". Birds of the nominate subspecies (no matter which taxonomic arrangement is preferred) are extremely rare vagrants to western Europe; individuals from the Pacific coast of North America may similarly stray as far as Hawaiʻi.[1]
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[edit] Description
The green heron is relatively small; adult body length is about 44 cm. The neck is often pulled in tight against the body. Adults have a glossy, greenish-black cap, a greenish back and wings that are grey-black grading into green or blue, a chestnut neck with a white line down the front, grey underparts and short yellow legs. The bill is dark with a long, sharp point. Female adults tend to be smaller than males, and have duller and lighter plumage, particularly in the breeding season. Juveniles are duller, with the head sides, neck and underparts streaked brown and white, tan-splotched back and wing coverts, and greenish-yellow legs and bill. Hatchlings are covered in down feathers which are light grey above and white on the belly.[1]
The Green Heron's call is a loud and sudden kyow; it also makes a series of more subdued kuk calls. During courtship, the male gives a raah-rahh call with wide-open bill, makes noisy wingbeats and whoom-whoom-whoom calls in flight, and sometimes calls roo-roo to the female before landing again. While sitting, an aaroo-aaroo courtship call is also given.[2]
[edit] Systematics
As noted above, this species was formerly included in B. striata, which at that time was erroneously known as B. striatus. The Early Pleistocene B. validipes, whose fossil remains were discovered in Florida, might have been the ancestor of the Green Heron as the living species seems to replace the extinct relative in the fossil record.
Subspecies distinction is uncertain at best. The color variation between populations is less pronounced than between birds of the same population. Migratory populations are longer-winged than those which are resident all year round, but this cannot be used to delimit subspecies as it is quite obviously is a consequence of differing habits and can be expected to undergo convergent evolution in unrelated populations of this species which just share the same habits. Thus, thorough molecular phylogenetic studies would be required to resolve the question of subspecies delimitation.
The following subspecies are commonly listed, though the validity of most of them is seriously disputed:[3]
- Butorides virescens anthonyi (Mearns, 1895)
- Breeds in the USA west of the Rocky Mountains, south to northern Baja California Peninsula, Mexico. Some resident, most migrate to western Mexico in winter.
- Butorides virescens bahamensis (Brewster, 1888)
- Bahamas. Resident.
- Butorides virescens frazari (Brewster, 1888)
- Southern Baja California Peninsula, Mexico. Resident.
- Butorides virescens maculata (Boddaert, 1783)[verification needed] – formerly maculatus
- Southernmost USA through Central America to central Panama, Caribbean. Resident.
- Butorides virescens virescens (Linnaeus, 1758)
- Breeds from southeastern Canada to central and southern USA east of the Rocky Mountains. Winters from southernmost USA to northern South America.
Much of the dispute hinges upon the distinctness of the Caribbean and Central American populations, the second taxon in this species to be described. To describe the two most extreme views, some authors assemble the bulk of the mainland population in the nominate subspecies but treat the parapatric populations as distinct subspecies, while others place all resident populations in maculata and all migratory ones in virescens.
[edit] Ecology
Their habitat is small wetlands in low-lying areas. Green Herons are most conspicuous during dusk and dawn, and if anything these birds are nocturnal rather than diurnal, preferring to retreate to sheltered areas in daytime. They will feed actively during the day however if hungry or provisioning young. Shore-living individuals adapt to the rhythm of the tides. They mainly eat small fish, frogs and aquatic arthropods, but may take any invertebrate or vertebrate prey they can catch, including such animals like leeches and mice. Green Herons are intolerant of other birds - including conspecifics - when feeding and are not seen to forage in groups. They typically stand very still on shore or in shallow water or perch upon branches and await prey. Sometimes they drop food, insects, or other small objects on the water's surface to attract fish, making them one of the few known tool-using species. They are also able to hover briefly to catch prey.[1]
The northern population move to their breeding ranges during March/April; near the northernmost limit of the Green Heron's range, breeding is well underway by the end of May. The migration to the winter quarters starts in September; by late October, the birds are absent from regions where they do not stay all year. At least the northward migration does not seem to be affected by global warming; birds appear in their breeding ranges at the same time they did 100 years ago.[4]
Individuals of non-migratory populations will abandon their territories after breeding season to roam about in the region. They may or may not return to the previous year's breeding location, depending on whether they found better habitat during these wanderings. In these populations, the breeding season is determined by rainfall and consequent prey availability.[1]
Green herons are seasonally monogamous. The pairs form in the breeding range, after an intense courtship display by the males which select the nesting sites and fly in front of the female noisily and with puffed-up head and neck plumage. They nest in forest and swamp patches, over water or in plants near water. Nests are a platform of sticks, often in shrubs or trees, sometimes on the ground. Locations in trees are preferred, with some nests built 20 meters off the ground; heights of several meters above ground are more common though. Rarely, large numbers of these birds congregate in heronries for nesting.[1]
The clutch is usually 2-5, rarely up to 8[verification needed] bluish-white[verification needed] glossy eggs which are laid in 2-day intervals. After the last egg has been laid, both parents incubate for about 19-25[verification needed] days until hatching, and feed the young birds. The frequency of feedings decreases as the offspring near fledging. The young sometimes start to leave the nest at 16 days of age, but are not fully fledged and able to fend for themselves until 30-35 days old. Sometimes - particularly in the tropical parts of its range - the Green Heron breeds twice a year.[1]
[edit] Gallery
 Green Heron in Seattle, Washington. |
 In May, migrant Green Herons have left Guadaloupe; remaining birds belong to B. v. maculata (if that subspecies is valid). |
 Juvenile soon after fledging. |
 Subadult retaining some juvenile plumage. |
[edit] Footnotes
[edit] References
- BirdLife International (2004). Butorides virescens. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006. Database entry includes justification for why this species is of least concern.
- Cornell Lab of Ornithology (CLO) (1994): Bird Guide - Green Heron. Retrieved 2008-FEB-10.
- Dewey, T. & Butzbaugh, J. (2001): Animal Diversity Web: Butorides virescens. Retrieved 2008-FEB-12.
- Henninger, W.F. (1906): A preliminary list of the birds of Seneca County, Ohio. Wilson Bull. 18(2): 47-60. DjVu fulltext PDF fulltext
- Jutglar, Francesc (1992): 33. Green-backed Heron. In: del Hoyo, Josep; Elliott, Andrew & Sargatal, Jordi (eds.): Handbook of Birds of the World (Volume 1: Ostrich to Ducks): 417, plate 28. Lynx Edicions, Barcelona. ISBN 84-87334-10-5
- National Geographic Society (2002): Field Guide to the Birds of North America. National Geographic, Washington DC. ISBN 0-792-26877-6
- Ohio Ornithological Society (OOS) (2004): Annotated Ohio state checklist. Version of April 2004. PDF fulltext
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