The rufous-necked Green Heron is resident in North
America, Central America and throughout the Caribbean, with
northernmost populations wintering as far south as northern South
America. The Striated Heron occurs throughout South America, where it
is relatively sedentary, as well as the Old World. The two forms meet
in southern Central America, islands of the southern Caribbean, and in
coastal northern South America, where hybridization occurs as inferred
by increased intermediacy and variability (Payne 1974, Hayes 2002).
Because seemingly pure phenotypes occur within the hybrid zone,
implying assortative rather than randon mating, the two taxa are
probably best regarded as specifically distinct, although more study is
needed (Hayes 2002).
In the southeastern
Caribbean, the Lesser Antilles and Tobago are inhabited by the Green
Heron whereas the Striated Heron occurs in Trinidad and the adjacent
mainland. The Striated Heron may be expected to wander northward; the
only Lesser Antillean records are a specimen from St. Vincent (click here) and a recent record from the Virgin Islands (click here).
The Green Heron occurs rarely in Trinidad, presumably as a migrant from
North America or perhaps from Tobago. In Tobago, the population is
composed predominantly of Green Heron with a small proportion of
Striated Heron and intermediate individuals. In a comprehensive study
of geographic variation within the New World, Payne (1974) used a
series of nine voucher specimens as a hybrid index to score neck
coloration in adults (see Fig. 4 in page 2).
Photographs of these specimens can be used to score live individuals in
the field (Hayes in prep.). Given the frequent occurrence of
intermediate phenotypes in the southeastern Caribbean and the confusion
regarding their identity, the variability of these taxa is discussed in
this photo essay.
Juveniles and immatures.--Juveniles
and immatures of both species are readily distinguished from adults by
their heavy streaking on the brownish neck and upper breast, a brownish
cast to the upperparts, a pale triangular spot at the tip of each
covert and remige, and a whitish belly. Immatures have a more solid,
black crown, whiter chin and throat, and lighter streaking on the neck
than juveniles. The thighs may be barred and the bill is usually paler
than in adults. Distinguishing between the two taxa at this age is
extremely difficult. The only apparent distinction is neck colouration,
in which Green Heron tends to be more rufous and Striated Heron tends
to be more grey, but there is considerable overlap. Observers should be
careful not to mistake brown-necked juvenile and immature Striated
Heron for adult Green Heron.
Subadults.--First-alternate
(first-summer) birds resemble adults but retain pale-tipped juvenal
wing feathers and have shorter back plumes. Adult neck colouration is
aquired, being solid except for a pale malar stripe and a narrow pale
line with rufous streaking on the foreneck. The two taxa are usually
distinguished by neck coloration, which is more rufous in Green Heron
and greyer in Striated Heron. However, considerable variation occurs
and intermediate individuals may represent hybrids.
Adults.--Adults
are readily distinguished from immatures by their darker bill, solid
colouration on the sides of the neck, greenish-grey upperparts lacking
brown lones, longer back plumes, more pointed scapulars and wing
coverts, narrower buffy margins to the wing coverts (which lack white
spots), narrower white tips of flight feathers restricted to the inner
primaries and secondaries, and darker brown-gray underparts. The only
apparent distinction between the two taxa is neck coloration, which is
more rufous in Green Heron and greyer in Striated Heron.
Considerable variation occurs, with neck coloration ranging from grey
to brownish grey in Striated Heron throughout its range, and from
grayish red-brown to dark purplish brown in the Green Heron.
Intermediate appearing individuals in the southeastern Caribbean may
represent hybrids and obviously cannot be identified. Hindneck
coloration of adults often provides the best clue for identification.
In Green Heron, the hindneck is always rufous without any hint of grey,
except when feathers are ruffled or moulting, revealing the grey bases
of adjacent feathers. In browner-necked Striated Herons, the brown is
usually (though not always) more intense toward the front of the neck,
where it merges with the rufous streaking, and is often greyer on the
hindneck.
LITERATURE CITED
Hayes, F. E. 2002. Geographic variation, hybridization, and taxonomy of New World Butorides herons. North
American Birds 56:4-10.
Payne, R. B. 1974. Species limits and variation of the New World Green Herons Butorides virescens and Striated
Herons B. striatus. Bulletin of the British Ornithologists' Club 94:81-88. |
