JOHNSTON: CAYMAN ISLAND AVIFAUNA


turbance, or a combination thereof. Another thrush, Mimocichla plumbea
rubripes, became extinct on Swan Island, possibly because of forest de-
struction (Paynter 1956). Both species on Grand Cayman, incidentally,
were undoubtedly in Stage IV of a taxon cycle because, among other
characteristics, each was highly restricted in habitat distribution, the
thrush probably to limestone forests and the oriole to sites on the north-
ern coast. (I. 1. leucopteryx is still common on Jamaica as is 1. 1.
lawrencii on St. Andrews Is.) Conversely, the White-winged Dove
(Zenaida asiatica) has invaded all three Cayman Islands since about
1935 and is now a well-established and common resident, especially in
more peripheral habitats of the islands. A spurious attempt at coloniza-
tion by an unmated female Mimocichla plumbea on Grand Cayman was
earlier documented by Johnston (1969). If one can include this last
occurrence as a bona fide instance of immigration, then within the last
40 years Grand Cayman alone has experienced two instances of avian
extinction and two of immigration. Even if one considers the Mimocichla
incident as being an unsuccessful immigration, the data still fit the Mac-
Arthur-Wilson model for a small island where the probability of extinc-
tion increases with decreasing island size.
 Immigration and extinction are not well documented for birds on the
smaller islands (Cayman Brac and Little Cayman), chiefly because few
ornithologists have carefully studied the birds of these islands until re-
cently (Johnston et al. 1971). The interrupted distribution of certain
species on these islands is both curious and unexplained (see Table 14),
especially since histories underlying the current distributional patterns are
largely unknown, and little information is available on food, habitat, and
other ecological factors. Evidently Anmazona leucocephala does not
presently breed on Little Cayman (if it ever did, despite old specimens
and observations to the contrary), but small flocks regularly fly over
from Cayman Brac during the day to feed on ripe fruits. Conversely,
the older ornithological records indicate that Quiscalus niger was once
common on Little Cayman and Cayman Brac; today, small flocks fly
from Little Cayman to Cayman Brac to spend the day feeding, but
evidently this grackle does not now breed on Cayman Brac. Minmo-
cichla plumbea is moderately common at least on the lowland perimeter
of Cayman Brac (Table 11); yet it has never been recorded from nearby
Little Cayman but has attempted to breed on Grand Cayman.
 As late as 1956, the Mockingbird (Mimus polyglottos) was unknown
 from both Little Cayman and Cayman Brac (Bond 1956), even though
 the species has been a common resident of ruderal and roadside habitats
 on Grand Cayman at least since the earliest days of ornithological in-
 vestigations in 1886. By 1956 Mockingbirds were reported on Cayman