location and apparently paired preferentially with their own species rather than
with Black-headed Gulls.
Harris (1970) swapped the eggs of the Lesser Black-backed Gulls Larus fuscus
with Herring Gulls Larus argentatuson Skokholm Island, Wales. Some 496
Lesser Black-backed Gull eggs were placed in the nests of Herring Gulls and 389
Herring Gull eggs were placed in the nests of Lesser Black-backed Gulls. Cross-
fostered gulls were subsequently found breeding on Skokholm and, of these, 71
were in mixed species pairs and 44 were breeding with their own species. Harris
found that cross-fostered females usually mated with the males of their foster
parent while the males mated with either species.
In an attempt to reintroduce Peregrine Falcons into their former range, young
were fostered in several areas into the nests of Prairie Falcon Falco mexicanus. In
California 113 nestling Peregrine Falcons were fostered into nests of wild Prairie
Falcons, and all or most fledged. About ten of these cross-fostered Peregrines
were later found breeding normally in the wild and none was seen mated to a
Prairie Falcon (Walton and Thelander 1988; Cade and Temple 1995).
In the Mauritius Kestrel, inexperienced wild pairs were, on four occasions,
given pipping eggs of Common Kestrels Falco tinnunculusto gain experience of
hatching and rearing. One pair proved competent enough for the Common
Kestrels to be replaced with Mauritius Kestrels ( Jones et al. 1992). Similarly,
Chatham Island Tits Petroica macrocephalawere fostered under the rarer
Chatham Island Black Robins to give the robins rearing experience (Butler and
Merton 1992).
The cross-fostering of Chatham Island Black Robins has been the most suc-
cessful use of this technique for the conservation of a critically endangered
species. Black Robin eggs and young were cross-fostered under Chatham Island
Tits and Chatham Island Warblers Gerygone albofrontata, a procedure which
encouraged Robin pairs to lay replacement clutches. Although the warblers
hatched the eggs and reared the young robins successfully for the first week, they
could not bring sufficient food to rear them beyond 10 days old. Subsequently, if
warblers were used as foster parents, the young were moved back to robins after
a week. The tits could rear the Black Robins successfully to fledging, but these
young imprinted to, and attempted to breed with tits. This was overcome by:
(1) swapping the cross-fostered young back to robins before fledging so that they
developed the appropriate species fixation; and (2) translocating robins that
fledged under tits to an island lacking tits so that the robins had no option but to
breed with each other, which they did with some initial reluctance. Of 180 black
robin eggs incubated by tits, 156 (87%) hatched—a figure comparable to that
from eggs incubated entirely by robins (Butler and Merton 1992).
286 |Conservation management of endangered birds