Experience with wild Mauritius Kestrels and Echo Parakeets has shown that,
if birds fail in breeding, but are going to be required to foster young later, then
they can be given dummy eggs, even for up to 5 days after the young have
hatched. They will incubate these for up to 5 days and when given the foster
young will look after them.
Swapping. In some species, such as raptors and parrots, where hatching is asyn-
chronous, the smaller young have poorer survival. The swapping of young
between broods, so that all in each brood are about the same size, can enhance the
survival of the compromised young and increase brood size at fledging. This has
worked with several species including Kakapo, Echo Parakeet, Mauritius Kestrel,
Pink Pigeon, and Spanish Imperial Eagle Aquila adalberti(Meyburg 1978).
Cross-fostering of eggs and young. Cross-fostering the young (or eggs) from
one species to another has been tried in many taxa. Usually a common species is
used to rear the young of a rarer species, freeing up the rarer species to lay addi-
tional clutches. Sometimes, however, the young of a common species have been
fostered to a rarer species to test parental abilities and to provide rearing experi-
ence before a fostering attempt with a conspecific.
For centuries strains of domestic chickens have been used to incubate the eggs
and rear the chicks of captive game birds and waterfowl. Domestic Bengalese
Finches Lonchuria striatahave been used to rear rarer estrildid finches, especially
Gouldian Finches Chloebia gouldiae. But most of the conservation-orientated cross-
fostering studies on wild birds have involved diurnal birds of prey and the Chatham
Island Black Robin, where cross-fostering has been attempted using a common
species to rear the young of a rarer species. Olendorff et al. (1980) and Barclay
(1987) review cross-fostering studies in raptors involving 12 different species.
Intra-generic cross-fostering.McIlhenny (1934) pioneered cross-fostering as a
successful conservation technique on wild birds. Snowy Egret Egretta thulaeggs
were harvested and cross-fostered to the nests of the commoner Little Blue Herons
E. caeruleaand Tricolored Herons E. tricolor. The Snowy Egrets recycled and were
left to incubate their second clutches and rear the young. The cross-fostering was
successful and the Snowy Egret population rapidly increased.
Subsequently the cross-fostering of wild birds was attempted in ethological
studies. Schutz (1940 quoted by Cade 1978) placed the eggs from a tree-nesting
colony of Common Gulls Larus canusin the nests of Black-headed Gulls
L. ridibundusamong reeds. On reaching sexual maturity, the Common Gulls
returned to their hatching and rearing location and formed a small colony with-
in the Black-headed Gull colony. The Common Gulls adopted a new breeding
Intensive management of focal pairs| 285