Fostering
Three main types of fostering can be distinguished:
- Augmentation. The addition of young, thus increasing the size of the
brood. - Replacement. The replacement of a clutch of eggs with a brood of young,
or the replacement of one brood with another. - Swapping. The swapping of young between broods, so that all the young
are about the same size, thus reducing the risk of mortality.
Fostering has been widely practiced in captivity in a range of species from many
different orders. Work on wild birds has been limited, and the most detailed and
successful studies have involved birds of prey.
In general, the more experienced the pair, the more liberties can be taken.
Some pairs are poor at rearing and can never be trusted with their own or fostered
young. The young to be fostered should not have developed fear reactions or they
may refuse to accept food from the adults. In species that produce altricial young,
fear reactions do not usually develop until the second half of the nestling period.
Fostering attempts with species that produce precocial young are usually done as
eggs since the young form attachments to their parents soon after hatching.
Augmentation fostering.The candidates for augmentation fostering are usually
birds with a smaller than normal broods. The fostered young should be close to
the age and size of the young that the adults are rearing.
The enlargement of normal brood size by adding extra chicks to the nests of
altricial species has given variable results. In 11 out of 40 brood enlargement
experiments reviewed by Dijkstra et al. (1990), enlarged broods suffered greater
mortality and yielded fewer fledglings than control broods, suggesting that in
these cases food was limiting. In the remaining 29 experiments, the enlarged
broods produced more fledglings, on average, than control ones, showing that
many species were able to raise larger than normal broods. If birds are to be given
extra young, extra food provision is a good precaution.
Replacement fostering. The replacement of whole clutches of eggs with young
is usually applied to birds that have been incubating non-viable eggs or whose
eggs are needed for other purposes. The young do better if they are several days
old and hence are stronger and easier to feed than newly hatched chicks. Large
falcons accept young up to about 3 weeks old (Fyfe et al. 1978). In Echo
Parakeets the optimal age for fostering is 4–7 days, although experienced females
will accept and rear younger or older chicks.
284 |Conservation management of endangered birds