Fostering of eggs
Eggs may be fostered to nests to add to those already there or to replace non-viable
eggs. Fostered eggs should be at the same stage of development as the rest of the
clutch.
Clutch augmentation is used with captive birds where pairs may incubate and
hatch larger clutches than normal, but is limited by the number of eggs that the
incubating bird can effectively cover. Wild birds on clutches larger than normal
are likely to succeed in rearing larger broods only when natural food is not limiting
or when extra food is provided.
Egg augmentation and replacement is an easy way to ensure that all nests in a
population have the possibility to hatch young, and is an useful technique to
introduce captive–produced eggs into a wild population.
Sequential egg removal
If an egg is removed soon after it is laid, the bird keeps laying further eggs in an
attempt to complete a clutch, sometimes producing more eggs than the usual.
This technique only works on birds that have an indeterminate clutch size. It is a
technique most often used on captive birds, where the laying of eggs can be care-
fully monitored. Captive Sandhill Cranes Grus canadensishave exceptionally laid
18 and 19 eggs in succession, yet the normal clutch size does not exceed four.
Single egg removal resulted in an average of 6.4 eggs per bird per year compared
with 5.3 if the birds were “double clutched” (i.e. removal of a complete clutch to
stimulate the laying of another). These egg removal studies did not have any
marked effect upon egg viability (Ellis et al. 1996).
Multiple and double clutching
Many species of birds that normally produce only one clutch per season can lay
a replacement clutch if the first nesting attempt fails, and some species can lay
several clutches in a season. This ability to recycle can be exploited by removing the
first clutch, and sometimes successive clutches, for artificial rearing (or fostering)
and then leaving the pair with a final clutch to incubate and to rear themselves.
This technique was used for California Condors in the wild prior to the last birds
being brought into captivity. Californian Condors lay single egg clutches and if
they successfully rear a chick it is dependent on its parents for so long that they do
not breed the following year; hence successful wild Condors can produce only a
single independent young every other year. Between 1983 and 1986, 16 eggs were
taken for artificial incubation from five different pairs. Thirteen (81%) of the eggs
produced surviving chicks, far exceeding the 40–50% fledging success of wild
282 |Conservation management of endangered birds