Cross-fostering should proceed with care because of the very real problems of
sexual imprinting to the foster species (Immelman 1972). Due to the taxonomic
closeness of species within the same genus there is the possibility that they would
produce fertile hybrids. For example, Scarlet Ibises Eodocimus ruberwere intro-
duced to Florida by cross-fostering the eggs to White Ibises E. albus. This proce-
dure resulted in considerable hybridization, producing “pink “ ibises (Long 1981).
Inter-generic cross-fostering. After a series of cross-fostering experiments in
captivity, Fyfe et al. (1978) experimented by placing broods of Prairie Falcons
under three species of Buteohawks. This was done to test the suitability of these
different Buteospecies as foster parents, but also to see if the procedure could be
used to extend the Prairie Falcon’s distribution. Two pairs of Ferruginous Hawks
Buteo regalisraised four out of five young, three pairs of Red-tailed Hawks B.
jamaicensisraised ten out of eleven young and a pair of Swainson’s Hawks B.
swainsoniraised four out of five young in a single brood. Subsequently three pairs
of Prairie Falcons were found breeding near to the cross-fostering sites but out-
side of the natural range of Prairie Falcons; they were assumed to have been the
young from the cross-fostering experiments. One pair even nested for two sea-
sons in an old Buteonest, the first time this behavior had been recorded in Prairie
Falcons. Clearly this trial, although modest in terms of sample size, demonstrates
the potential of the technique. Similarly, in Germany Common Buzzards Buteo
buteo, Goshawks Accipiter gentilis, and Common Kestrels nesting in trees have
been used to rear young Peregrine Falcons in an attempt to re-establish a tree
nesting tradition in the latter species (Saar 1988). Several pairs of Peregrine
Falcons have subsequently nested in trees in the area concerned (Cade 2000).
Where possible, cross-fostering should be of whole broods so that the siblings
can socialize with each other. Where single young are raised by foster parents of
another species, then the only possibilities of early socializing and sexual
imprinting are with the foster parents, as indeed happened when Whooping
Cranes were fostered under Sandhill Cranes Grus canadensis(Ellis et al. 1996).
Cross-fostering has worked well in some inter-generic attempts, where hybridiza-
tion between the species was highly unlikely, as demonstrated by the successful
rearing of young falcons by ButeoandAccipiterhawks.
There are clear behavioral constraints on which species will tolerate the young
of another species. In small passerines, neither domestic Bengalese Finches
(Estrildidae) nor domestic Canaries (Fringillidae) reared the young of Madagascar
Fodies Foudia madagascariensis(Ploceidae) even though they incubated and
hatched the eggs. Neither species responded to the begging cries and gaping of
the young fodies. The Bengalese Finches did, however, rear the young of the
Intensive management of focal pairs| 287