which they lived. Such natural instances of cross fostering can also lead to hybridization between species that would not normally breed. A case of this is offered by the Galapagos finches. Two species of the genus Geospiza, the medium ground-finch (Geospiza fortis) and the common cactus-finch (Geospiza scandens) occasionally hybridize. The birds' songs are a barrier to interbreeding, but sometimes young birds will not learn their own species song, e.g. if their father dies and they are nesting near another species. Another situation where birds can imprint on the wrong song is when one species takes over the nest of another, but fails to remove all of its eggs. Cross fostered young can then hybridize with their foster parents' species, allowing gene flow between the two populations. Hybrids experience reduced fitness, however, so the two species can remain separate. See also Brood parasite Phenotypic plasticity Twin studies References Primary sources http://www.southalabama.edu/biology/faculty/obrien/teaching/bly122/Chapter%2047.pdf Alcock, J. (1998) Animal Behavior: An Evolutionary Approach (6th edition). Sinauer Associates, Inc. Sunderland, Massachusetts. Zoology Conservation biology