hosts of brood parasites have evolved egg-discrimination ability as a
defence that allows them to reject parasitic eggs laid in their nests.
Twenty-five years ago, Stephen Rothstein emphasised that rejection rates
differed markedly between potential host species in Europe and America. The
much more complete information available today supports Rothstein’s
conclusions, but also allows new ones, especially when considering host size.
For instance, successful resistance, one of the three potential long-term outcomes of brood parasite–host coevolution, is
considerably more frequent in small-sized European host species and in
medium-sized and large-sized Nearctic host species, while this evolutionary
outcome is rare among Neotropical hosts regardless of their size. These results
have never before been discussed, despite the differences being spectacular: 17
out of 19 small hosts presenting successful resistance are from Europe and 16
out of 17 medium-sized and 11 out of 13 large hosts presenting successful
resistance are from North America. Interestingly, many large Nearctic hosts
with a rejection rate close to 100% are corvids. The high rejection capacity shown by large Nearctic
potential hosts probably evolved as a response to a highly virulent extinct
brood parasite, either a large extinct cowbird or an extinct cuckoo species, which went extinct after losing the arms
race against its large hosts.
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