Does cowbird parasitism create ecological traps?
Decision rules in response to nest predation and the number of broods fledged suggest a long evolutionary history of responding to predation. For many North American species, however, cowbird parasitism has become a threat only recently, as the Brown-headed Cowbird increased both its geographical range and global population (reviewed in Robinson et al. 1995a). This phenomenon is so recent (<50 years in some areas of the West) that many species may not yet have had the opportunity to evolve resistance (evolutionary "lag": Rothstein 1975). Birds of the eastern deciduous forest, in particular, seem to lack any defenses against parasitism. They tend not to eject cowbird eggs or abandon parasitized nests, nor do they select nest sites that are inaccessible to cowbirds (e.g., cavities). Many prairie and shrubland birds, in contrast, have a variety of defenses against parasitism (reviewed in Robinson et al. 1995a), which suggest a long history of coevolutionary interactions with cowbirds (Rob
