We re-evaluate the plausibility that five species of birds that breed in late summer in northwestern Mexico are migratory double breeders that first bred earlier in the same season to the north. We use data aggregated from scientific collections to generate abundance indices that adjust counts of specimens in collections by collecting effort, which we measure as the number of passerines collected in the same region and time period as the species of interest. Our abundance indices generally show displaced phenologies, such that presumed double breeders arrive and breed early in the north, then later in northwestern Mexico. We also compare breeding records for these regions, but these records could not be corrected for effort. Our phenologies suggest that the breeding populations of the Yellow-billed Cuckoo (Coccyzus americanus) from the western U.S. and northwestern Mexico may be derived from birds that bred earlier in eastern North America. Similarly, Orchard Orioles (Icterus spurius) breeding in late summer in northwestern Mexico and on the Mexican plateau may be derived from birds that attempted to breed earlier in North America. Our abundance indices and other new data suggest migratory double breeding is less likely in the Hooded Oriole (I. cucullatus) and probably not occurring in the Yellow-breasted Chat (Icteria virens) and Cassin's Vireo (Vireo cassinii).
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