Rohwer et al. (2009) present isotopic evidence of migratory double breeding by five species of birds that were breeding in July and August in northwestern Mexico. The presence of old brood patches in these species, combined with their late breeding in Mexico, suggested that these species had bred earlier in the north. Here we present data showing that Orchard Orioles and Yellow-breasted Chats breed as commonly in coastal Sinaloa in late May and early June as they do later in the summer; further, many females of these species examined in early June 2011 had downy ventral apteria, indicating that they were breeding for the first time in their annual cycle in Sinaloa. Thus, the old brood patches observed in these species in July and August, when they are still breeding in northwestern Mexico, may reflect earlier breeding attempts by those same individuals in Mexico. Yellow-billed Cuckoos seldom call and are uncommon in northwestern Mexico until late June and likely do not begin breeding until July. For Yellow-billed Cuckoos there had been no description of how the ventral apterium changes with breeding, so Rohwer et al. (2009) assumed that it followed a passerine pattern of re-feathering during the post-breeding molt. To test this assumption we examined the ventral apterium in cuckoo specimens collected throughout the winter and found that it remains featherless throughout the year, including immediately after the complete mid winter molt. Thus, the bare ventral apteria of cuckoos arriving in northwestern Mexico in June and July do not constitute evidence of prior breeding in that year.
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Vol. 125 • No. 2