Resident or short-distance migrant birds that occupy urban habitats are known to breed earlier than nearby rural populations, but similar data on long-distance tropical migrants are lacking. To understand the relationship between urbanization and reproductive phenology of a tropical migratory bird species, we (1) described variation in patterns and consequences of breeding phenology across an urban-to-rural gradient and (2) assessed underlying reasons why phenology may be related to urbanization. We studied Acadian Flycatchers (Empidonax virescens) between 2001 and 2007 in 35 forests spanning an urban-to-rural gradient in central Ohio. The general pattern of breeding phenology was opposite of that described for most resident and short-distance migrant birds. At higher levels of urbanization, site arrival dates and clutch initiations were later and cessation of breeding was earlier than in forests of more rural landscapes. The phenological shifts reduced the length of the nesting season in urban landscapes, with the result that birds in more urban landscapes initiated fewer nests and fledged fewer young than their rural-breeding counterparts. Underlying causes of phenological differences are less clear. One possibility supported by our data is that urban forests are less desirable and selected later by smaller females, which initiated first clutches later in the nesting season than larger females. This study provides the first evidence that urbanization is associated with attenuated nesting seasons for tropical migratory birds and that this shift in breeding phenology may have reproductive consequences.
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