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ABSTRACT.-
Recent studies have suggested climate change could amplify the differences between arrival dates of male and female passerines. We investigated the generality of this finding and additional questions related to protandry by analyzing 32 years of banding data for seven species of migratory passerines. Six species exhibited significant protandry with males arriving on average between 2 and 6 days earlier than females. Only Baltimore Orioles (Icterus galbula) did not have significant differences between average arrival dates of males and females. The magnitude of protandry did not change in response to warming spring temperatures during the period of study, and none of the ecological variables examined explained variation in the amount of protandry. Males of all species studied were significantly larger than females. However, the magnitude of size difference also did not explain the amount of protandry observed. Arrival dates of males and females within each species tended to follow similar trends over time and sex ratios did not change over time for any species. Changes in sex ratios of Mourning Warblers (Geothlypis philadelphia)-more females in warmer years-however, were significantly related to mean temperature in the year of migration. Protandry may remain fairly consistent as the climate changes, although further research is needed to test the generality of this pattern.
Received 15 May 2011. Accepted 24 December 2011.
Evidence shows that male passerines tend to arrive earlier each spring than females of the same species, a phenomenon known as protandry (Rubolini et al. 2004). The generality of this pattern is still uncertain and mechanisms underlying this difference have not been definitively established although several hypotheses exist (Morbey and Ydenberg 2001, Coppack and Pulido 2009, Saino et al. 2010, Spottiswoode and Saino 2010).
Recent studies have suggested that climate change could amplify the difference between male and female arrival dates, because male and female migratory birds may respond differently to warming climate (M0ller 2004, Spottiswoode et al. 2006; but see Rainio et al. 2007). Males may be under selection to arrive as early as possible (Crick 2004) while, for females, it may be advantageous to remain in wintering areas until conditions have improved (Spottiswoode et al. 2006). Thus, males could gain an advantage over other males by arriving earlier in warmer spring conditions, allowing them...