Clutch size but not egg size associates with migration distance in South American land birds

Abstract

The association between migratory strategies and life-history traits helps explain how migratory organisms balance the energetic costs and survival risks with the benefits of migration. However, there is no consensus on how life-history traits associate with migration, and on migrant’s position at the slow–fast continuum of life history. Birds subject to different selective pressures are likely to show distinct patterns from each other. We used data from egg collections to investigate the relationship between reproduction and migration by assessing clutch size and egg size of 58 migratory and non-migratory tyrant flycatchers breeding in South America. We first compared clutch size and egg size of migrants and non-migrants, and then we assessed how migrants balance these reproductive traits with migration distance. Despite energy expenditure faced by migrants during their journey, migratory behavior was not a factor influencing clutch size and egg size of migrants and non-migrants. On the other hand, migration distance positively correlated with clutch size in migrants. Our study provides evidence that migration distance may constrain migrants in terms of costs and pressure reproduction in the direction of a faster life-history strategy, while migratory behavior per se may not be a determinant to place migrants in the slow–fast continuum of life history. Thus, among tyrant flycatchers breeding in South America variation in migratory strategies might be more important than migratory behavior in interacting with life-history traits. This study also demonstrates the potential of museum egg collections to test ecological hypotheses that investigate large-scale variation in breeding parameters of birds.