Intrinsic traits mediate variation in the latitudinal survival gradient
We find that the association between body mass and survival and reproduction and survival ― two of the cornerstone trade-offs of life history theory (Stearns 1992) ― are well supported by our meta-analysis, suggesting higher survival for larger birds and those with smaller clutch sizes. Notably, when mass and clutch size were included in the joint model, the strength of the latitudinal survival gradient was diminished (Table 1). Similarly, we found general support for the idea that sedentary behavior favors higher survival and, hence, shifts towards slower life-histories often associated with tropical latitudes. These results highlight the importance of considering the interplay between intrinsic and extrinsic variables when investigating macroecological processes. Latitude of course is associated with the changes in many aspects of avian life history, including migratory behavior (Alerstam et al. 2003), clutch size (Cardillo 2002; Jetzet al. 2008) and body mass (Olson et al. 2009), all of which have been demonstrated to increase globally with increasing latitude. Combined with these findings, our results are in accordance with the theory of a slow-fast life-history continuum (Ricklefs & Wikelski 2002) and suggest that while birds at tropical latitudes tend to be longer lived and have reduced clutches given their body size, this is far from the full picture. Global patterns of avian survival are driven by interactions between intrinsic traits and lineage-specific effects of latitude and their associated climatic factors.