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.