Influence of Climate on Survival
Our results suggest that temperature seasonality, at least at the
resolution that we examined it, is a poor predictor of avian survival.
Indeed, latitude-only models out-performed single-predictor models of
extrinsic climate factors for each of the moderators we considered by a
minimum of >4.88 ΔAICC (Table S2). Although
temperature seasonality was not significant, our finding of higher
survival in the southern hemisphere, but only for New World birds, is in
accordance with reported asymmetries in climate between hemispheres.
Compared to north-temperate latitudes, austral latitudes are
characterized as less seasonal in general, having higher minimum winter
temperatures and higher, less variable patterns of precipitation (Chownet al. 2004). That said, South America does posses environments
with climates closer to those of the northern hemisphere (e.g., mean
temperatures ≤0°C, higher temperature seasonality) compared to Africa
and Asia, which lack such climate analogs at their southern-most
latitudes. Although latitudinal variation in life history traits arises
in part from natural selection imposed by complex interactions among
environmental factors, latitude as a ‘catch-all’ variable provided a
more complete picture of global variation in survival. For example,
temperature seasonality fails to capture the negative latitude-survival
relationship in passerines because this effect is counter-acted by
pooling data for taxa from different regions; specifically, combining
data with estimates for southern hemisphere passerines from the Old
World. It appears, therefore, that latitude remains one of the best
methods to portray the suite of climatic constraints that characterize a
species’ environment and leads to variation in life histories, but only
when northern and southern hemispheres are examined independently.