Global-scale patterns of avian survival with latitude
We found support for the oft-toted latitudinal survival gradient, but
this depended on both the geographic region and taxa being considered.
Specifically, we demonstrate that the previously noted inverse
relationship between latitude and survival is only weakly borne out
across northern hemisphere avifauna overall, but that this effect is
strengthened when considering only passerines or species inhabiting the
mainland. In contrast, the relationship was only evident in the southern
hemisphere for survival estimates from New World birds, the vast
majority of which were passerines. When considered independently, there
was no indication that nonpasserines had higher survival with decreasing
latitude in either hemisphere. Overall, our meta-analysis reveals that
while some tropical birds may be longer lived than their temperate
counterparts, the shape of the latitude-survival response is likely to
differ among species and between hemispheres.
Our synthesis is the first to assess global-scale patterns in avian
survival rates; previous studies have either been limited geographically
(Karr et al. 1990; Peach et al. 2001; Lloyd et al.2014), or have focused on a narrower range of species, such as raptors
(Newton et al. 2016) or shorebirds (Méndez et al. 2018).
To date, the most extensive analysis of avian survival and latitude
comes from a study of 12 locations spanning 60° across the Americas
(Muñoz et al. 2018). Our global-scale analysis compliments that
of Muñoz et al. (2018), who reported a linear decrease in
survival of roughly 2.1% for every 10° increase in latitude for
passerine birds from Alaska to Peru, similar to what we observed for
northern hemisphere species worldwide. Granted both our studies used a
meta-analytical approach, Muñoz et al. (2018) conducted their
analysis using a Bayesian mode of inference and considered only
forest-dwelling passerines, while our study includes survival estimates
of both passerines and nonpasserines from a variety of habitats, which
we investigated using a maximum-likelihood approach. We also fit
regression lines for latitude both north and south of the equator rather
than testing the relationship between survival and absolute latitude.
This latter point is particularly important, given that one general
explanation for spatial patterns in life-history traits is that they
arise from natural selection imposed by latitudinal gradients in
environmental conditions (Cardillo 2002), which differ between
hemispheres (Chown et al. 2004). Despite our use of different
methods, the fact that we obtained some common results lends increased
support to the overall relationship. Moreover, with our analysis, we
provide a strong mechanistic basis for understanding variation in
survival rates, as it better reflects the climatic variables that
underlie latitude in the northern and southern hemispheres.
Hemispheric asymmetries in other patterns of avian life-history traits,
such as timing of reproduction (Covas et al. 1999), clutch size
(Moreau 1944; Martin et al. 2006; Lloyd et al. 2014), and
parental care (Russell et al. 2004; Llambías et al. 2015),
are well documented. The global patterns we identified are also
congruent with the idea of a differential response of life-histories
between hemispheres ― we detected an inverse relationship between
survival and latitude in the northern hemisphere but found little
indication that this association was mirrored by southern hemisphere
species overall. Only when we analyzed biogeographic realms in the
southern hemisphere separately did we find that New World birds showed
higher survival with decreasing latitude. This pattern is deceptive,
however, since southern hemisphere nonpasserines account for little more
than 1% of the effect sizes analyzed in the New World data subset. We
therefore interpret this result as evidence of the latitudinal survival
gradient in South American passerines. This means that for Old World
birds, tropical species had similar survival rates to birds from the
austral zone, and this was likely to be true regardless of whether they
were passerines or nonpasserines. Survival estimates from Australasia
and Oceania, biogeographic realms not traditionally included in the New
/ Old world classification, also reflected this same pattern and showed
no evidence of a negative relationship with latitude.
Such differences may be explained, in part, by the historical geography
and latitudinal positions of the continents. For the last 15 million
years, South America has extended roughly 20° further into the southern
hemisphere than continental landmasses in the Old World. Thus, one
reason we may have detected a negative trend in survival for southern
hemisphere birds, but only in the New World, could simply be due to the
greater range of latitudes and climatic conditions available to
landbirds from South America with which to adapt. For example, latitudes
greater than 35° S are characterized by higher seasonality and mean
annual temperatures ≤0°C (Chown et al. 2004); thus, this result
may be indicative of a threshold response of avian survival to freezing
temperatures and / or a more seasonal environment. Supporting this idea,
mean survival of South American passerines that occurred at latitudes
higher than 35° S (survival rate = 0.38, n = 8) was lower on average
than those from the highest latitudes occupied by birds in Africa (Old
World survival at 34° S = 0.69, n = 19). Only one other study has
addressed the question of a latitudinal survival gradient in the
southern hemisphere; Lloyd et al. (2014) found no indication of
higher survival for birds living in tropical Malawi compared to austral
South Africa. Our results are congruent with those findings and suggest
that higher survival of tropical birds may be a pattern localized
primarily to passerines from the northern hemisphere and in South
America, where factors such as a more seasonal environment may limit
resource availability and constrain species survival.