INTRODUCTION
Aves, a class represented by around 10,000 species, display a broad
diversity of morphologies and behaviors, and also show considerable
variation in their lifespan and annual survival. For example, in
large-bodied landbirds, such as some raptors and parrots, annual
survival is often high (Newton et al. 2016; Maestri et al.2017) and individuals are long lived, but for small-bodied species like
warblers and kinglets, rates of annual survival can be low (DeSanteet al. 2015; Johnston et al. 2016). While differences in
body mass account for some of this variation ― larger species tend to
live longer than smaller ones (Lindstedt & Calder 1976, 1981; Promislow
1993; Speakman 2005) ― many species live longer or shorter lives than
predicted given their body mass (Healy et al. 2014). Other
aspects of a species’ life history, particularly the demographic cost of
reproduction, may explain this residual variation in survival rates
(Williams 1966; Stearns 1992; Roff 2002). This view stems from the
hypothesis that limited resources (i.e., time and / or energy) result in
an allocation trade-off between two competing vital functions;
specifically, current reproduction reduces future reproduction and
survival. The pivotal survival-reproduction trade-off has been well
documented in birds (Ricklefs 1977, 2000; Saether 1988; Linden & Møller
1989; Martin 1995; Ghalambor & Martin 2001), and with the observations
of early investigators that the number of eggs laid declines from the
poles towards the equator (Moreau 1944; Lack 1947; Skutch. 1949), it has
given rise to the expectation that tropical species should offset a
reduced clutch size by having higher adult survival (Murray 1985).
There are many studies that suggest high adult survival in tropical
birds, the majority of which focus on comparisons between
north-temperate systems and the tropics (Martin 2004). Early reports of
high survival came from studies equating survival estimates with return
rates (Snow 1962; Fogden 1972; Fry 1980; Bell 1982; Dowsett 1985). While
these studies deepened our understanding of life-history strategies in
tropical birds, estimates based on return rates are problematic because
they confound estimation of complicated functions of survival rate and
capture probability (Nichols & Pollock 1983; Krementz et al.1989; Sandercock 2006). More recently, studies employing improved
methods for estimating survival via Jolly-Seber (JS) and
Cormack-Jolly-Seber (CJS) models, which separate apparent survival
(i.e., Φ: the product of true survival and site fidelity) from encounter
probability (Sandercock 2006), have reinforced the idea of higher adult
survival at lower latitudes (Faaborg & Arendt 1995; Johnston et
al. 1997; Francis et al. 1999; Peach et al. 2001;
McGregor et al. 2007). The generality of these findings, however,
has been questioned based on comparisons showing negligible differences
in survival between Central and North American birds (Karr et al.1990), and lower than expected survival rates for birds from South
America (Blake & Loiselle 2008). Other studies have even found higher
survival rates for south temperate birds compared to tropical species in
Africa (Lloyd et al. 2014). Only one quantitative review has
formally addressed latitudinal patterns in adult survival rates of birds
across a broad range of latitudes. Muñoz et al. (2018) showed
that adult survival was higher for species in the tropics compared to
those in five sites across the north temperate zone, supporting the
hypothesis of a latitudinal gradient in survival, at least for
forest-dwelling passerines in the western hemisphere. Yet, despite
longstanding interest in the idea of a latitudinal gradient in survival,
we still lack an empirical synthesis at the global scale, which stands
as a limiting factor in our ability to generalize these relationships to
the diverse life history of birds found worldwide (Martin 2004).
Most explanations for a latitudinal survival gradient are based on the
assumption of consistent latitudinal variation in survival and other
life history traits with which it covaries, such as clutch size (Karret al. 1990; Faaborg & Arendt 1995; Johnston et al. 1997;
Peach et al. 2001; McGregor et al. 2007). Indeed, most
comparative studies of variation in life history traits treat northern
and southern latitudes equivalently (Jetz et al. 2008; Muñozet al. 2018; Terrill 2018). However, this assumption may not
always be met, since latitude itself does not directly influence
organisms per se; rather, environmental factors that covary with
latitude exert selective pressures on life history traits. For example,
although there exists a global latitudinal gradient in clutch size
(Cardillo 2002; Jetz et al. 2008), this trend is dampened in the
southern hemisphere ― south temperate species lay smaller clutches than
those in the north temperate hemisphere (Yom-Tov et al. 1994;
Martin 1996; Evans et al. 2005). Consistent with this pattern,
south temperate birds in Africa also tend to be longer lived than their
north temperate European counterparts (Lloyd et al. 2014). This
hemispheric asymmetry may in part be due to differing climatic
conditions between northern latitudes and equivalent southern ones.
Namely, south temperate latitudes are less seasonal and have higher
minimum winter temperatures, both of which have been hypothesized to
decrease adult mortality and lead to smaller clutch size (Ricklefs
1980). Similarly, clades and their intrinsic traits that may influence
survival rates are also distributed nonrandomly across environmental
gradients (Jetz et al. 2008; Sibly et al. 2012). Migratory
habit, for instance, arises at least in part from species occupying
higher latitudes and experiencing seasonal environments with lower
minimum winter temperatures, and there can be substantial deleterious
effects on survival over the migratory phase of the annual cycle
(Sillett & Holmes 2002; Rockwell et al. 2017). Thus, the
geographic variation in survival rates reflects a composite of extrinsic
factors, intrinsic traits, and historical events related to a species’
lineage.
Because previous analyses of the latitudinal gradient in survival have
focused on the north-temperate / tropical model (Martin 2004; Muñozet al. 2018) and have relied on a relatively narrow group of
taxa, our current perspective of the biological underpinnings of the
geographic variation in survival rates remains somewhat limited. Here,
we present data on survival rates for 681 species of landbirds gathered
from around the world (Fig. 1). The purpose of our analysis was to test
the relative importance of latitude and extrinsic climate factors
(temperature, precipitation, and seasonality) in explaining geographic
patterns of avian survival rates, and to ask whether including intrinsic
traits (body mass, clutch size, migratory habit) improved model
predictions. Specifically, we ask: (1) Is there a latitudinal gradient
in adult survival and, if so, are there differences between hemispheres?
(2) Do climate measurements (extrinsic factors) explain differences in
survival rates as well as latitude? (3) Do intrinsic traits explain
additional variation in species-level survival rates? We tested for
these relationships in both passerines and nonpasserines and between Old
World and New World birds from mainland and island populations.
By integrating data on
macroecological processes with comparative biology, our modeling
approach provides a powerful tool for understanding the diversity of
life histories that have evolved across the globe.