Challenges in evaluating avian survival
Adult survival estimates are affected by several methodological caveats
that we consider here. First, a general problem with comparing survival
studies is that differences between estimates derived from old versus
new methods and between live recaptures and dead recoveries may mask
trends in the data (Roodbergen et al. 2012). Our dataset
consisted primarily of studies that used live capture-mark-recapture
techniques (83% of effect sizes) and most of these were conducted since
2000; nearly all studies were conducted after 1990 when modern
statistical tools for analyses of marked animals were developed
(Lebreton et al. 1992). One of the drawbacks of
capture-mark-recapture data is that the reported metric, apparent
survival, is a product of true survival and site fidelity and as such
will always be biased low, whereas estimates of survival from dead
recovery models are often interpreted as true survival (Sandercock
2006). Biases in survival estimates may therefore be strong for birds
from tropical regions, which consisted exclusively of live-recapture
data, and where behaviors such as altitudinal migration are more common
than in temperate regions (Barçante et al. 2017) and can lead to
temporary emigration from study plots. Another issue affecting the
comparison of survival studies is the study duration. This, too, may be
particularly problematic for tropical regions, where data collection is
often hampered by sampling conducted over irregular or insufficiently
long intervals to produce robust estimates of survival (Ruiz-Guitérrezet al. 2012). For example, in our meta-analysis 62% and 69% of
effect sizes from austral and temperate latitudes, respectively, were
calculated from datasets spanning >10 years, compared to
only 34% from tropical latitudes. However, in a study of tropical birds
comparing survival estimates derived from 6 vs.12 years of data, Blake
& Loiselle (2013) reported an improvement in precision, but no change
in point estimates for survival. Still, other authors argue that longer
time frames are needed to generate reliable survival estimates for
tropical resident species (i.e., 10–30 years), given their expected
longevity and low recapture probabilities (p <0.25;
Ruiz-Guitérrez et al. 2012). Despite these problems with the
comparability of the data, we found no indication that difference in
methodological approaches strongly biased our results (Fig. S3).