DOI: 10.5061/dryad.zs7h44j56
title: Reduced avian body condition due to global warming has little reproductive or population consequences

description: Climate change has strong effects on traits such as phenology and
 physiology. Studies typically assume that climate-induced trait changes
 will have consequences for population dynamics, but explicit tests are
 rare. Body condition reflects energy storage and may directly affect how
 much can be invested in reproduction and survival. However, the causal
 pathway by which decreased body condition impacts population dynamics has
 never been quantified across multiple populations and species. Therefore,
 we lack a general understanding of the consequences of changes in
 condition for variables more relevant for conservation, such as population
 size. Using structural equation modeling, we investigate how
 temperature-induced changes in body condition affect reproduction, and the
 subsequent impact on population growth rates of 19 bird species across 80
 Dutch sites over a 21-year period. Warmer temperatures were associated
 with decreased body condition, which led to both decreased and increased
 reproduction at different sites, cancelling out any overall effect. The
 indirect effect of temperature on population growth (via body condition
 and reproduction) only explained within-species variation in the total
 effects of temperature on population growth. Instead, the direct effect of
 temperature on population growth (unrelated to condition and reproduction)
 was the most important pathway underlying the total effects of temperature
 on population growth, suggesting that unknown variables are mediating this
 effect. About half of the species are expected to increase under global
 warming, but this variation was not associated with any species
 characteristic. Overall, body condition responses to global warming are
 common, but their consequences on reproduction and subsequently population
 growth contribute relatively little to the total temperature impacts on
 population dynamics. Given that warming temperatures have strong effects
 on population dynamics, understanding the pathways via which temperature
 impacts population dynamics will be crucial for our ability to predict
 climate change effects in the future and improve conservation efforts.
description: Body mass, body size and
 reproduction data were collected as part of the Dutch Constant Effort Site
 (CES) program, which covers 80 sites across the Netherlands, spanning 21
 years (1994-2014; see Appendix 1 Fig S1 for map). The CES-project follows
 a standardised protocol where birds are captured using mist nets from the
 13<sup>th</sup> April until the 13<sup>th</sup>
 August every year (Robinson et al. 2009). Most sites were sampled 12 times
 per year (range 9-12). Captured birds are ringed and morphometric
 measurements taken, including body mass (grams) and wing length (maximum
 chord measurement; Svensson 1992). The sex and age-class (juvenile or adult)
 are classified based on plumage (wherever possible). For all
 species we consider juveniles to be less than one year old and born in the
 year of capture. Independent annual population
 growth rates were calculated using abundance count data from the Dutch
 Breeding Bird Monitoring Program (BMP) which has been running since 1984.
 This bird counting methodology uses fixed study plot territory mapping, a
 bird counting methodology that produces reliable estimates of total bird
 abundance compared to point/transect counts, and makes year-to-year
 comparisons possible at the site level (Van Turnhout et al. 2010).  Study
 plots are visited 5-10 times per year, with every visit covering the whole
 plot. We
 calculated the weighted mean abundance for each of the CES sites by
 averaging the 5 closest BMP sites that had the same habitat type
 (typically within a 15km radius of the CES sites). Estimates in this data set are
 calculated per the methodology in the article. Please see the metadata tab
 for specific information.