Disentangling the effects of spring anomalies in climate and net primary production on body size of temperate songbirds

Abstract

Body size is implicated in individual fitness and population dynamics. Mounting interest is being given to the effects of environmental change on body size, but the underlying mechanisms are poorly understood. We tested whether body size and body condition are related to ambient temperature (heat maintenance hypothesis), or/and explained by variations in primary production (food availability hypothesis) during the period of body growth in songbirds. We also explored whether annual population-level variations of mean body size are due to changes of juvenile growth and/or size-dependent mortality during the first year. For 41 species, from 257 sites across France, we tested for relationships between wing length (n = 107 193) or body condition (n = 82 022) and local anomalies in temperature, precipitation and net primary production (NDVI) during the breeding period, for juveniles and adults separately. Juvenile body size was best explained by primary production: wings were longer in years with locally high NDVI, but not shorter in years with low NDVI. Temperature showed a slightly positive effect. Body condition and adult wing length did not covary with any of the other tested variables. We found no evidence of climate-driven size-dependent mortality for the breeding season. In our temperate system, local climatic anomalies explained little of the body size variation. A large part of wing length variance was site-specific, suggesting that avian size was more dependent on local drivers than global ones. Net primary production influenced juvenile size the most through effects on body growth. We suggest that, during the breeding season in temperate systems, thermoregulatory mechanisms are less involved in juvenile growth than food assimilation.

Thermal constraints on body size depend on the population’s position within the species’ thermal range in temperate songbirds

“…Warmer years induce a reduction of the cost of body heat maintenance in cool areas, enabling a higher energetic allocation to body growth (Gillooly et al, ). An alternative, non‐exclusive hypothesis is that warming may also increase net primary production, thus improving invertebrate abundance (Dubos et al, ; Yom‐Tov & Geffen, ). In the present study, all study species are insectivorous during spring.…”

“…In other studies focussing on temporal variation in bird size, the effect of mean temperature of the breeding period was about 1% of the juvenile wing length in Australia, and the effect of summer temperature ranged between −0.63 and 0.15% per C in eastern North America (Collins et al, ). Recently, Dubos et al () reported that, when analysing additive effects of annual weather on wing length across sites and species, they could explain 5% of the total variance in juvenile wing length at best, that is, the additive and species‐specific variation between years. All the remaining variance actually corresponds to site‐specific annual variations (7%) or site‐ and species‐specific annual variations (88%).…”

“…More importantly, the effect of temperature fluctuations probably differs between climatic regions. For instance, the physiological and ecological consequences of annual weather deviation are likely to depend on the average local climatic regime: in cool or cold regions (e.g., Collins, Relyea, Blustein, & Badami, 2017a;Dubos et al, 2018;Pérez et al, 2016), or during cool periods of the year (e.g., Gardner, Amano, Mackey, et al, 2014), hot temperatures indeed affect body growth positively. In addition, events of extreme heat are also scarcer in temperate climates than in arid systems (Garcia, Cabeza, Rahbek, & Araújo, 2014).…”

“…In addition, events of extreme heat are also scarcer in temperate climates than in arid systems (Garcia, Cabeza, Rahbek, & Araújo, 2014). Observed body size variations are therefore expected to represent the response to fluctuations in average temperatures in temperate climates (Dubos et al, 2018), whereas the influence of extreme hot events is expected to be particularly strong in arid, semi-arid and tropical climates (Gardner et al, 2017;Holmgren et al, 2016). In arid or semi-arid climates, larger individuals are expected to survive heat waves better, presumably because the smallest individuals show higher rates of evaporative water loss, and would be more exposed to dehydration and mortality during heat waves (McKechnie & Wolf, 2010).…”

“…For the same species, temporal variation in body size can differ between sites (e.g., Collins et al, 2017a;Dubos et al, 2018;Meiri et al, 2009). Within a species' distribution range, populations are likely to have adapted locally to average meteorological conditions (e.g., Both & te Marvelde, 2007).…”