Beyond average temperatureDistribution of wintering birds at multiple scales / más allá de la temperatura promedio: Distribución de aves invernantes a múltiples escalas

Resumen

Aim - To disentangle the subtleties of the relationship between the thermal environment and the distribution of wintering birds, at multiple spatial scales. Specifically, to analyze the relative influence of temperature on species distributions, its interaction with various thermal and non-thermal factors, and the context-dependence of these relationships (i.e., species, season and geographical location). Location - Iberian Peninsula and Guadarrama Mountain range (central Spain). Methods - I use observational and experimental field approaches to control for habitat structure, topography, food abundance and predation risk, from forest patches to landscape, regional and peninsular scales. Fieldwork is carried out with woodland passerines inhabiting mountain oakwoods of Quercus pyrenaica, and subtle variations in temperature and wind are precisely measured with data loggers. Main results and conclusions - Wintering birds respond to natural variations in temperature at multiple scales, selecting the warmest forest patches to forage and the warmest mountain areas and peninsular regions to overwinter. However, temperature per se accounts for a relatively small proportion of the variation in species distribution patterns. Indeed, a sudden extreme drop in temperature is not enough to alter species abundances, or to promote their general redistribution to minimize wind chill. Other non-thermal environmental factors like vegetation structure, predation risk and availability of predictable food resources, seem to be more deterministic in driving winter species distribution than direct measurements of temperature. On the other hand, the influence of the thermal environment goes far beyond average temperature: minimum night temperature, incident sun radiation and altitude related with snow cover are key drivers of species distributions. The relative relevance of all these factors depends on the ecological scenario, varying across species, seasons and geographical locations. I conclude that if we disregard other environmental effects, we will overestimate the influence of temperature on species abundance and underestimate the plasticity of these to respond to temperature changes. Applications and prospects - Fine-grained approaches, based on direct measures of the study organisms and carried out through precise local measurements of environmental variables, are needed to understand the functional mechanisms driving species distribution patterns at wider scales. At all scales, we need to control for other environmental factors when predicting the responses of birds to temperature, under either current or future scenarios of global warming. Otherwise, we are at risk of generating strongly biased predictions that will inflate the magnitude of climate change effects.