Analysing spatial and temporal variation in colony size: an approach using autoregressive mixed models and information theory

Abstract

The spatial and temporal variation in population sizes of animal colonies are rarely studied simultaneously. I examined factors determining colony size (number of nests) for 23 colonies from the only breeding population of rook Corvus frugilegus in Spain over 7 years. Population sizes within colonies were highly predictable over time, with autocorrelations up to a distance (lag) of 6 years. Autoregressive mixed models were used to explain colony size as a function of environmental factors, while controlling for temporal autocorrelation. These factors included refuse tips, widely used as food resource, and a derived variable that incorporated the two factors most often related to avian colony size (inter-colony competition and foraging habitat around colonies). Autoregressive models provided a better fit to the data than models which did not consider temporal autocorrelation. The information-theoretic (AIC c -based) approach revealed uncertainty in the selection of the best model explaining colony-size, but relatively strong support for certain variables. The highest weights of evidence were for year (x i = 0.90) and the number of competitors per unit of foraging habitat (i.e., derived variable; x i = 0.63), showing that the size of rook colonies in Spain was negatively affected by inter-colony competition relative to the foraging habitat surrounding the colonies. This variable measured within a 6-km radius from the colonies had *30 times higher weight of evidence (more plausible) than the same variable measured within 3 km, indicating that food limitation may occur outside the breeding period. Sizes of colonies tended to decrease when distance between the colony and the nearest refuse tip increased. There was some evidence supporting the idea that the effect of the number of competitors per unit of foraging habitat on colony size varied from year to year, but statistical power was weak. These findings suggest that variation in number of rook nests within colonies reflects spatial and temporal heterogeneity of net food via both inter-colony competition and foraging habitat around the colony.

The importance of social information in breeding site selection increases with population size in the Eurasian Griffon VultureGyps fulvus

“…Doligez et al. 2004, Fletcher 2006), incorporating a temporal component into the analysis of habitat selection would enable a better understanding of the process behind the pattern and more realistic predictions of future expansions (Olea 2009). Our results confirm that habitat models may reveal different habitat selection drivers along the progressive stages of a species’ colonization process.…”

“…The variables affecting breeding habitat suitability (e.g. food availability, human disturbance, conspecific presence/abundance) can vary both spatially and temporally (Olea 2009), conditioning the type of information to be used by a species in settlement decisions (Doligez et al. 2004, Serrano et al.…”

Opposing selective pressures may act on the colony size in a waterbird species

“…For this purpose we induced formation of breeding colonies that varied markedly in size by providing different size patches of attractive nesting area (floating rafts) at a site with limited availability of natural nesting habitat. By inducing establishment of all the colonies at the same site we could control for differences in environmental conditions that could affect breeding performance of birds, an assumption that is difficult to satisfy when comparing colonies from different locations (Ambrosini et al 2002;Olea 2009). Usually, breeding aggregations of colonial birds are spread over large distances, and thus are often subjected to varying environmental and ecological pressures, which act simultaneously on reproducing birds and are difficult to disentangle in comparative analyses.…”

As the Sword Grows: Individual Variation and Ontogenetic Effects of a Sexually Selected Trait on Locomotor Performance inXiphophorus hellerii

“…To calculate AICc, we used a sample size of 76 (77 for SW): nine individuals over 9 wk result in 81 individual # week samples, but we had missing data for five individual # week (table A1), resulting in a sample size of 76 (Olea 2009). …”