The effect of insularity on avian growth rates and implications for insular body size evolution

Abstract

Island populations often differ in consistent ways from their mainland counterparts with respect to their ecology, behaviour, morphology, demography and life-history characteristics; a set of changes referred to as the 'island syndrome'. To understand the ecological and evolutionary drivers of the island syndrome requires characterization of suites of interacting traits. While patterns in some types of traits, e.g. body size, are well characterized across a range of taxa, key gaps remain. Growth rate is one such trait, being an important determinant of both increases and decreases in body size, and can vary with changes in predation pressure and food limitation; two factors that are known to differ between mainland and island environments. Using a phylogenetic meta-analytic approach, we characterize differences in growth rates among mainland and island altricial bird populations, controlling for environmental factors. We found a trend towards slower growth on islands in small-bodied (less than 1 kg) bird species. This is consistent with the idea that the pattern of body size increases in small-bodied island colonists is associated with the evolution of slower growth combined with shifts in age and size at maturity in relaxed predation regimes.

Keywords: bird; growth rates; island biology; island syndrome; life history; meta-analysis.

Figure 1.

Theoretical prediction of avian body size changes from mainland to island environments as a product of shifts in growth rates and the reaction norm determining age and size at maturity. This is adapted from Palkovacs [17] general description of changes in adult body size on islands (BSI) compared to the mainland (BSM), but modified for avian growth rates specifically. (a) When predation release is the dominant ecological driver, the effect of reduced extrinsic mortality (shift from mainland to island reaction norm: RNM and RNI, respectively) is larger than that of lower growth rate (shift from mainland to island growth rate: GRM and GRI, respectively) resulting in increases in insular body size. (b) When food limitation is the dominant ecological driver, the effect of reduced growth rates is larger than that of reduced extrinsic mortality, resulting in smaller insular body sizes. Note that body size at maturity is determined by the intersection of the growth rate curve and the reaction norm of age and size at maturity (A and B).

Figure 2.

Growth rates (k) of 213 altricial bird species depicted on a phylogenetic tree (for methods of tree construction see [46]). Growth rates range from 0.097 to 0.736 (growth rates were averaged for species with multiple estimates) and show a strong phylogenetic signal (35% total variance explained). Dark red depicts fast growth rates and light yellow depicts slower growth rates.