In most animals, especially those that must swallow food items whole, prey size is related directly to predator size. This paper examines gape limitation and the influence of fruit size on diet in fruit-eating birds, drawing on data gathered over a 5-yr period on 70 bird species and 171 plant species in the lower montane forests of Monteverde, Costa Rica. The results suggest that fruiteating birds face many of the constraints imposed on other gape-limited foragers, but have an unusual ~inim~m-size relationship with their food because of the unique characteristics of fruits. Fruit-eating btrds With broad gapes consumed more lauraceous fruit species and a larger mean and maximum size of fruits overall than narrow-gaped birds. However, the size of the smallest fruits eaten was not correlated with gape width; large-gaped species commonly fed on diminutive fruits. Birds effectively selected among individual fruits within a tree on the basis of fruit size, dropping bulky fruits beneath the tree. Effective size selectivity also occurred among trees of different species in the same family and among plant species in various families. The diet of broad-gaped birds was not comprised differentially of large fruit species. For example, Three-wattled Bellbirds favored medium-sized fruits whereas Long-tailed Manakins took individual fruits in the same proportions as they took fruit specie~ of different mean fruit diameters. Gape limitations and effective size selectivity have obvious consequences for seed dispersal patterns: plants with large fruits attracted fewer species of birds than plants with small fruits. Moreover, the broad-gaped bird species on which large-fruited plants specialized were those with the most generalized diets.

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Telomeres are highly conserved regions of DNA that protect the ends of linear chromosomes. The loss of telomeres can signal an irreversible change to a cell's state, including cellular senescence. Senescent cells no longer divide and can damage nearby healthy cells, thus potentially placing them at the crossroads of cancer and ageing. While the epidemiology, cellular and molecular biology of telomeres are well studied, a newer field exploring telomere biology in the context of ecology and evolution is just emerging. With work to date focusing on how telomere shortening relates to individual mortality, less is known about how telomeres relate to ageing rates across species. Here, we investigated telomere length in cross-sectional samples from 19 bird species to determine how rates of telomere loss relate to interspecific variation in maximum lifespan. We found that bird species with longer lifespans lose fewer telomeric repeats each year compared with species with shorter lifespans. In addition, phylogenetic analysis revealed that the rate of telomere loss is evolutionarily conserved within bird families. This suggests that the physiological causes of telomere shortening, or the ability to maintain telomeres, are features that may be responsible for, or co-evolved with, different lifespans observed across species.This article is part of the theme issue ‘Understanding diversity in telomere dynamics'.

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No abstract

Over a 9-yr period, we studied dispersal of young banded Savannah Sparrows (Passerculus sandwichensis) from their natal nest to the site where they first bred 1 yr later in a population on an isolated archipelago in the Bay of Fundy, Canada. On a broad geographic scale, young birds were highly philopatric, returning from wintering grounds several thousand kilometers away to nest on the same island and often in the same field where they had hatched the year before. In some cases, birds chose nest sites within a few meters of their natal nest. The median dispersal distance between a bird's natal nest and its first nest as an adult was 228 m, a distance equivalent to about six times the diameter of an average territory. Nearly three-quarters of the breeding birds in the study population had been banded as nestlings or fledglings within a 10-ha area on one island, which suggests that most birds in the population originated within the 127-ha archipelago.Within the archipelago, males and females dispersed similar distances from their natal site. There was no correlation between natal dispersal distances of parents and their offspring, nor was there a correlation between natal dispersal distances of siblings, which indicated that natal dispersal has low heritability. Sex, hatching date, fledging mass, and population densities during the previous and current years were all poor predictors of natal dispersal distance. Males that were strongly philopatric recruited significantly more offspring during their lifetime than males raised outside the study area, although there were no differences between philopatric and dispersing females.Birds tended to shift to distinct parts of the island to breed if their parents of the opposite sex still occupied the territory where they had hatched. Dispersal was not affected by return of same-sex parents. Distances between nests of siblings raised the same year were farther apart than expected by chance, based on Monte Carlo simulations. Although many birds in the population had the opportunity to pair with kin, some mechanism, yet to be determined, enabled birds to avoid inbreeding: in 1073 nesting attempts involving birds of known parentage, no individuals were known to have paired with close relatives (coefficient of kinship Ͼ0.125). Because complete inbreeding avoidance occurred in Ͻ1% of Monte Carlo simulations, the absence of inbreeding among Kent Island Savannah Sparrows is unlikely to be due to chance.Understanding natal dispersal in birds requires a combination of models: ecogenetic models at broad geographic scales (e.g., adaptation to local environments), ecological constraints and neutral models at smaller spatial scales (e.g., unavailability of territories within particular habitats), and genetic models (e.g., inbreeding avoidance).

No abstract

Female birds often copulate outside the pair-bond to produce broods of mixed paternity, but despite much recent attention the adaptive significance of this behaviour remains elusive. Although several studies support the idea that extra-pair copulations (EPCs) allow females to obtain 'good genes' for their offspring, many others have found no relationship between female mating fidelity and traits likely to reflect male quality. A corollary to the good genes hypothesis proposes that females do use EPCs to increase the quality of young, but it is the interaction between maternal and paternal genomes - and not male quality per se - that is the target of female choice. We tested this 'genetic compatibility' hypothesis in a free-living population of Savannah sparrows (Passerculus sandwichensis) by determining whether females mated nonrandomly with respect to the major histocompatibility complex (Mhc). During both the 1994 and 1995 breeding seasons, female yearlings (but not older birds) avoided pairing with Mhc-similar males (P < 0.005). The Mhc similarity between mates also predicted the occurrence of extra-pair young in first broods (P < 0.007) and covaried with estimates of genome-wide levels of similarity derived from multilocus DNA fingerprinting profiles (P = 0.007). The overall genetic similarity between adults tended to predict female mating fidelity, but with less precision than their Mhc similarity (P = 0.09). In contrast, females appeared insensitive to the size, weight or age of males, none of which explained variation in female mating fidelity. Taken together, these results are consistent with the hypothesis that females sought complementary genes for their offspring and suggest either that the benefits of heterozygosity (at the Mhc) drive female mating patterns or that the avoidance of inbreeding is an ultimate cause of social and genetic mate choice in Savannah sparrows.