1. Habitat structure has been considered as a main factor shaping the evolution of bird song acoustics. 2. Based on expected differential patterns of sound degradation in different habitats, the ...acoustic adaptation hypothesis (AAH) proposes that songs with lower frequencies, narrower frequency ranges and longer inter-element intervals should occur more frequently in densely vegetated compared with herbaceous habitats. 3. Empirical tests of the AAH have provided mixed results. Here, we review for the first time the literature on this topic using a meta-analytical, quantitative approach. 4. Maximum, minimum, peak frequency and frequency range were found to be significantly lower in closed compared with open habitats, but the mean size of the effect of habitat was small. Inter-element intervals were not affected by habitat structure. 5. The AAH implicitly assumes that birds are selected to maximize song broadcast range and minimize degradation, while it neglects the potential role of energetic costs of singing and selection by eavesdroppers (e.g. predators and parasites). 6. Our meta-analysis supports the AAH, but habitat structure only weakly predicts the acoustical properties of bird songs. Thus, other potentially relevant factors should be included in realistic models of the evolution of bird song acoustics.
Comparative studies of trait evolution require accounting for the shared evolutionary history. This is done by includ- ing phylogenetic hypotheses into statistical analyses of species＇ traits, for ...which birds often serve as excellent models. The online publication of the most complete molecular phylogeny of extant bird species （www.birdtree.org, BirdTree hereafter） now allows evolutionary biologists to rapidly obtain sets of equally plausible phylogenetic trees for any set of species to be incorporated as a phylogenetic hypothesis in comparative analyses. We discuss methods to use BirdTree tree sets for comparative studies, either by building a consensus tree that can be incorporated into standard comparative analyses, or by using tree sets to account for the ef- fect of phylogenetie uncertainty. Methods accounting for phylogenetic uncertainty should be preferred whenever possible because they should provide more reliable parameter estimates and realistic confidence intervals around them. Based on a real compara- tive dataset, we ran simulations to investigate the effect of variation in the size of the random tree sets downloaded from BirdTree on the variability of parameter estimates from a bivariate relationship between mass-specific productivity and body mass. Irre- spective of the method of analysis, using at least 1,000 trees allows obtaining parameter estimates with very small （〈 0.15%） co- efficients of variation. We argue that BirdTree, due to the ease of use and the major advantages over previous ＇traditional＇ meth- ods to obtain phylogenetic hypotheses of bird species （e.g. supertrees or manual coding of published phylogenies）, will become the standard reference in avian comparative studies for years to come.
ABSTRACT
Sexual selection arises from competition among individuals for access to mates, resulting in the evolution of conspicuous sexually selected traits, especially when inter‐sexual competition ...is mediated by mate choice. Different sexual selection regimes may occur among populations/subspecies within the same species. This is particularly the case when mate choice is based on multiple sexually selected traits. However, empirical evidence supporting this hypothesis at the among‐populations level is scarce. We conducted a meta‐analysis of the intensity of sexual selection on the largest database to date for a single species, the barn swallow (Hirundo rustica), relying on quantitative estimates of sexual selection. The intensity of sexual selection was expressed as the strength (effect size) of the relationships between six plumage ornaments (tail length, tail asymmetry, size of white spots on tail, ventral plumage colour, throat plumage colour and throat patch size) and several fitness proxies related to reproduction, parental care, offspring quality, arrival date from spring migration, and survival. The data were gathered for four geographically separated subspecies (H. r. rustica, H. r. erythrogaster, H. r. gutturalis, H. r. transitiva). The overall mean effect size (Zr = 0.214; 95% confidence interval = 0.175–0.254; N = 329) was of intermediate magnitude, with intensity of sexual selection being stronger in males than in females. Effect sizes varied during the breeding cycle, being larger before egg deposition, when competition for access to mates reaches its maximum (i.e. in the promiscuous part of the breeding cycle), and decreasing thereafter. In addition, effect sizes from experiments were not significantly larger than those from correlative studies. Finally, sexual selection on different sexually dimorphic traits varied among subspecies. This last result suggests that morphological divergence among populations has partly arisen from divergent sexual selection, which may eventually lead to speciation.
Captive experiments have shown that many species regulate their macronutrient (i.e. protein, lipid and carbohydrate) intake by selecting complementary food types, but the relationships between ...foraging strategies in the wild and nutrient regulation remain poorly understood. Using the pine marten as a model species, we collated available data from the literature to investigate effects of seasonal and geographic variation in diet on dietary macronutrient balance. Our analysis showed that despite a high variety of foods comprising the diet, typical of a generalist predator, the macronutrient energy ratios of pine martens were limited to a range of 50–55% of protein, 38–42% of lipids and 5–10% of carbohydrates. This broad annual stabilisation of macronutrient ratios was achieved by using alternative animal foods to compensate for the high fluctuation of particular prey items, and sourcing non-protein energy (carbohydrates and fats) from plant-derived foods, particularly fruits. Macronutrient balance varied seasonally, with higher carbohydrate intake in summer–autumn, due to opportunistic fruit consumption, and higher protein intake in winter–spring. In terms of their proportional dietary carbohydrate intake the pine marten’s nutritional strategy fell between that of true carnivores (e.g. the wolf) and more omnivorous feeders (e.g. the European badger). However, in terms of energy contributed by protein pine martens are equivalent to obligate carnivores such as the wolf and domesticated cat, and different to some omnivorous carnivores such as the domesticated dog and grizzly bears.
Climate is changing at a fast pace, causing widespread, profound consequences for living organisms. Failure to adjust the timing of life-cycle events to climate may jeopardize populations by causing ...ecological mismatches to the life cycle of other species and abiotic factors. Population declines of some migratory birds breeding in Europe have been suggested to depend on their inability to adjust migration phenology so as to keep track of advancement of spring events at their breeding grounds. In fact, several migrants have advanced their spring arrival date, but whether such advancement has been sufficient to compensate for temporal shift in spring phenophases or, conversely, birds have become ecologically mismatched, is still an unanswered question, with very few exceptions. We used a novel approach based on accumulated winter and spring temperatures (degree-days) as a proxy for timing of spring biological events to test if the progress of spring at arrival to the breeding areas by 117 European migratory bird species has changed over the past five decades. Migrants, and particularly those wintering in sub-Saharan Africa, now arrive at higher degree-days and may have therefore accumulated a ‘thermal delay’, thus possibly becoming increasingly mismatched to spring phenology. Species with greater ‘thermal delay’ have shown larger population decline, and this evidence was not confounded by concomitant ecological factors or by phylogenetic effects. These findings provide general support to the largely untested hypotheses that migratory birds are becoming ecologically mismatched and that failure to respond to climate change can have severe negative impacts on their populations. The novel approach we adopted can be extended to the analysis of ecological consequences of phenological response to climate change by other taxa.
Organisms are routinely confronted with crucial decisions on the best time and place to perform fundamental activities. However, unpredictable spatio-temporal variation in ecological factors makes ...life-history optimization difficult particularly for long-distance migrants, which are putatively blind of conditions thousands of kilometers and weeks ahead along their journey. Here we challenge, on a hierarchy of geographical scales, the common wisdom that migratory birds have no clue to ecological conditions at destination. Using ringing data of the inter-continental migrating barn swallow (Hirundo rustica), we show that temperatures at breeding sites and at times of arrival from migration are more correlated with those at actual wintering sites and at times of departure than with those at other sites and at periods before/after departure. Hence, individual swallows have clues to adjust timing of spring migration based on expected conditions at destination, and they apparently choose wintering sites to increase availability of such information.
1. Life of many organisms flows as a sequence of annual cycles. Timing of cyclical events is shaped by natural selection also via the domino effects that any life history stage has on the stages that ...follow. Such 'carry-over effects' have major consequences for evolutionary, ecological and demographic processes, but the causes that generate their individual-level variation, including the effect of sex, are poorly understood. 2. We used light-level geolocators to study carry-over effects on the year-round life cycle of the long-distance migratory barn swallow (Hirundo rustica) and sex-dependent variation in their strength. 3. Correlation analyses showed that timing of breeding influenced departure time for autumn migration in females but not in males. In addition, strong, time-mediated carry-over effects of timing of departure from the wintering areas in sub-Saharan Africa for spring migration on timing of arrival to the breeding grounds in Italy and Switzerland operated in both sexes. However, carry-over effects of spring migration phenology on breeding date and seasonal fecundity were observed among females but not among males. 4. We used partial least squares path modelling to unveil the complex carry-over effects of phenology during the non-breeding season in combination with the ecological conditions experienced by individual swallows in the wintering area, as gauged by Normalized Difference Vegetation Index values (NDVI), on breeding performance. Phenology during the non-breeding season combined with NDVI during wintering accounted for as much as 65-70% of variation in subsequent seasonal fecundity in females, while such carry-over effects on breeding success of males were weaker. 5. Intense, sex-specific carry-over effects can have impacted on evolutionary processes, including sexual selection, and affected phenological response to climate change, causing the large population decline observed in this species.
Territoriality entails demanding social interactions with competing individuals, typically males. Variation in quality of males can be predicted to affect the spatial arrangement of territories. We ...present a model aimed at understanding the spatial properties of territories on leks, where the presence of a hierarchy in a population of males leads to the clustering of individuals around high-ranking 'hotshot' males. The hierarchy results in a decrease in the number of nearest neighbors interacting directly with high-ranking males, with potential socio-sexual benefits for such males.
Conditions experienced during early-life can cause the onset of oxidative stress, resulting in pervasive effects on diverse life-history traits, including lifespan. In birds, maternally-transferred ...egg substances may exert positive or negative influence over the offspring phenotype. Among these, testosterone can upregulate the bioavailability of certain antioxidants but simultaneously promotes the production of pro-oxidants, leading to an oxidative stress situation, which is one of the main forces causing telomere attrition However, no study has investigated the role of this androgen on telomere dynamics in birds and little is known about the effects of yolk testosterone on oxidative status in early-life of these species. We physiologically increased the levels of yolk testosterone by in ovo injections in yellow-legged gull (Larus michahellis) to evaluate the effects induced by this androgen on hatchlings plasma total antioxidant capacity, amount of pro-oxidant molecules and telomere length at hatching. Testosterone supplementation did not increase hatchling body growth, did not result in the overproduction of pro-oxidant molecules nor a reduction of antioxidant capacity. Accordingly, telomere length at hatching was not affected by testosterone treatment, although hatchlings from the third-laid eggs showed shorter telomeres than their siblings from first- and second-laid eggs, independently of testosterone treatment. Our results suggest that injection of physiological levels of testosterone does not induce oxidative stress to hatchlings and, consequently do not affect telomere dynamics during early post-natal periods.
Avian communication has been traditionally believed to be mainly mediated by visual and auditory channels. However, an increasing number of studies are disclosing the role of olfaction in the ...interaction of birds with their social environment and with other species, as well as in other behaviors such as nest recognition, food location and navigation. Olfaction has also been suggested to play a role in parent-offspring communication not only in the post- but also in the pre-hatching period. Volatile compounds produced during embryogenesis and passively released through the eggshell pores may indeed represent the only cue at parents' disposal to assess offspring quality, including the sex composition of their clutch before hatching. In turn, sex identification before hatching may mediate adaptive strategies of allocation to either sex. In the present study, we analyzed odour composition of barn swallow eggs incubated in their nest in order to identify any sex-related differences in volatile compounds emitted. For the first time in any bird species, we also investigated whether odour composition is associated with relatedness. The evidence of differences in odour composition among eggs containing embryos of either sex indicates that parents have a cue to identify their brood sex composition even before hatching which can be used to modulate their behavior accordingly. Moreover, odour similarity within nests may represent the prerequisite for kin recognition in this species.
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