Peter A. Cotton

Avian migration phenology and global climate change

There is mounting evidence that global climate change has extended growing seasons, changed distribution patterns, and altered the phenology of flowering, breeding, and migration. For migratory birds, the timing of arrival on breeding territories and over-wintering grounds is a key determinant of reproductive success, survivorship, and fitness. But we know little of the factors controlling earlier passage in long-distance migrants. Over the past 30 years in Oxfordshire, U.K., the average arrival and departure dates of 20 migrant bird species have both advanced by 8 days; consequently, the overall residence time in Oxfordshire has remained unchanged. The timing of arrival has advanced in relation to increasing winter temperatures in sub-Saharan Africa, whereas the timing of departure has advanced after elevated summer temperatures in Oxfordshire. This finding demonstrates that migratory phenology is quite likely to be affected by global climate change and links events in tropical winter quarters with those in temperate breeding areas.

Chick Begging Strategies in Relation to Brood Hierarchies and Hatching Asynchrony

Altricial offspring solicit food by begging, and their parents feed them according to begging intensity, which has been shown to be positively related to offspring need. Parent‐offspring genetic conflict calls for analyses of evolutionary stability, and various theoretical models have shown that stability is possible in the framework of handicap theory. The models predict that a negative relationship exists between offspring condition and begging and that offspring in poorer condition should be fed preferentially. However, these predictions depend on two unsatisfactory assumptions. First, they assume a monotonically decelerated relation between condition and fitness (this function is more likely to be sigmoid); second, they ignore physical competition between siblings, which is known to be important. We examined the significance of these issues by manipulating hatching asynchrony in broods of starlings Sturnus vulgaris, thus controlling competitive asymmetries between nest mates. We created broods with senior (older) and junior (younger) chicks and control broods with synchronous chicks. In field and laboratory experiments, we found that seniors begged less than juniors and controls, whereas juniors did not differ significantly from controls. However, seniors received more food from their parents and grew better than juniors or controls (hence, they were in better condition). These results violate the predictions of available theoretical models and, together with limitations in the universality of their assumptions, indicate that fundamental aspects of parent‐offspring communication are not yet understood.

Food allocation among nestling starlings : sibling competition and the scope of parental choice

Parental provisioning for nestling birds is generally considered to be an interactive, conflictive process because the optimal provisioning rate differs between parents and young and because nestlings are engaged in intersibling competition. Understanding the evolution of communication in such a situation presents unusual problems because the scope for parental strategies may be limited by competitive behaviour of the chicks. We substantiate this view by studying parent-offspring feeding interactions between chicks and provisioning adults in the European starling Sturnus vulgaris in relation to chick state and intersibling competition. The state of one target chick in each nest was manipulated in the field by temporarily placing it in enlarged, reduced or normal-sized broods before returning it to its original nest. Conditions in the original nests were standardized during manipulation by using substitute chicks. Once returned to its original brood, the probability of the target chick being fed increased if it increased its begging intensity and/or it positioned itself closer to the entrance of the nest. Both begging intensity and position were functions of the treatment previously experienced, with target chicks begging more and attaining positions closer to the nest entrance after they had spent time in larger broods. We postulate that these factors must be included in theoretical analyses of the evolution of food-solicitation signalling because, although the effect of begging on feeding probability may be mediated by parental choice, the effect of position depends on between-chick dynamics, and the parents apparently accept the outcome of these interactions.

Specialization in Plant-Hummingbird Networks Is Associated with Species Richness, Contemporary Precipitation and Quaternary Climate-Change Velocity

Large-scale geographical patterns of biotic specialization and the underlying drivers are poorly understood, but it is widely believed that climate plays an important role in determining specialization. As climate-driven range dynamics should diminish local adaptations and favor generalization, one hypothesis is that contemporary biotic specialization is determined by the degree of past climatic instability, primarily Quaternary climate-change velocity. Other prominent hypotheses predict that either contemporary climate or species richness affect biotic specialization. To gain insight into geographical patterns of contemporary biotic specialization and its drivers, we use network analysis to determine the degree of specialization in plant-hummingbird mutualistic networks sampled at 31 localities, spanning a wide range of climate regimes across the Americas. We found greater biotic specialization at lower latitudes, with latitude explaining 20–22% of the spatial variation in plant-hummingbird specialization. Potential drivers of specialization - contemporary climate, Quaternary climate-change velocity, and species richness - had superior explanatory power, together explaining 53–64% of the variation in specialization. Notably, our data provides empirical evidence for the hypothesized roles of species richness, contemporary precipitation and Quaternary climate-change velocity as key predictors of biotic specialization, whereas contemporary temperature and seasonality seem unimportant in determining specialization. These results suggest that both ecological and evolutionary processes at Quaternary time scales can be important in driving large-scale geographical patterns of contemporary biotic specialization, at least for co-evolved systems such as plant-hummingbird networks.

Cue association and antipredator behaviour in a pulmonate snail, Lymnaea stagnalis

Associative learning may help to offset costs of unnecessary escape behaviour by providing accurate information about the current risk to potential prey. We investigated innate antipredator behaviour and cue association learning in naive gastropods. Juvenile laboratory-reared great pond snails, Lymnaea stagnalis (L.), were exposed to odour cues from a natural predator, tench, Tinca tinca (L.), and crushed conspecifics. The snails showed an innate antipredator behaviour to odour from T. tinca, by crawling above the water line (crawl-out response). This crawl-out response was significantly increased in the presence of alarm cues (crushed conspecifics). In a second experiment, juvenile L. stagnalis were exposed to tench odour and alarm cues in aquaria before being tested in behavioural assays. The behavioural responses to tench cue alone were similar to those seen in response to tench plus alarm cues presented together during the first experiment. Hence, L. stagnalis is apparently capable of relating potential predation risk to recent experience. In a final experiment snails were removed from pre-exposure cues for periods of 1, 4 and 8 days prior to behavioural assays. A raised level of avoidance persisted for at least 8 days, suggesting that this behaviour may be retained over timescales relevant to predation risk in the natural environment. The ability of organisms to modify antipredator behaviour based on recent experience, as found in L. stagnalis, would allow costs associated with unnecessary responses to be reduced while still allowing the organisms to avoid active predators.

TRAIT COMPENSATION IN MARINE GASTROPODS: SHELL SHAPE, AVOIDANCE BEHAVIOR, AND SUSCEPTIBILITY TO PREDATION

Many organisms have evolved morphological and behavioral traits that reduce their susceptibility to predation. However, few studies have explicitly investigated the relationships between defensive traits and susceptibility. Here we demonstrate a negative correlation between morphological defenses and behavioral avoidance across several species of marine gastropod that is linked to vulnerability to crab predation. Snails that had relatively taller shell spires (high aspect ratio) showed greater responsiveness when exposed to predation cues than did species with disc-like shells (low aspect ratio). Our results suggest that the snail species most vulnerable to predation compensated by showing the highest levels of behavioral avoidance, and hence may be at a disadvantage in competition with less vulnerable species. This has important implications because the behavioral response of herbivorous gastropods to predation cues may play a central role in structuring rocky intertidal communities through trait-mediated indirect effects.

Consistent crustaceans: the identification of stable behavioural syndromes in hermit crabs

Consistent individual variation in behaviour has been termed ‘animal personality’ and has been identified across a range of behavioural contexts including aggression, boldness in response to a threatening stimulus and exploration. When behaviours are correlated across multiple functional behavioural categories or ‘contexts’, ‘behavioural syndromes’ are said to be present. It is possible, however, that behavioural syndromes may also show consistencies. Here we investigated the presence of behavioural syndromes linking startle responses, exploration and aggression in hermit crabs and assessed their stability across two situations (low versus high predation risk). Correlation analyses detected behavioural syndromes between startle responses, a measure of ‘boldness’, and the latency to investigate a novel object, as well as the latency to attack an opponent in an aggressive context. The startle response–investigation and startle response–aggression syndromes were stable between situations, whilst there was a lack of relationship between investigation and aggression in each situation. Here we propose that these consistent individual differences in the expression of behavioural syndromes reveal the presence of animal personality, manifesting in not just one, but a suite of interacting traits.

Nestling sex ratio in the European Starling Sturnus vulgaris

We used a PCR-based molecular DNA sexing technique to sex European Starling Sturnus vulgaris chicks from a nestbox colony at Wytham, Oxford, UK. The nestling sex ratio among 350 one-week-old chicks from 108 broods was significantly femalebiased. There was no significant heterogeneity in brood sex ratio. We found no significant correlations between sex ratio and several environmental and parental variables, nor were there any significant differences in egg dimensions or growth rates of the sexes. Mortality, but not fledging sex ratio, was strongly correlated with hatching order, suggesting that the sex ratio imbalance was not due to sex-biased chick mortality. The possibility that the overall sex ratio bias is dictated by maternal adjustment of the sex ratio is discussed.

Whole-organism performance capacity predicts resource-holding potential in the hermit crab Pagurus bernhardus

Animal contests often involve the production of repeated signals, which are assumed to be energetically demanding to perform and advertise the performer’s fighting ability through a demonstration of stamina. Although the costs of repeated signals have been investigated by the postfight analysis of metabolites, single physiological measures may not adequately reflect an individual’s overall stamina. An alternative approach is to investigate whole-organism performance capacities. We examined the relationship between locomotor capacities and the vigour of repeated bouts of shell-rapping signals during fights over gastropod shells in the hermit crab Pagurus bernhardus. In these fights only attacking crabs perform shell rapping while defenders withdraw into their shell. Successful attackers and defenders had higher mean and maximum walking speeds than losers. Furthermore, maximum speed was a better correlate of fighting ability in defenders, whereas mean speed was better in attackers. This study demonstrates that whole-organism performance capacities correlate with success in agonistic encounters, and that different aspects of performance capacity appear to be important depending on the behaviours performed by different roles. Thus, this study supports the assumptions of theoretical models such as the cumulative assessment model and the energetic war of attrition, which predict that repeated signals advertise stamina and fighting ability, by demonstrating that mean rates of performance, a key component of stamina, are related to success in hermit crab shell fights.

Susceptibility to predation affects trait-mediated indirect interactions by reversing interspecific competition.

Numerous studies indicate that the behavioral responses of prey to the presence of predators can have an important role in structuring assemblages through trait-mediated indirect interactions. Few studies, however, have addressed how relative susceptibility to predation influences such interactions. Here we examine the effect of chemical cues from the common shore crab Carcinus maenas on the foraging behavior of two common intertidal gastropod molluscs. Of the two model consumers studied, Littorina littorea is morphologically more vulnerable to crab predation than Gibbula umbilicalis, and it exhibited greater competitive ability in the absence of predation threat. However, Littorina demonstrated a greater anti-predator response when experimentally exposed to predation cues, resulting in a lower level of foraging. This reversed the competitive interaction, allowing Gibbula substantially increased access to shared resources. Our results demonstrate that the susceptibility of consumers to predation can influence species interactions, and suggest that inter-specific differences in trait-mediated indirect interactions are another mechanism through which non-consumptive predator effects may influence trophic interactions.