Aim Parental care improves the survival of offspring and therefore has a major impact on reproductive success. It is increasingly recognized that coordinated biparental care is necessary to ensure the survival of offspring in hostile environments, but little is known about the influence of environmental fluctuations on parental cooperation. Assessing the impacts of environmental stochasticity, however, is essential for understanding how populations will respond to climate change and the associated increasing frequencies of extreme weather events. Here we investigate the influence of environmental stochasticity on biparental incubation in a cosmopolitan ground‐nesting avian genus. Location Global. Methods We assembled data on biparental care in 36 plover populations (Charadrius spp.) from six continents, collected between 1981 and 2012. Using a space‐for‐time approach we investigate how average temperature, temperature stochasticity (i.e. year‐to‐year variation) and seasonal temperature variation during the breeding season influence parental cooperation during incubation. Results We show that both average ambient temperature and its fluctuations influence parental cooperation during incubation. Male care relative to female care increases with both mean ambient temperature and temperature stochasticity. Local climatic conditions explain within‐species population differences in parental cooperation, probably reflecting phenotypic plasticity of behaviour. Main conclusions The degree of flexibility in parental cooperation is likely to mediate the impacts of climate change on the demography and reproductive behaviour of wild animal populations.

Social behaviours are highly variable between species, populations and individuals. However, it is contentious whether behavioural variations are primarily moulded by the environment, caused by genetic differences, or a combination of both. Here we establish that biparental care, a complex social behaviour that involves rearing of young by both parents, differs between closely related populations, and then test two potential sources of variation in parental behaviour between populations: ambient environment and genetic differentiation. We use 2904 hours behavioural data from 10 geographically distinct Kentish (Charadrius alexandrinus) and snowy plover (C. nivosus) populations in America, Europe, the Middle East and North Africa to test these two sources of behavioural variation. We show that local ambient temperature has a significant influence on parental care: with extreme heat (above 40°C) total incubation (i.e. % of time the male or female incubated the nest) increased, and female share (% female share of incubation) decreased. By contrast, neither genetic differences between populations, nor geographic distances predicted total incubation or female's share of incubation. These results suggest that the local environment has a stronger influence on a social behaviour than genetic differentiation, at least between populations of closely related species.

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EFFECTS OF FOOD SUPPLEMENTATION ON FEMALE NEST ATTENTIVENESS AND INCUBATION MATE FEEDING IN TWO SYMPATRIC WREN SPECIES AARON T. PEARSE,'-'' JOHN F. CAVITT,' '» AND JACK F. CULLY, JR. BSTRACT.-We examined effects of incubation mate feeding on female incubation behavior and correlates of fitness by providing female Bewick's Wrens {Thryomanes bewickii) and House Wrens {Troglodytes aeclon) with food supplements. Males of these species vary in their rates of feeding; Bewick's Wrens feed their incubating mates frequently, whereas House Wrens seldom engage in this behavior. Average length of incubation bout and nest attentiveness (proportion of time spent on the nest) were higher for supplemented female Bewick's Wrens and House Wrens compared to controls. Furthermore, mates of supplemented Bewick's Wrens provisioned females at lower rates than controls, and their rate of feeding was inversely correlated with ambient temperature. Incubation length and hatching success were not significantly different between treatments for either species. These results suggest that incubation mate feeding can increase female nest attentiveness and perhaps enhance fitness of both males and females. In House Wrens, potential tradeoffs between the benefits of parental care and opportunities to obtain additional mates may explain why males rarely feed incubating females.

Adult passerines commonly lose mass during the course of the breeding cycle. This loss has been ascribed to an energy deficit incurred during nesting, but recently two adaptational hypotheses have been proposed. The first, the wing-loading hypothesis, proposes that adults actively reduce mass in order to increase flight efficiency. The second, the reserve-mobilization hypothesis, proposes that females store reserves during the early stages of the breeding cycle, which they mobilize to sustain them during later stages. We tested these hypotheses by providing food supplements to House Wrens (Troglodytes aedon) breeding on a study area north of Bloomington, Illinois, during the 1990-1992 breeding seasons. Additional food had no effect on mass except late in one season. Females that were given additional food during the late-young stage in 1992 had significantly higher mass than controls, but their mass was not greater than that normally obtained by females not receiving food supplements in other years. We conclude that the results from this experimental study are consistent with the wing-loading hypothesis, and that House Wrens do not lose mass because of an energy deficit or because they mobilize reserves.