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Biparental care for offspring requires cooperation, but it is also a potential source of conflict, since one parent may care less at the expense of the other. How, then, do parents respond to the reduction of their partner’s care? Theoretical models predict that parents that feed offspring should partially compensate for the reduced care of their partner. However, for incubating birds partial compensation is unlikely the optimal strategy, because the entire brood can fail with reduced care. Although biparental incubation dominates in non-passerine birds, short-term manipulations of parental care and evaluations of individual differences in the response, both crucial to our understanding of parental cooperation, are scarce. Here, we describe the response of semipalmated sandpiper (Calidris pusilla) parents to a 24-hour removal of their partner during the incubation period, explore factors that can explain individual variation in the response, and describe how incubation rhythms changed after the removed parent returned. On average, the parents compensated partially for the absence of their partner’s care (59%; 95%CI: 49-70%,N= 25 individuals). The level of compensation did not differ between the sexes. However, individual responses varied from no to full compensation (2-101%). In seven nests where the removed parent never returned, the widowed parent attended the nest for 0-10 days (median: 4 days). In contrast to patterns observed in undisturbed nests or in uniparental species, nest attendance during compensation tended to be higher during the warmer part of the day. Although the level of compensation was not related to the before-experimental share of incubation, more responsive parents (that left the nest earlier upon human approach) compensated more. The quality of incubation in the after-experimental period was lower than usual, but improved quickly over time. Our findings suggest that full compensation might be limited by energetic constraints or by variation in responsiveness to the absence of the partner. Nevertheless, semipalmated sandpiper parents are able to adjust their subsequent incubation behaviour to take full responsibility for the nest when widowed. Because (nearly) full compensation was the most common response, we speculate that all individuals attempt full compensation, but that some fail because their energy stores get depleted, or because they are less responsive to the absence of their partner.

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment1,2,3,4. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions1,5, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators6,7,8,9,10. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring)6,7,8,9,11. The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood5,6,7,9. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization12 where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the median length of one parent’s incubation bout varied from 1–19 h, whereas period length—the time in which a parent’s probability to incubate cycles once between its highest and lowest value—varied from 6–43 h. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Rhythms entrainable to the 24-h light–dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity5,6,7,9. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms., The study was supported by the Max Planck Society (to B.K.). M.B. is a PhD student in the International Max Planck Research School for Organismal Biology.

Eurasian Oystercatchers Haematopus ostralegus ostralegus are a classic example of individual feeding specialization. Feeding specialization causes morphological differentiation in Oystercatchers' bill shapes due to varying degrees of abrasion associated with specific handling techniques for each prey species. Consequently, the Oystercatcher's bill shape has been used as a proxy for diet specialization, as it provides a quick and easy way to assess the diet choice of this marine top-predator. However, bill shapes of Oystercatchers are categorized visually in distinct types, while it has been argued that the relevant variation is continuous. Also, it is unclear how comparable the bill-shape classification is among studies and between the sexes and how universal bill shape-diet relationships are. Here we investigate the usefulness of bill-shape types as a proxy for diet choice in Oystercatchers, using four new and two published datasets. We show that quantitative bill-morphometrics provide no evidence that b...

Intake rate maximization alone is not always sufficient in explaining prey size selection in predators. For example, bivalve-feeding oystercatchers regularly select smaller prey than expected if they aimed to maximize their intake rate. It has been proposed that to these birds large prey are ‘‘risky,’’ in the sense that birds may damage their bills when feeding on large bivalves. Large bivalves yield more energy, but according to this hypothesis this is achieved at the expense of energy yield in the long term when (1) the risk of bill damage increases with prey size and (2) foraging with a damaged bill is less effective. In accordance with this hypothesis, we show that captive oystercatchers feeding on large cockles experienced a high probability of bill tip damage, while bill damage was absent when cockles were small. Moreover, among free-living oystercatchers the prevalence of bill damage was correlated with mean cockle size near the capture site, and the data on captive birds fit in this pattern. Food intake of captive oystercatchers feeding exclusively on cockles was reduced by 23% after bill damage, and free-living birds with damaged bills had 14 g lower mass. Because lower body mass was associated with higher mortality probability, these results indicate long-term costs associated with feeding on large cockles. We conclude that the risk of bill damage can potentially explain why oystercatchers avoid large bivalves and that oystercatchers may maximize long-term intake rate by selecting prey sizes that are ‘‘suboptimal’’ from a short-term rate-maximizing point of view. Key words: prey choice, prey profitability, prey size selection, risky prey. [Behav Ecol]