Piet J. van den Hout

Disentangling the roles of frequency-vs. state-dependence in generating individual differences in behavioural plasticity

Theoretical work suggests that both negative frequency-dependent payoffs and state-dependent payoffs can lead to individual variation in behavioural plasticity. We investigated the roles of both frequency- and state-dependence on the occurrence of individual variation in behavioural plasticity in a series of experiments where we manipulated perceived predation danger for red knots (Calidris canutus islandica). We found individual variation in plasticity in a trait with negative frequency-dependent payoffs (vigilance), but not in a trait with positive frequency-dependent payoffs (escape flights). Furthermore, there was no correlation between the average level of vigilance under low predation danger and the magnitude of response to increased predation danger, as would be expected under state-dependence. Thus, our results provide support for the hypothesis that negative-frequency dependence favours individual variation in plasticity. However, negative-frequency dependence alone cannot explain why plasticity would be consistent within individuals, and future studies should address the factors that might favour individual consistency.

Differential responses of red knots, Calidris canutus, to perching and flying sparrowhawk, Accipiter nisus, models

According to the threat-sensitive predator avoidance hypothesis, prey should match the intensity of their antipredation response to the degree of threat posed by predators. We used controlled indoor experiments to investigate the ability of red knots to discern between high- and low-threat encounters with a representative predator, the sparrowhawk. The behaviour of knots was compared across three conditions: no predators present (very low predation threat), presentation of a perching sparrowhawk model (low predation threat) and presentation of a gliding sparrowhawk model (high predation threat). In all behavioural parameters measured, red knots showed evidence of discriminating between the different levels of predation risk. Knots responded immediately to the presence of sparrowhawks with escape flights, and the duration of escape flights was longer following the gliding sparrowhawk events than following perching events. Similarly, the proportion of time spent vigilant increased with increasing level of predation threat, while the proportion of time spent feeding decreased. These results show that knots recognize variations in the level of predation threat, and adjust their antipredator responses accordingly. Furthermore, model sparrowhawks were introduced into the experimental arena at similar distances to the knots, which suggests that knots are able to use cues other than distance to predator to gauge the immediate level of threat that a predator poses.

Interference from adults forces young red knots to forage for longer and in dangerous places

In birds and mammals, juvenile and adult foragers are often found apart from each other. In this study, we found this is also true for red knots, Calidris canutus canutus, wintering on the intertidal flats of Banc d’Arguin, Mauritania. Not only did juveniles feed separately from adults, they also fed at places where they were more vulnerable to predation by large falcons. That the dangerous areas used by juveniles were no better feeding areas led us to reject the foodesafety trade-off that explained age-related distribution differences in many earlier studies. Instead, juveniles were displaced by adults in dyadic interactions which suggests that they suffered from interference from adults. Juveniles retreated to feeding areas that were more dangerous and yielded lower intake rates, and coped by extending foraging time by using higher, nearshore intertidal areas that were exposed for longer. When disturbed by predators in these nearshore areas, juveniles continued feeding whereas adults left. Thus, rather than compensating for increased predation danger by higher intake rates, on the Banc d’Arguin red knot juveniles foraged for longer.

Individual shifts toward safety explain age-related foraging distribution in a gregarious shorebird

Although age-related spatial segregation is ubiquitous, the underlying mechanisms are poorly understood. Here, we aim to elucidate the processes behind a previously established age-related foraging distribution of red knots (Calidris canutus canutus) in their main wintering area in West Africa (Banc d’Arguin, Mauritania). Based on 10 years of observations of 1232 uniquely color-ringed individuals of 1 to 18+ years old, we examined whether the observed age-related foraging distribution resulted from 1) spatial differences in mortality or 2) age-related shifts in habitat use. Using multistate capture–recapture modeling, we showed that with age foraging red knots moved away from the shoreline, that is, to areas with fewer surprise attacks by raptors. Considering uncertainties in the subjective gradient in predation danger with increasing distance from shore (as assessed from correlations between vigilance and distance from shore in foraging birds), we applied 2 different danger zone boundaries, at 40 m and 500 m from shore. Between years, red knots had a much higher chance to move from the dangerous nearshore area to the “safe” area beyond (71–78% and 26% for 40-m and 500-m danger zone boundary, respectively), than vice versa (4% and 14%). For neither danger zone boundary value did we find differences in annual mortality for individuals using either dangerous or safe zone, so the move away from the shore with age is attributed to individual careers rather than differential mortality. We argue that longitudinal studies like ours will reveal that ontogenetic shifts in habitat use are more common than so far acknowledged.