About eating and not eaten. Vigilance and foraging strategies in wintering Eurasian siskins (Carduelis spinus) = Sobre menjar i no ser menjar: estratègies de vigilància i alimentació en lluers hivernants (Carduelis spinus)

Animals have to eat a certain amount of food every day to survive. Since trophic resources are usually found in exposed locations, animals are in risk of predation while feeding, and face the well known trade-off between predation and starvation risks. Animals may reduce predation risk by scanning for predators. However, an increase in vigilance entails an increase in exposure time to predators. Therefore, animals may increase food intake to reduce foraging bouts at the cost of vigilance or may do the opposite. Animals may also reduce predation risk without increasing vigilance by joining groups. However, when food items are concentrated in some defendable patches, individuals will fight to access them, leading to interference competition. Birds have developed different vigilance and foraging strategies to cope with all these trade-offs. These strategies can be adapted to the environmental conditions and type of predators, and will highly differ according to the genetic, phenotypic and ecological characters of the individuals. In this thesis I investigated how wintering Eurasian siskins (Carduelis spinus) adjusted their vigilance and foraging behaviour to predation risk and competition, and how this adjustment varied according to conspicuousness, residence status and personality of focal birds. I also studied how resident and transient siskins regulated their body mass to predation risk. In order to determine the differential effects of predation risk and competition on the behaviour of siskins, I designed an experiment with three feeders differing in predation risk and interference competition. I captured the siskins every week and marked them with aluminium rings (at first capture) and colour rings (if residents). From each bird, I measured body mass, wing length and plumage coloration cues. I filmed the siskins foraging at feeders from a hide. Then I analyzed the video recordings and selected a sample of birds at each feeder to study the effect of environmental factors, and selected pairs of birds belonging to different cohorts of sex, residence and personality to compare their vigilance and foraging behaviour. I also compared body mass of a large sample of residents and transients in two periods with and without avian predators. I found that siskins adjusted their vigilance behaviour specifically to the environmental conditions. Vigilance to predators, achieved by reducing inter-scan durations, was less costly in terms of food intake rate than vigilance to flockmates, achieved by increasing scan durations. Siskin males were more brightly coloured and detectable than females, and showed a vigilance system more oriented to predator detection, supporting (together with the correlation between coloration and vigilance in males) the view of a predation cost to conspicuousness. Resident and transient siskins showed a different vigilance and foraging strategy. Residents were more confident in vigilance to reduce predation risk while transients preferred to reduce foraging bout lengths. Transient siskins were in higher predation risk because of their vigilance and foraging behaviour, and this was related both to their unfamiliarity with the area and their subordination to residents. Proactive male siskins showed a vigilance system that improved their ability to detect predators as compared with reactive males, supporting the view of a behavioural compensation for their personality trait (and not the existence of behavioural carryovers). Resident siskin males adjusted their body mass to the presence of predators at the foraging area, something that transients could not do because of their site unfamiliarity. In general, dominant individuals (whether males, residents or proactive birds) showed a foraging behaviour that prioritized the reduction in predation risk and not the increase in food intake rate, as compared to subordinates. Dominance alone cannot account for our results, although it probably had some effect in all the comparisons.