Interindividual spacing affects the finder’s share in ring-tailed coatis (Nasua nasua)

Social foraging models are often used to explain how group size can affect an individual’s food intake rate and foraging strategies. The proportion of food eaten before the arrival of conspecifics, the finder’s share, is hypothesized to play a major role in shaping group geometry, foraging strategy, and feeding competition. The variables that affect the finder’s share in ring-tailed coatis were tested using a series of food trials. The number of grapes in the food trials had a strong negative effect on the finder’s share and the probability that the finder was joined. The effect of group size on the finder’s share and foraging success was not straightforward and was mediated by sociospatial factors. The finder’s share increased when the time to arrival of the next individual was longer, the group was more spread out, and the finder was in the back of the group. Similarly, the total amount of food eaten at a trial was higher when more grapes were placed, arrival time was longer, and the number of joiners was smaller. Individuals at the front edge of the group found far more food trials, but foraging success was higher at the back of the group where there were fewer conspecifics to join them. This study highlights the importance of social spacing strategies and group geometry on animal foraging tactics and the costs and benefits of sociality.

Dependence of feeding rates on body mass when food density is limiting to growth

Bioenergetics models are commonly used to predict effects of changes in metabolic rates and food availability on growth. However, food intake rate generally is assumed to vary as Wd, where d = 2/3, an assumption based on observations from feeding trials in laboratory studies. Further, the von Bertalanffy growth function (VBGF) is specifically integrated using this assumption. We argue that when considered from an ecological perspective, d is highly uncertain, dependent on how swimming speed, reactive distance, and prey biomass varies ontogenetically with the growth of a predator. Incorrectly specifying d leads to incorrect predictions of consumption and metabolism, especially at younger ages that are typically under-sampled. Three alternate means of detecting departures from d = 2/3 are provided, the most promising of which involves fixing initial length of the generalized VBGF to the length at endogenous feeding and directly calculating von Bertalanffy parameters (L∞, K, t0). Using this approach, it may be possible to more accurately estimate consumption and metabolism and to characterize lifetime growth.

Differential effects of predation risk and competition over vigilance variables and feeding success in Eurasian siskins (Carduelis spinus)

Many investigations have studied the effects of predation risk and competition over vigilance and feeding success, but they have proven to be difficult to discriminate. Moreover, none of the studies that have avoided the confusion has considered all the vigilance variables, food intake rate and time spent in the foraging patch. In this study, we designed an experiment with Eurasian siskinsCarduelis spinusforaging on three bird table feeders: one with low predation risk and competition, one with low predation risk and high competition and one with high predation risk and intermediate competition. Birds responded to increasing interference competition by increasing mean scan durations (probably due to the birds having to be vigilant for both other flock members and predators) and maintaining the length of mean inter-scan durations, while they responded to increasing predation risk by reducing mean inter-scan durations (probably in order to detect the predator sooner) while maintaining similar length of mean scan durations. Birds were often ejected from the feeder or departed because of disturbances, so time spent on feeders was reduced both because of competition and predation risk. Pecking rates were affected by competition but not by predation risk. Our results clearly show that birds vigilance strategy while foraging might be very different when they are mainly concerned with scanning for predators or when they primarily monitor competing flock companions. In addition, they stress the importance of recording all the vigilance and feeding variables when studying the effect of ecological factors over the foraging behaviour of birds.