Abstract: Risk-sensitive foraging theory is based on the premise that unpredictable runs of good or bad luck can cause a variable food source to differ in fitness value from a fixed food source yielding the same average rate of gain but no unpredictability. Thus, risk-sensitive predictions are dependent on the food intake from variable sources being not only variable but also unpredictable or `risky' in outcome. This study tested whether unpredictability is a component of the value that foraging starlings,Sturnus vulgarisattribute to food sources that are variable in the delay to obtain food. Two groups of birds chose between a fixed and a variable delay option; the variable option was unpredictable in the risky group and predictable in the risk-free group in the overall rate of intake it yielded. In both groups the fixed option was adjusted by titration to quantify the magnitude of preference for predictable and unpredictable variance. On negative energy budgets both groups were significantly risk-prone, with the risky group being significantly more risk-prone than the risk-free group. Switching the birds to positive budgets by doubling the size of each food reward had no significant effect on preference, and similar trends to those found with negative budgets were observed. These results are not readily explained by risk-sensitive foraging theory, but may be explained by the algorithm used by the birds to attribute value to average expected rewards.

Abstract: The links between ecology, behavioural plasticity and brain size are often tested via the comparative method. Given the problems in interpretating comparative tests of learning and cognition, however, alternative measures of plasticity need to be developed. From the short notes section of nine ornithological journals, two separate, exhaustive data sets have been collated on opportunistic foraging innovations in birds of North America (1973-1993;N=196) and the British Isles (1983-1993;N=126). Both the absolute and relative frequencies (corrected for species number per order) of innovations differ between bird orders in a similar fashion in the two geographical zones. Absolute and relative frequency of innovations per order are also related to two measures of relative forebrain size in the two zones. The study confirms predicted trends linking opportunism, brain size and rate of structural evolution. It also suggests that innovation rate in the field may be a useful measure of behavioural plasticity.

Abstract: This is the first study to investigate the short-term effects of high population density on captive chimpanzees (Pan troglodytes). Subjects of the study were 45 chimpanzees living in five different groups at the Yerkes Regional Primate Research Center. The groups were observed under two conditions: 1) when they had access to both the indoor and outdoor sections of their enclosures; 2) during cold days when they were locked into the indoor runs, which reduced the available space by more than half. Under the high-density condition, allogrooming and submissive greetings decreased, but juvenile play increased. Remarkably, the rate of various forms of agonistic behavior, such as aggression, bluff charge, bluff display, and hooting, occurred less frequently under the high-density condition. This general decrease in adult social activity, including agonistic behavior, can be interpreted as an inhibition strategy to reduce opportunities for conflict when interindividual distances are reduced. This strategy is probably effective only in the short run, however. Behavioral indicators of anxiety, such as rough scratching and yawning, showed elevated rates, suggesting increased social tension under the high-density condition.