Group foraging in starlings: distributions of unequal competitors

Effects of predictability and competition on group and individual choice in a free-ranging foraging environment

The present study examined the social foraging of rats in an open arena. The relative quantity of food varied across two food sources, or "patches." Five food quantity ratios (1:1, 1:2, 1:8, 8:1, 2:1) were presented in a series of 30-min sessions. Ratios varied randomly across 6-min components within sessions (Phase 1), or in a consistent order across sessions (Phase 2). Group and individual preferences were well described by the ideal free distribution and the generalized matching law, respectively, with evidence of undermatching at both group and individual levels. Sensitivity of individual and collective behavior to the relative quantities of food was higher in Phase 2 than in Phase 1. Competitiveness rankings, assessed before and after experimental sessions by delivering food in rapid succession from a single feeder, was positively related to sensitivity values in Phase 1, but less consistently so in Phase 2. This study illustrates a promising experimental method for investigating foraging in a social context.

Foraging behaviour in tadpoles of the bronze frog Rana temporalis: experimental evidence for the ideal free distribution

The ability of bronze frog Rana temporalis tadpoles (pure or mixed parental lines) to assess the profitability of food habitats and distribute themselves accordingly was tested experimentally using a rectangular choice tank with a non- continuous input design. Food (boiled spinach) was placed at two opposite ends of the choice tank in a desired ratio (1 : 1, 1 : 2 or 1 : 4) to create habitat A and B. The tadpoles in Gosner stage 28-33, pre-starved for 24 h, were introduced in an open ended mesh cylinder placed in the center of the choice tank, held for 4 min (for acclimation) and then released to allow free movement and habitat selection. The number of tadpoles foraging at each habitat was recorded at 10, 15, 20, 25 and 30 min time intervals. The actual suitability, Si (the food available in a habitat after colonization of tadpoles) of each habitat was obtained from the equation Si = Bi - fi (di) where Bi is basic suitability (amount of food provided at each habitat before release of tadpoles), fi is the rate of depletion of food (lowering effect) with introduction of each tadpole, and di is the density of tadpoles in habitat i. The expected number of tadpoles at each habitat was derived from the actual suitability. With no food in the choice tank, movement of the tadpoles in the test arena was random indicating no bias towards any end of the choice tank or the procedure. In tests with a 1 : 1 food ratio, the observed ratio of tadpoles (11.71 : 12.28) was comparable with the expected 12 : 12 ratio. The observed number of tadpoles in the habitats with a 1 : 2 food ratio was 8.71 : 15.29 and 7.87 : 16.13 for pure and mixed parental lines respectively. In both cases, the observed ratios were close to the expected values (7 : 17). Likewise, in experiments with a 1 : 4 food ratio, the observed number of tadpoles in the two habitats (10.78 : 37.22) did not differ significantly from the expected ratio of 7 : 41. In all tests, the number of R. temporalis tadpoles matched ideally with habitat profitability (undermatching index K approximate, equals 1). The study shows that tadpoles of the bronze frog exhibit an ideal free distribution while foraging regardless of whether they are siblings or non-siblings in a group, which correlates well with their group living strategy in nature

Patch choice and risk: relative competitive ability is context dependent

The relative abilities of individual cichlids, Tilapia zillii to obtain food under scramble competition was highly repeatable between trials using a single input source, regardless of whether the input was constant or variable. However, when given a choice between two patches differing only in their temporal variability in input about an identical mean, an individual's rank based on intake in one patch was uncorrelated with either its intake in the other patch or its intake in the single-patch trials. In the two-patch trials, certain individuals both spent more time in food patches and visited patches more often than others, and overall the fish spent more time in the constant rate patch than the variable patch, leading to more items being consumed from the constant rate patch. We discuss possible causes and consequences of this dependence of relative competitive ability on the context of the foraging situation. Copyright 1999 The Association for the Study of Animal Behaviour.

Relative competitive ability changes with competitor density: evidence from feeding blackbirds

How individual competitive ability varies with competitor density in systems where interference competition occurs has been studied in few systems, despite its importance in determining the fundamental predictions of influential phenotype-limited ideal free distribution models. I measured individual variation in competitive ability in wild wintering blackbirds, Turdus merula. Variation in feeding rate of single birds, and the decline in their feeding rate in the presence of competitors, were measured in experimental patches. Individuals varied significantly both in their feeding rate when alone and in the decline in that feeding rate in the presence of competitors. Individuals that had high absolute feeding rates did not tend to have smaller declines in feeding rate in the presence of competitors. The relative ranking among birds in their feeding rate was strongly dependent on competitor density. This result has important implications for the expected distributions of competitors between patches. (c) 1998 The Association for the Study of Animal Behaviour.