Predation Costs Research Articles

Locatability of begging calls in nestling altricial birds

Nestling begging calls of altricial species of birds have design features (wide frequency range, abrupt onsets and modulation in amplitude and frequency) that make them easily located by birds and mammals, and so may attract predators to the nest. To be maintained by natural selection, such calls must also be beneficial. It is argued here that sibling competition for food during the early stages of nestling development favours locatability of begging calls, presumably because noisy nestlings attract the attention of parents. In the magpie, Pica pica, begging calls of nestlings have a wider frequency spectrum before the nestlings' eyes have opened, a trait that increases their locatability. A strategy of having locatable calls should spread if favoured by mechanisms that overcome the increased predation risk associated with such calls. Two mechanisms are proposed: increased attenuation of the signals by emphasizing higher frequencies (a feature commonly found in begging calls) and dispersion of energy over a wide time-frequency range, a trait that, because of sound degradation, probably masks the estimate of source distance by predators. This hypothesis agrees with predictions of models of intra-brood conflict: when predation costs are higher, level of solicitation (locatability) should decrease. Hole-nesting species, which have a lower risk of predation, have calls with wider frequency ranges and lower (less attenuable) medium frequencies than those of open-nesting species of a similar weight.

Patch use as an indicator of habitat preference, predation risk, and competition

A technique for using patch giving up densities to investigate habitat preferences, predation risk, and interspecific competitive relationships is theoretically analyzed and empirically investigated. Giving up densities, the density of resources within a patch at which an individual ceases foraging, provide considerably more information than simply the amount of resources harvested. The giving up density of a forager, which is behaving optimally, should correspond to a harvest rate that just balances the metabolic costs of foraging, the predation cost of foraging, and the missed opportunity cost of not engaging in alternative activities. In addition, changes in giving up densities in response to climatic factors, predation risk, and missed opportunities can be used to test the model and to examine the consistency of the foragers' behavior. The technique was applied to a community of four Arizonan granivorous rodents (Perognathus amplus, Dipodomys merriami, Ammospermophilus harrisii, and Spermophilus tereticaudus). Aluminum trays filled with 3 grams of millet seeds mixed into 3 liters of sifted soil provided resource patches. The seeds remaining following a night or day of foraging were used to determine the giving up density, and footprints in the sifted sand indicated the identity of the forager. Giving up densities consistently differed in response to forager species, microhabitat (bush versus open), data, and station. The data also provide useful information regarding the relative foraging efficiencies and microhabitat preferences of the coexisting rodent species.

Search and procurement in a simulated natural environment

Differences in food availability and predation risk can influence how herbivores use landscapes. As a result, trade-offs between costs and benefits can influence habitat and patch selection. To determine how oribi antelope, Ourebia ourebi, weigh up costs and benefits when making habitat and patch level foraging decisions, we measured giving-up densities in artificial patches. First, we determined large-scale habitat use, and then explored how different variables explained patch use within habitats. When the availability of food within patches across the different habitats was equal, oribi preferred to feed in short and tall grasslands and avoided woodlands. Furthermore, the avoidance of woodlands extended into the surrounding grasslands, resulting in oribi feeding less intensively in grassland areas within 15 m of the woodlands. Within the safe grassland habitats, oribi preferred to feed in patches close to tall grass (i.e. escape cover), and where they could see beyond 2 m. These results suggest that oribi select habitats and patches in relation to perceived predation risk (i.e. predation costs outweigh potential food intake benefits). However, when we increased food availability within woodlands, oribi increased their large-scale risk-taking behaviour and fed in these woodlands. Furthermore, this increased risk taking extended to small-scale foraging decisions whereby an increase in food availability within woodlands caused oribi to increase their relative usage of patches that had sight lines as well as patches that were closer to potential ambush sites. Ultimately, these results highlight how changes in food availability can determine the degree to which herbivores are willing to increase their risk-taking behaviour, and how these changes can affect overall landscape use.

Costs and Returns of Angora Goat Enterprises with and without Coyote Predation

Costs and Returns of Angora Goat Enterprises with and without Coyote Predation

Field study of Haplochromis burtoni: habitats and co-habitant

This report describes the natural habitat of the cichlid fish Haplochromis burtoni along the Burundi coast of Lake Tanganyika in central Africa. Parameters of the physical environment as well as more general biological factors are described qualitatively and quantitatively. Temporary shore pools, a previously undescribed habitat, are examined in detail. These pools, appear to play an important role in the life cycle of Haplochromis burtoni and other mouthbrooding cichlids by providing, for a large part of each year, a protected area 0 which is particularly suited for reproduction, possibly at the cost of high predation. As a result of circaannual changes in the lake level, the shore pools are alternately isolated from and rejoined with the lake, allowing a regular exchange of fauna.