Ease Of Capture Research Articles

Prey choice by predators: effect of prey vulnerability

We examined the effect of prey vulnerability on the prey choice behaviour of the turtle Phrynops gibbus in a series of laboratory experiments using two sizes and states (mobile and immobile) of prey. Our results suggest that prey vulnerability (ease of capture) alters the apparent choice of prey by these turtles. When vulnerability was low, turtles consumed significantly more small prey and when vulnerability was high they consumed significantly more large prey exhibiting what has been respectively referred to in the foraging literature as passive and active prey choice. We suggest that the distinction between passive and active prey choice may be misleading and that the interaction between forager behaviour and prey antipredator behaviour should be considered in studies of nonrandom predator diets.

Subsistence Hunting in Arabuko-Sokoke Forest, Kenya, and Its Effects on Mammal Populations.

Mammal populations in Arabuko-Sokoke Forest, Kenya, provide an important source of protein and income for local communities. The harvested biomass in 1991 was over 350 kg/km2 , and the economic value of the harvest was over 1.3 million KShs (approximately US$35,000). Bushpigs, aardvarks, and primates comprise 95% of the harvested biomass, but the main prey in terms of number of animals killed (35% of total) was the four-toed elephant shrew (Petrodomus tetradactylus). Trapping is concentrated on the periphery of the forest and reduces densities of four-toed elephant shrews within 1-2 km of the forest edge by 41%, squirrels by 66%, and Syke's monkeys by 55%. Yellow baboons (Papio cynocephalus), Syke's monkeys (Cercopithecus mitis), and the larger ungulates are also hunted with dogs throughout the forest. The results of our study suggest that yellow baboons and Syke's monkeys are overharvested, and current offtake rates of elephant shrews, squirrels, and duikers (Cephalophus spp.) are sustainable. Overharvesting has reduced the densities of large ungulates, including bushbuck (Tragelaphus scriptus) and buffalo (Syncerus caffer), to low levels. Differences in reproductive rates, abundance, population, dispersion, ease of capture, and relative vulnerability to hunting, as compared to trapping, influence the extent to which different prey species are affected by harvesting. Caza de subsistencia en la selva de Arabuko-Sokoke, en Kenia, y sus efectos sobra las poblaciones de mamíferos.

Diet of house wrens (Troglodytes aedon) and the abundance of the invertebrate prey in the dune-ridge forest, Delta Marsh, Manitoba

The diet of house wrens (Troglodytes aedon) was determined from gut contents of wrens collected on the forested dune ridge, Delta Marsh, Manitoba, in 1981 and 1982. Despite a preponderance of midges (Diptera: Chironomidae) in the food resources that were available to the wrens, the wrens consumed representatives of most of the invertebrate taxa that were sampled by sweep netting in the feeding habitat. The ranked importance of 13 taxa in the wren diet was correlated most closely with the ranks of their biomass available. When larger individuals within a taxa were available they were consumed in greater proportion by wrens. Chironomids were an exception; small individuals were usually eaten in greater proportion than expected from their abundance in sweep-net samples. The percentage of small chironomids eaten, however, decreased as their abundance increased. Prey selection apparently depended on abundance, size, and ease of capture. House wrens apparently were less selective when suitable prey were abundant.

The Petroleum-Inducible Mixed-Function Oxidase of Cunner (Tautogolabrus adspersus Walbaum 1792): Some Characteristics Relevant to Hydrocarbon Monitoring

The hepatic aryl hydrocarbon hydroxylase of the cunner (Tautogolabrus adspersus) is a sensitive indicator of petroleum contamination. This fish is a particularly favorable species because of its limited home range and ease of capture. Oil concentrations of 1–2 mg/L in water caused two- to sixfold induction of the enzyme above uninduced levels; induction decayed in < 7 d after exposure ceased. Feeding of crude oil or mussel tissue contaminated with oil at a concentration of 500 mg/kg caused up to fivefold induction. Males maintained a constant specific activity of aryl hydroxylase through the season, but females had a decreased activity before spawning. Starvation may also have reduced AHH activity. Correlations with liver and body weight were not observed. 2,5-Diphenyloxazole was found to be a safe substitute for benzo[a]pyrene in the assay. Key words: cunner, mixed function oxidase, aryl hydrocarbon hydroxylase, oil pollution, monitoring, biochemistry

Feeding habits and diet overlap of three species of garter snakes (Thamnophis) on Vancouver Island

Closely related, sympatrtc species of organisms often show resource partitioning. In this study, diet partitioning is examined in three sympalric species of garter snakes (Thamnophis) on Vancouver Island. Thamnophis sirtalis feeds mainly on amphibians and earthworms, Thamnophis ordinoides on earthworms and slugs, and Thamnophis elegans on slugs, fish, and small mammals. Thamnophis sirtalis and T. ordinoides, which are mote widely distributed on Vancouver Island than is T. elegans, have not been shown to modify their diets in the absence of T. elegans, but it is suggested that this lack of niche shift may be due to variations in prey availability. Neither innate differences in food preferences among the species nor interspecific competition for food have been shown to explain the observed differences in diet among the three species. Certain types of prey appear more likely than others to turn up in large numbers in stomach samples and it is suggested that this may be due to variations in ease of capture and (or) abundance. Possible explanations of the observed diets are discussed.

SELECTIVE PREDATION BY TWO SWIFTS AND A SWALLOW IN CENTRAL AMERICA

SummarySamples of flying insects were made at three sites and during dry and wet seasons in Costa Rica and at two sites during the wet season in Panama. Compared with these were the diets of three aerially foraging birds: the Band‐rumped Swift Chaetura spinicauda and Rough‐winged Swallow Stelgidopteryx ruficollis in Costa Rica and the Short‐tailed Swift Chaetura brachyura in Panama.Among insects flying, Diptera accounted for between 52 and 75%, with smaller numbers of Hymenoptera (mean = 15%) and Coleoptera (10%). In bird stomach contents, however, Hymenoptera predominated (59–81%), Coleoptera were less common (mean = 18.5%) and Diptera least common (3.4%) of the three orders. The discrepancy between diets and samples of flying insects could not be accounted for solely on the basis of differential digestion. The composition of the swift and swallow diets was similar; there were no significant seasonal differences in the taxonomic composition of the diet of Chaetura spinicauda.Size distributions of beetle prey fitted log‐normal distributions in every case. Mean size of prey was significantly larger than that of insects available in all comparisons. Although there was no seasonal difference in the mean size of prey of Chaetura spinicauda, the variance in prey size, used as a measure of niche width, was significantly larger in the wet than in the dry season. Although insect densities may be higher, foraging time is reduced and the wet season is likely the worse season. This result therefore supports Emlen's hypothesis of narrower niches under optimal conditions and broader niches under suboptimal conditions.For increasing altitude above ground published data show a decrease in the density of insects flying, a decrease in the proportions of taxa favoured by the birds, and a significant decrease in the mean size of beetles. None of these trends can explain the differences between the birds' diets and the samples of flying insects.Preference of prey by these birds is concluded not to be a function of prey size alone, but to involve interaction among prey size, ease of capture, and local density. Because of different flight abilities, insect taxa will differ in ease of capture and, consequently, in mean size preferred by the predator, as well as proportions in the diet. This factor favours Hymenoptera and Coleoptera over Diptera as prey. Local concentrations of winged ants, bees and social wasps, and fig wasps make these prey differentially easy to exploit and explain the preference of Hymenoptera over Coleoptera. Large species of swifts are hypothesized to be the evolutionary consequence of the tendency of aerially foraging birds to exploit local concentrations of single prey species.

Habits of the Swamp Rice Rat, Oryzomys Palustris Palustris (Harlan)

Along the South Atlantic coast line, illimitable marshes of swamp grass cover the mud flats of lowlands of the tidal zone (Fig. 1). Here, in the cover of dense vegetation, rice rats occur in great numbers, their runway threading the black mud covered daily by the tides, or rising into the higher ground which skirts the tidal zone. The abundance of these rodents, the ease of capture and their docile behavior in captivity, make them ideal laboratory animals. It is therefore surprising that so little has been recorded of their habits. Observations on rice rats were made by Audubon and Bachman (1854), to which little has been added to the present day. Goldman (918) has presented a brief review of the habits of these rodents. Harper (1927) gives some interesting data on the nests and behavior of this species in the Okefinokee Swamp of Georgia, while Svihla (1931) has recorded observations on the Texas rice rat (Oryzomys palustris texensis). Other than these accounts, published data on the life history of this widespread genus is all but lacking. Rice rats range from New Jersey and the central part of the Mississippi Valley southward, being well represented in nearly every part of South America and reaching their greatest development in the tropics. More than 150 species and subspecies have been described, but little other than a name has been assigned to the majority of these, the habits having been totally unexplored.

Investigations of the Herring of Passamaquoddy and Adjacent Regions

The young herring ("sardine") fishery is seen as partly dependent on extremely local conditions. The fish first appear in the catches when about 12 months old in August. They are generally segregated into shoals of similar length (within an age-group). Certain areas tend to have herring of particular sizes. The "sardine" region is poorly supplied with herring fry rather than well supplied. Turbidity of the water is the only physical factor found possibly rendering the region specially attractive to herring. Euphausiids at the surface are one of the striking features of the heart of the "sardine" region. The principal species of plankton animals (Thysanoessa, Calanus, Sagitta) behave in a manner best explained by diurnal quiescence and nocturnal activity. Large landings of "sardines" in the "sardine" region as compared with other places is partly to be explained as due to especial ease of capture in that region, but it is uncertain whether there is a larger population of fish. It is clear that the proposed dams across the mouths of Passamaquoddy and Cobscook bays would make considerable havoc of the exceptionally rich fishery in their neighbourhood. The fishery inside the dams would almost certainly be reduced to negligible proportions, since it seems dependent on immigration. It cannot be foretold whether the total effect on capture immediately outside the dams would be deleterious or not. There appears little possibility of a wide-spread effect, for example along the coast of Maine, or even seriously at Grand Manan.