Berry Pinshow

Habitat Selection: An Experimental Field Test with Two Gerbil Species

We provide experimental evidence for the isoleg theory of habitat selection in a pair of psammophilic gerbil species. Gerbillus allenbyi (mean mass: 26 g) and G. pyramidum (mean mass: 40 g) coexist in Israels Negev desert in areas that may contain three distinct sandy habitats: stabilized sand fields, semistabilized dunes, and drifting dunes. When all three habitat types are available, coexistence between the two species has been explained by a centrifugal model of community organization that has been untested until now. To begin testing it, we conducted a field experiment in six 1 ha enclosures, each containing similar proportions of two of the sandy—habitat types (stabilized sand and semi—stabilized dune). This experiment tested the following hypotheses concerning the coexistence of the two species: (1) both species prefer the same primary habitat type; (2) G. allenbyi and G. pyramidum exhibit intraspecific density—dependent habitat selection; (3) habitat preference of both G. allenbyi and G. pyramidum...

The Effect of Competition on Foraging Activity in Desert Rodents: Theory and Experiments

We studied, both theoretically and empirically, the effect of intra- and in- terspecific competition on the foraging effort of individuals. We considered two models, one for a time-minimizer satisfying an energy requirement, the other for an animal max- imizing fitness as a function of multiple inputs subject to a time constraint. The goal of satisfying an energy constraint predicts that foraging effort should increase with increased competition. The goal of maximizing fitness subject to a time constraint on multiple inputs may also predict that foraging effort should increase with increased competition because of the missed opportunity cost that results when different inputs are complementary. However, if the fitness-maximizer with multiple inputs incurs an energy cost of foraging (in addition to missed opportunity costs), then it should often reduce foraging effort in response to an increase in competition. We experimentally tested the foraging response to increased competition by two species of gerbils, Gerbillus allenbyi and G. pyramidum, over a range of manipulated population densities in field enclosures located in the Negev Desert of Israel. Our results support the cost-benefit model when the additional energy cost of foraging is important. Per capita activity (as measured by spoor) declined as a function of intraspecific density for each species and as a function of interspecific density for G. allenbyi. We detected no interspecific effect, however, on G. pyramidum.

Water and Energy Limitations on Flight Duration in Small Migrating Birds

-We examined the physiological limitations to flight duration in small migrating birds with a computer-simulation model. Given preflight body mass, fat and water contents, and flight-path meteorological data, we calculated water and energy budgets and possible flight time. The model can be applied to birds of any size that migrate by flapping flight. As an example, we simulated the flight of small Palearctic passerines (body mass = 10 g) during their annual migration over the Sahara desert. Sensitivity analysis of model input variables indicated that oxygen extraction and expired air temperature are the most important physiological variables in a birds water budget and can profoundly influence flight duration. This manifests the importance of: (1) efficient cooling in the nasal passages of flying birds; and (2) the choice of flight altitude (which affects both ambient air temperature and expired air temperature). The model predicted that: (1) Prior to migration, these birds must have stored fat comprising at least 22% of their initial body mass; otherwise, they cannot complete their journey. (2) In relatively fat birds (stored fat > 0.22 body mass), dehydration rather than energy will limit flight duration. (3) Birds should fly at an altitude not exceeding 1,000 m to cross the Sahara successfully. (4) Even in low-flying fat birds, flight duration will be limited by their stringent water budget. The model further predicted that small passerines cannot cross the Sahara in a 30to 40-h nonstop flight, as commonly accepted, but should confine flying to the cooler hours (i.e. nights) and rest during the day in order to avoid elevated rates of water loss due to higher ambient air temperatures. Available data and observations of birds trapped at stopover sites in the Sahara support these predictions. Received 26 February 1991, accepted 13 January 1992. LONG-DISTANCE flight over seas or deserts is perhaps the most risky and physiologically challenging event in the life cycle of migratory birds (Moreau 1961, Wilson 1981). Reports of high attrition among migrants and of exhausted birds found severely dehydrated and/or fat depleted (Serle 1956, Odum et al. 1964, Rogers and Odum 1964, Johnston 1968, Wilson 1981) raised the question: Is water or energy the greater physiological limitation to bird flight duration? Energy, rather than water, is currently considered the major factor limiting bird flight duration because dehydration can be avoided by flying at high altitudes where air temperatures are low (Blem 1976, Torre-Bueno 1978, Skadhauge 1981, Dawson 1982, Biesel and Nachtigall 1987, Biebach 1990). However, this paradigm has not been substatiated with empirical field

Anatomical and histological changes in the alimentary tract of migrating blackcaps (Sylvia atricapilla): a comparison among fed, fasted, food-restricted, and refed birds.

During northward migration, blackcaps that arrive to refuel at stopover sites in Israel’s Negev Desert have reduced masses of organs that are important in food digestion and assimilation. We tested several predictions from the general hypothesis that smaller organs of digestion (small intestine and pancreas) and nutrient assimilation (liver) bring about a lower capacity to consume food and that the organs must be restored before blackcaps can feed and digest at a high rate. We used a fasting protocol to create a group of blackcaps with reduced intestine and liver mass (reduced by 45% and 36%, respectively) compared with controls fed ad lib. Because most of the small intestine’s biochemical digestive capacity reside in enterocytes found on villi, we predicted and found that reduced intestinal mass in fasted blackcaps related mainly to changes in enterocytes rather than other cells and tissues such as nonabsorptive crypt cells or underlying muscle. Because migrating blackcaps that stop over to feed begin to increase in body mass only 2 d after arrival, we predicted and found a similar recovery period in blackcaps that were first fasted but then refed—the organ mass, structure, function, and ability to consume food was restored after 2 d of feeding. Another group of food‐restricted blackcaps (fed at one‐third ad lib. level) lost similar amounts of body mass as fasted blackcaps but had much greater capacity to consume food than fasted blackcaps, and so we predicted that they would exhibit little or no reduction in alimentary organs relative to controls fed ad lib. A surprising result was that, as in fasted blackcaps, in food‐restricted blackcaps, the decreases in masses of small intestine, liver, and pancreas were proportionally greater than the decreases in body mass or in masses of nonalimentary organs (heart, pectoralis). Food restriction, like fasting, caused a decrease in amount of intestinal mucosa and an alteration in the phenotype of enterocytes. These results are thus not consistent with the general hypothesis, and although they can be rationalized by assuming that blackcaps fed ad lib. have excess digestive capacity, it may also be that the physiological process or processes limiting very high feeding rate lie elsewhere than in the digestive system.

The Shape of a Gerbil Isocline Measured Using Principles of Optimal Habitat Selection

The isoclines of Gerbillus allenbyi competing with G. pyramidum are negatively sloped and at least approximately parallel. They are not linear, however; they are steeper over both low and high G. pyramidum densities and only very slightly negative at intermediate densities. This nonlinearity agrees with the predictions of the theory of optimal density—dependent habitat selection in a two—species, shared—preference system, the system that obtains here. The isocline field was measured by a new method based on the single—species habitat selection theory of Fretwell. The species whose isoclines are being measured (the target species) is given a chance to equilibrate its per capita reproductive rate across a fence that separates two densities of its competitors. Equilibration requires that the target species distribute its density unequally on the two side of the fence. The general features of the method may be useful in other animal studies where densities can be experimentally manipulated.

Guild structure, foraging space use, and distribution in a community of insectivorous bats in the Negev Desert

Large-scale anthropogenic habitat degradation is taking place in the Negev Desert and such degradation is considered to be the primary cause of population decline in many bat species. A study of bat community structure in the central Negev Desert Highlands was undertaken to examine habitat associations, activity patterns and the distribution of bats in relation to landscape structure. During 1999 and 2000, mist-nets and bat detectors were used to capture bats and to monitor their presence in natural and artificial sites. In total, 12 species of insectivorous bats were found. Communities of insectivorous bats were divided into three guilds based on foraging space: open space foragers, background-cluttered space foragers, and highly cluttered space foragers. Bats of the background-cluttered space guild foraged over a variety of habitats while bats from the other guilds were more restricted with regard to their foraging spaces. In both years, season had a significant effect on bat activity. Despite there being significantly more bat activity in artificial sites than in natural sites, bat species richness was least in the artificial sites. For most species of desert-dwelling bats, areas that are typified by dense vegetation cover and have a perennial water source are of primary importance.

Density-dependent habitat selection in migratory passerines during stopover: what causes the deviation from IFD?

We studied the distribution of migratory warblers (genus: Sylvia) in poor and high quality habitat patches at a stopover site in the northern Negev, Israel. The purpose of our study was to test predictions based on the ideal free distribution (IFD) model by using a natural ecosystem which has a high turnover of individuals moving between unfamiliar foraging patches. We trapped birds in two groves of Pistacia atlantica embedded within a coniferous forest. The fruit-density ratio between these groves was 45:1. We compared bird density, body condition and habitat matching (the ratio between bird density and resource density) at the two sites. To analyse the data we integrated two approaches to density-dependent habitat selection: the isodar method and the habitat matching rule. As predicted by the IFD model, we found that habitat suitability decreased with bird density with a high correlation between warbler densities in the two habitat patches. Contrary to IFD predictions, warbler density in the poor patch was higher than expected by the habitat-matching rule. This habitat under-matching, had a cost: in the rich habitat the average energy gain per individual bird was higher than in the poor habitat. Further analysis suggests that the apparent habitat under-matching is not due to interference or differences in warbler competitive abilities. Therefore, we suggest that this migratory bird community is not at equilibrium because the birds possess imperfect knowledge of resource distribution. We propose that this lack of knowledge leads to free, but not ideal distributions of migrant birds in unfamiliar stop over sites.

Flight Modes in Migrating European Bee-Eaters: Heart Rate May Indicate Low Metabolic Rate during Soaring and Gliding

Background Many avian species soar and glide over land. Evidence from large birds (m b>0.9 kg) suggests that soaring-gliding is considerably cheaper in terms of energy than flapping flight, and costs about two to three times the basal metabolic rate (BMR). Yet, soaring-gliding is considered unfavorable for small birds because migration speed in small birds during soaring-gliding is believed to be lower than that of flapping flight. Nevertheless, several small bird species routinely soar and glide. Methodology/Principal Findings To estimate the energetic cost of soaring-gliding flight in small birds, we measured heart beat frequencies of free-ranging migrating European bee-eaters (Merops apiaster, m b∼55 g) using radio telemetry, and established the relationship between heart beat frequency and metabolic rate (by indirect calorimetry) in the laboratory. Heart beat frequency during sustained soaring-gliding was 2.2 to 2.5 times lower than during flapping flight, but similar to, and not significantly different from, that measured in resting birds. We estimated that soaring-gliding metabolic rate of European bee-eaters is about twice their basal metabolic rate (BMR), which is similar to the value estimated in the black-browed albatross Thalassarche (previously Diomedea) melanophrys, m b∼4 kg). We found that soaring-gliding migration speed is not significantly different from flapping migration speed. Conclusions/Significance We found no evidence that soaring-gliding speed is slower than flapping flight in bee-eaters, contradicting earlier estimates that implied a migration speed penalty for using soaring-gliding rather than flapping flight. Moreover, we suggest that small birds soar and glide during migration, breeding, dispersal, and other stages in their annual cycle because it may entail a low energy cost of transport. We propose that the energy cost of soaring-gliding may be proportional to BMR regardless of bird size, as theoretically deduced by earlier studies.

Test for physiological limitation to nutrient assimilation in a long-distance passerine migrant at a Springtime stopover site.

During northward migration, blackcaps arrive at stopover sites in Israels Negev Desert with reduced masses of organs that are important in food digestion and assimilation. Blackcaps that stay to refuel (largely on fruits) do not gain mass rapidly until after 3 d at the stopover site. We hypothesized that (1) it may take several days to rebuild these reduced organs, (2) during this recovery interval high feeding rates might not be possible, and (3) this could be the basis for the absence of immediate body mass gain in blackcaps at stopover sites. To test predictions from this hypothesis we used an established fasting protocol to create a group of blackcaps with reduced intestinal and liver mass compared with ad lib. fed controls. Migrants were captured and caged in the laboratory, where they were habituated to a fruit mash diet for 8 d. One experimental group was then fasted 2 d, one was fed at a restricted level (one‐third ad lib. food intake) for 4 d, and one was held as ad lib.–fed controls. The fasted and restricted birds were then allowed to feed again ad lib. Birds that were experimentally fasted progressively increased their daily assimilation rate and achieved the highest rate (one‐third higher than controls) 3 d after the end of their fast. Birds that were restricted achieved high rates immediately once ad lib. food was provided. Increased assimilation rate was achieved via hyperphagia and not increased assimilation efficiency. The response of the fasted birds supports the hypothesis that there may be physiological constraints to the rate of refueling during migratory stopover.

The Analysis of 13C/12C Ratios in Exhaled CO2: Its Advantages and Potential Application to Field Research to Infer Diet, Changes in Diet Over Time, and Substrate Metabolism in Birds

Abstract Stable isotopes are becoming an increasingly powerful tool for studying the physiological ecology of animals. The 13C/12C ratios of animal tissues are frequently used to reconstruct the diet of animals. This usually requires killing the subjects. While there is an extensive medical literature on measuring the 13C/12C ratio of exhaled CO2 to determine substrate digestion and oxidation, we found little evidence that animal physiologists or physiological ecologists have applied 13C/12C breath analysis in their studies. The analysis breath 13C/12C ratios has the advantage of being non-invasive and non-destructive and can be repeatedly used on the same individual. Herein we briefly discuss the medical literature. We then discuss research which shows that, not only can the breath13C/12C ratio indicate what an animal is currently eating, but also the animals diet in the past, and any changes in diet have occurred over time. We show that naturally occurring 13C/12C ratios in exhaled CO2 provides quantitative measure of the relative contribution of carbohydrates and lipids to flight metabolism. This technique is ripe for application to field research, and we encourage physiological ecologists to add this technique to their toolbox.