Red knots (Calidris canutus islandica) manage body mass with dieting and activity

ABSTRACTMass regulation in birds is well documented. For example, birds can increase body mass in response to lower availability and/or predictability of food and decrease body mass in response to increased predation danger. Birds also demonstrate an ability to maintain body mass across a range of food qualities. Although the adaptive significance of mass regulation has received a great deal of theoretical and empirical attention, the mechanisms by which birds achieve this have not. Several non-exclusive mechanisms could facilitate mass regulation in birds. Birds could regulate body mass by adjusting food intake (dieting), activity, baseline energetic requirements (basal metabolic rate), mitochondrial efficiency or assimilation efficiency. Here, we present the results of two experiments in captive red knots (Calidris canutus islandica) that assess three of these proposed mechanisms: dieting, activity and up- and down-regulation of metabolic rate. In the first experiment, knots were exposed to cues of predation risk that led them to exhibit presumably adaptive mass loss. In the second experiment, knots maintained constant body mass despite being fed alternating high- and low-quality diets. In both experiments, regulation of body mass was achieved through a combination of changes in food intake and activity. Both experiments also provide some evidence for a role of metabolic adjustments. Taken together, these two experiments demonstrate that fine-scale management of body mass in knots is achieved through multiple mechanisms acting simultaneously.

Risk allocation: acute and chronic predator exposure have contrasting effects on Song Sparrow (Melospiza melodia) singing behaviour

Increasing the danger posed by predators may cause prey animals to alter their behaviour. For example, they may be more vigilant and so feed more slowly. Breeding male Song Sparrows (Melospiza melodia (A. Wilson, 1810)) spend much time in conspicuous, loud song, which is an important behaviour for territorial defense and for mate attraction. We measured their singing behaviour in relation to both chronic (active Cooper’s Hawk (Accipiter cooperii (Bonaparte, 1828)) nest nearby) and acute (playback of hawk calls) predator exposure. We found that proximity to a Cooper’s Hawk nest had little or no influence. In contrast, the response to acute exposure was strong and immediate: Song Sparrows reduced the song rate and the proportion of time spent singing, lowered perch height, and increased concealment. The decline in the amount of song during the few minutes following playback attributable to the acute exposure was 34.6%. We analyze these results in light of theories about how animals adjust risk taking in response to predation danger. Given that the numbers of their predators have risen steadily for the past few decades and affect the level of singing, we consider the implications for trend estimates of songbird populations based on surveys using auditory methods.

Experimental addition of cover lowers the perception of danger and increases reproduction in meadow voles (Microtus pennsylvanicus)

Predation danger is pervasive for small mammals and is expected to select strongly for behavioural tactics that reduce the risk. In particular, since it may be considered a cost of reproduction, predation danger is expected to affect the level of reproductive effort. We test this hypothesis in a population of meadow voles (Microtus pennsylvanicus (Ord, 1815)) under seminatural conditions in field enclosures. We manipulated the voles’ perception of predation danger by adjusting the available cover and measured giving up density (GUD) in food patches to verify that the perception of danger differed between high- and low-cover treatments. Treatments did not differ in actual predation rate, in vole density, or in the quantity or quality of food. During the experiments, we measured indices of vole reproductive effort including activity (electronic detectors), foraging intensity (fecal plates), and the number of young produced (livetrapping). Voles in the high-cover (lower danger) treatments were more active, foraged more, and produced 85% more young per female per trap period than voles in the low-cover (higher danger) treatment. We briefly discuss the population consequences of this adaptive behavioural flexibility.

Does predation danger on southward migration curtail parental investment by female western sandpipers?

Theory predicts that if extending parental care delays migratory departure, and if later migration is more dangerous, then parental care should be curtailed to make an earlier departure. Adult western sandpipers (