Liana Zanette

FOOD SHORTAGE IN SMALL FRAGMENTS: EVIDENCE FROM AN AREA‐SENSITIVE PASSERINE

Many forest-interior songbirds are considered “area sensitive” because they are absent from smaller forest remnants in fragmented landscapes. Reductions in food abundance with fragment size could explain area sensitivity, but to date, only one empirical study has investigated this possibility. From 1995 to 1997, we tested the food abundance hypothesis in two small (∼55-ha) and two large (>400-ha) forest fragments located in a matrix of agricultural land in northeastern New South Wales, Australia. We measured differences in food abundance by comparing the biomass of surface-dwelling invertebrates in large and small fragments. We also determined whether food supply was associated with foraging efficiency and reproductive performance in an area-sensitive, ground-foraging insectivore, the Eastern Yellow Robin (Eopsaltria australis), breeding at these same sites. Invertebrate biomass in the small fragments was about half of that in the large fragments. Incubating female robins received 40% less food from males...

Perceived Predation Risk Reduces the Number of Offspring Songbirds Produce per Year

Fear itself reduces reproductive success in song sparrows. Predator effects on prey demography have traditionally been ascribed solely to direct killing in studies of population ecology and wildlife management. Predators also affect the prey’s perception of predation risk, but this has not been thought to meaningfully affect prey demography. We isolated the effects of perceived predation risk in a free-living population of song sparrows by actively eliminating direct predation and used playbacks of predator calls and sounds to manipulate perceived risk. We found that the perception of predation risk alone reduced the number of offspring produced per year by 40%. Our results suggest that the perception of predation risk is itself powerful enough to affect wildlife population dynamics, and should thus be given greater consideration in vertebrate conservation and management.

Balancing food and predator pressure induces chronic stress in songbirds

The never–ending tension between finding food and avoiding predators may be the most universal natural stressor wild animals experience. The ‘chronic stress’ hypothesis predicts: (i) an animals stress profile will be a simultaneous function of food and predator pressures given the aforesaid tension; and (ii) these inseparable effects on physiology will produce inseparable effects on demography because of the resulting adverse health effects. This hypothesis was originally proposed to explain synergistic (inseparable) food and predator effects on demography in snowshoe hares (Lepus americanus). We conducted a 2 × 2, manipulative food addition plus natural predator reduction experiment on song sparrows (Melospiza melodia) that was, to our knowledge, the first to demonstrate comparable synergistic effects in a bird: added food and lower predator pressure in combination produced an increase in annual reproductive success almost double that expected from an additive model. Here we report the predicted simultaneous food and predator effects on measures of chronic stress in the context of the same experiment: birds at unfed, high predator pressure (HPP) sites had the highest stress levels; those at either unfed or HPP sites showed intermediate levels; and fed birds at low predator pressure sites had the lowest stress levels.

Predator-induced stress and the ecology of fear

Summary Predator-induced stress has been used to exemplify the concept of stress for close to a century because almost everyone can imagine the terror of fleeing for ones life from a lion or a tiger. Yet, because it has been assumed to be acute and transitory, predator-induced stress has not been much studied by either comparative physiologists or population ecologists, until relatively recently. The focus in biomedical research has always been on chronic stress in humans, which most comparative physiologists would agree results from ‘sustained psychological stress – linked to mere thoughts’ rather than ‘acute physical crises’ (like surviving a predator attack) or ‘chronic physical challenges’ (such as a shortage of food). Population ecologists have traditionally focused solely on the acute physical crisis of surviving a direct predator attack rather than whether the risk of such an attack may have a sustained effect on other demographic processes (e.g. the birth rate). Demographic experiments have now demonstrated that exposure to predators or predator cues can have sustained effects that extend to affecting birth and survival in free-living animals, and a subset of these have documented associated physiological stress effects. These and similar results have prompted some authors to speak of an ‘ecology of fear’, but others object that ‘the cognitive and emotional aspects of avoiding predation remain unknown’. Recent biomedical studies on animals in the laboratory have demonstrated that exposure to predators or predator cues can induce ‘sustained psychological stress’ that is directly comparable to chronic stress in humans, and this has now in fact become one of the most common stressors used in studies of the animal model of post-traumatic stress disorder (PTSD). We review these recent findings and suggest ways the laboratory techniques developed to measure the ‘neural circuitry of fear’ could be adapted for use on free-living animals in the field, in order to: (i) test whether predator risk induces ‘sustained psychological stress’ in wild animals, comparable to chronic stress in humans and (ii) directly investigate ‘the cognitive and emotional aspects of avoiding predation’ and hence the ‘ecology of fear’.

Do stable isotopes reflect nutritional stress? Results from a laboratory experiment on song sparrows

Stable isotope analysis is an increasingly valuable tool in ecological studies and shows promise as a measure of nutritional stress in wild animals. Thus far, however, the only studies on endotherms that have conclusively shown changes in δ15N and δ13C values in response to nutritional stress were conducted on fasting animals and animals growing under extreme levels of food restriction. We conducted a laboratory experiment to test whether δ15N and δ13C values provide a general index of nutritional stress. We compared the isotopic composition of whole blood, liver, muscle and feathers between two groups of juvenile song sparrows (Melospiza melodia) hand-reared in captivity under identical conditions except for feeding regime. To verify that our experimental treatment induced a biologically meaningful level of nutritional stress, we simultaneously measured the effects on physiology, growth and development at multiple scales. While food-restricted birds were physiologically stressed, physically smaller, and showed poorer growth and brain development compared to ad libitum-fed birds, there was no effect of feeding regime on either δ15N or δ13C values in any tissue. Instead of a continuum where the level of change in 15N or 13C contents corresponds to the level of nutritional stress, we suggest there may be a threshold level of nutritional stress below which such isotopic changes are likely to be negligible.

Synergistic effects of food and predators on annual reproductive success in song sparrows.

The behaviour literature is full of studies showing that animals in every taxon balance the probability of acquiring food with the risk of being preyed upon. While interactions between food and predators clearly operate at an individual scale, population–scale studies have tended to focus on only one factor at a time. Consequently, interactive (or ‘synergistic’) effects of food and predators on whole populations have only twice before been experimentally demonstrated in mammals. We conducted a 2 × 2 experiment to examine the joint effects of food supply and predator pressure on the annual reproductive success of song sparrows (Melospiza melodia). Our results show that these two factors do not operate in an additive way, but instead have a synergistic effect on reproduction. Relative to controls, sparrows reared 1.1 more young when food was added and 1.3 more when predator pressure was low. When these treatments were combined 4.0 extra young were produced, almost twice as many as expected from an additive model. These results are a cause for optimism for avian conservation because they demonstrate that remedial actions, aimed at simultaneously augmenting food and reducing predators, can produce dramatic increases in reproductive success.

Early nutritional stress impairs development of a song-control brain region in both male and female juvenile song sparrows (Melospiza melodia) at the onset of song learning

Birdsong is a sexually selected trait and is often viewed as an indicator of male quality. The developmental stress hypothesis proposes a model by which song could be an indicator; the time during early development, when birds learn complex songs and/or local variants of song, is of rapid development and nutritional stress. Birds that cope best with this stress may better learn to produce the most effective songs. The developmental stress hypothesis predicts that early food restriction should impair development of song-control brain regions at the onset of song learning. We examined the effect of food restriction on song-control brain regions in fledgling (both sexes, 23–26 days old) song sparrows (Melospiza melodia). Food restriction selectively reduced HVC volume in both sexes. In addition, sex differences were evident in all three song-control regions. This study lends further support to a growing body of literature documenting a variety of behavioural, physiological and neural detriments in several songbird species resulting from early developmental stress.

What do artificial nests tells us about nest predation

Artificial nests are commonly used to evaluate predation, but the assumption that this method mimics predation on natural nests has seldom been tested. Natural and artificial nests of eastern yellow robins (Eopsaltria australis) were monitored in four, 55-ha plots over two breeding seasons. Overall, daily survival rates were higher (P<0.001) for natural (95%/day) than for artificial nests (88%/day). Among plots, daily survival rates for the two types of nests were not correlated with one another (P=0.72) indicating that the spatial pattern of predation on artificial nests did not mimic that for natural nests. Seasonal variation was evident for natural nests in one year, when they were more successful at the beginning and end of the breeding season. No seasonal patterns were observed for artificial nests in either year. Neither natural nor artificial nests showed annual variation in predation. Previous researchers concluded that large birds were important predators on robin nests. In this study, predation by large birds on artificial nests was positively correlated with the numbers of large birds counted on the plots (P=0.04). However, large birds depredated only 16% of artificial nests. Daily survival rates for artificial nests were recalculated using predation by large birds only. These rates were compared with natural nests, but there was still no correspondence in the spatial and temporal patterns of predation for the two types of nests. These results suggest that inferences about predation on natural nests based on artificial nest studies should be avoided.

Nesting success and nest predators in forest fragments: a study using real and artificial nests.

Abstract Area sensitivity in songbirds is commonly attributed to increased nest predation in forest fragments. In 1995 and 1996, we tested whether the nest predators and nesting success of an area-sensitive forest bird, the Eastern Yellow Robin (Eopsaltria australis), varied with fragment size, and we also conducted an artificial nest experiment. The study occurred in two small (55 ha) and two large (>400 ha) forest fragments in a matrix of agricultural land in New South Wales, Australia. Predation accounted for 95% of all failures of 282 robin nests, and the survival of robin nests was negatively correlated with how frequently we observed avian nest predators near nests (i.e. nest-predator activity). Of 461 artificial nests, 84% were depredated, nearly all (99%) by birds. Thus, birds were important predators of nests. The abundance, species richness, and activity of avian nest predators were not related to fragment size. Survival of robin nests averaged 19%; nests in small fragments had a 22% chance of producing at least one fledgling compared with 15% in large fragments, but the difference was not significant. Survival of artificial nests averaged 12% in both small and large fragments. Nest-predator activity accounted for the most variation (68%) in the fate of robin nests, followed by the cumulative density of open-cup nesters (16%). The placement of robin nests had no influence on nest fate. We conclude that nest predation was not area dependent and propose food supply as an alternative hypothesis to explain area sensitivity. We suggest that, rather than being related to fragment size, nest predation increases with decreasing forest cover in a landscape. Increased nest predation in fragmented compared with contiguous landscapes may lower the population viability of songbirds in a region, and hence regional numbers. Therefore, the spatial scale at which fragmentation influences nest predation and songbird populations must be considered carefully.

Song repertoire size varies with HVC volume and is indicative of male quality in song sparrows (Melospiza melodia)

Complex birdsong is a classic example of a sexually selected ornamental trait. In many species, females prefer males with large song repertoires, possibly because repertoire size is limited by the size of song control nuclei which reflect developmental success. We investigated whether song repertoire size was indicative of brain area and male quality in song sparrows (Melospiza melodia) by determining if repertoire size was related to the volume of song control nucleus HVC, as well as several morphological, immunological and genetic indices of quality. We found that males with large repertoires had larger HVCs and were in better body condition. They also had lower heterophil to lymphocyte ratios, indicating less physiological stress and a robust immune system as measured by the number of lymphocytes per red blood cell. Song repertoire size also tended to increase with neutral-locus genetic diversity, as assessed by mean d2, but was not related to internal relatedness. Our results suggest several mechanisms that might explain the finding of a recent study that song sparrows with large song repertoires have higher lifetime fitness.