Forest fragmentation and avian nest predation in forested landscapes

Summary The size of forest fragments, the use of land bordering fragments, and the distance of nests from an edge all affect the frequency of predation upon bird nests in Maine (USA), an area where the forest has been fragmented by roads, but not significantly reduced in area. We placed artificial nests containing quail eggs in forests of different sizes and at various distances from the edge to test which of these factors was most important in describing predation. Predation was greatest in small tracts surrounded completely by land. Large areas and those bordered on at least one side by a large water body had lower predation rates. This suggests that influx of predators from nearby habitats may be responsible for much of the nest predation in forest fragments.     

Animal culture research should include avian nest construction

Material culture—that is, group-shared and socially learned object-related behaviour(s)—is a widespread and diverse phenomenon in humans. For decades, researchers have sought to confirm the existence of material culture in non-human animals; however, the main study systems of interest—namely, tool making and/or using non-human primates and corvids—cannot provide such confirmatory evidence: because long-standing ethical and logistical constraints handicap the collection of necessary experimental data. Synthesizing evidence across decades and disciplines, here, I present a novel framework for (mechanistic, developmental, behavioural, and comparative) study on animal material culture: avian nest construction.     

Parent birds assess nest predation risk: influence of cavity condition and avian nest predator activity

Skutch hypothesized that nest predators visually assess parental activities to locate a prey nest, whereas parents modify fitness‐related traits to reduce the probability of nest predation. We examined how cavity condition and parental activity interact with avian nest predators to shape the nest success of two coexisting parid species, marsh tits Poecile palustris and oriental tits Parus minor, breeding in nest‐boxes during the incubation period. Nest‐boxes were manipulated to create a prolonged risk of nest predation (entrance diameter 2.6 cm control vs 5.5 cm treatment) soon after clutch completion. To measure changes in parental behavior, we also simultaneously simulated a pulsed risk of nest predation, using sound playbacks of a coexisting control bird and an avian nest predator. We found that the parent tits merely responded the pulsed risk, presumably due to an environment with high avian nest predator encounters, compared to the prolonged risk. Instead, both species spent more time on vigilance at the nest, only under prolonged risk conditions. The activity of corvids near the nest‐box was higher in the marsh tit than that in oriental tits. This activity was also higher in the treatment nest box than that in the control nest‐box. Nest predation during the incubation period was higher in marsh tits than in oriental tits, presumably due to higher and more plastic vigilance in oriental tits, compared to marsh tits. Our results highlight that the differences in cavity condition and parental activities at the nests of two coexisting non‐excavators may contribute to differential nest predation by attracting avian nest predators.     

Microhabitat structure and avian nest predation risk in an open Argentinean woodland: an experimental study

We studied the effect of the general structure of the nest plant, especially the presence of thorns, and the structural homogeneity of the nest patch, on the vulnerability of nests to predation, using natural and artificial nests. Artificial nests placed in non-thorny plants had a significantly lower predation rate and higher daily survival rate than those in thorny plants. The addition of a ‘thorny microhabitat’ around the immediate proximity of nests placed in non-thorny plants did not have any effect on vulnerability of nests to predation. Conversely, natural nests were located in patches of habitat with a higher density of the species of plant that supported the nest compared to patches selected at random. However, daily survival rate was similar for natural nests placed in patches with a higher or lower density of the species of nest plant in the four bird species analysed. Similarly, survival of artificial nests did not increase with the presence of a higher number of plants similar to the nest plant in the nest patch. Thus, the observed patterns of survival for natural and artificial nests did not seem to support the potential prey-site hypotheses. Birds appeared to be the main nest predators in this ecosystem. Behavioural aspects of the identified predators and habitat structure could explain the lack of effect of thorns and nest patch characteristics on nest survival.     

Understanding and managing enhancements: why fisheries scientists should care

Fisheries enhancements are a set of management approaches involving the use of aquaculture technologies to enhance or restore fisheries in natural ecosystems. Enhancements are widely used in inland and coastal fisheries, but have received limited attention from fisheries scientists. This paper sets out 10 reasons why fisheries scientists should care about understanding and managing enhancements. (1) Enhancements happen, driven mostly by resource users and managers rather than scientists. (2) Enhancements create complex fisheries systems that encompass and integrate everything fisheries stakeholders can practically manage. (3) Enhancements emerge in fisheries where the scope for technical and governance control is high, and they synergistically reinforce both. (4) Successful enhancements expand management options and achievable outcomes. (5) Many enhancements fail or do ecological harm but persist regardless. (6) Effective science engagement is crucial to developing beneficial enhancements and preventing harmful ones. (7) Good scientific guidance is available to aid development or reform of enhancements but is not widely applied. (8) Enhancement research advances, integrates and unifies the fisheries sciences. (9) Enhancements provide unique opportunities for learning about natural fish populations and fisheries. (10) Needs, opportunities and incentives for enhancements are bound to increase.     

How avian nest site selection responds to predation risk: testing an 'adaptive peak hypothesis'

1. Nest predation limits avian fitness, so birds should favour nest sites that minimize predation risk. Nevertheless, preferred nest microhabitat features are often uncorrelated with apparent variation in predation rates. 2. This lack of congruence between theory-based expectation and empirical data may arise when birds already occupy 'adaptive peaks'. If birds nest exclusively in low-predation microhabitats, microhabitat and nest predation may no longer be correlated even though predation ultimately shaped microhabitat selection. 3. This 'adaptive peak hypothesis' was tested for a population of Yellow Warblers (Dendroica petechia) focusing on two nest microhabitat features: concealment and height. Experimental nests measured relative predation risk both within and outside the microhabitat range typically occupied by natural nests to examine whether nest site choices made by birds restricted our ability to detect microhabitat effects on predation. 4. Within the natural range (30-80% concealment, >75 cm height), microhabitat-predation relationships were weak and inconsistent, and similar for experimental and natural nests. Over an extended range, however, experimental predation rates were elevated in exposed sites (<30% concealed), indicating a concealment-related 'adaptive plateau'. 5. Clay egg bite data revealed a concealment effect on avian predators, and the abundance of one avian predator group correlated with nest concealment among years, suggesting these predators may cue birds to modulate nest concealment choices. 6. This study demonstrates how avian responses to predation pressure can obscure the adaptive significance of nest site selection, so predation influences may be more important than apparent from published data.     

Power lines,roads, and avian nest survival: effects on predator identity and predation intensity

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Density-dependent nest predation in waterfowl: the relative importance of nest density versus nest dispersion

When nest predation levels are very high or very low, the absolute range of observable nest success is constrained (a floor/ceiling effect), and it may be more difficult to detect density-dependent nest predation. Density-dependent nest predation may be more detectable in years with moderate predation rates, simply because there can be a greater absolute difference in nest success between sites. To test this, we replicated a predation experiment 10 years after the original study, using both natural and artificial nests, comparing a year when overall rates of nest predation were high (2000) to a year with moderate nest predation (2010). We found no evidence for density-dependent predation on artificial nests in either year, indicating that nest predation is not density-dependent at the spatial scale of our experimental replicates (1-ha patches). Using nearest-neighbor distances as a measure of nest dispersion, we also found little evidence for "dispersion-dependent" predation on artificial nests. However, when we tested for dispersion-dependent predation using natural nests, we found that nest survival increased with shorter nearest-neighbor distances, and that neighboring nests were more likely to share the same nest fate than non-adjacent nests. Thus, at small spatial scales, density-dependence appears to operate in the opposite direction as predicted: closer nearest neighbors are more likely to be successful. We suggest that local nest dispersion, rather than larger-scale measures of nest density per se, may play a more important role in density-dependent nest predation.     

Avoid nest predation when predation rates are low,and other lessons: testing the tropical–temperate nest predation paradigm

Nest predation is the most important cause of nest failure in most birds and latitudinal differences in nest predation rates and life histories suggest that nest predation has been influential in life history evolution. All else equal, natural selection should favor reduction of nest predation, yet evidence is equivocal. We used Monte Carlo simulations to examine the combined effects of variation in nest predation rates, breeding season length and renesting intervals on the annual number of young fledged. Simulations suggest that selection most strongly favors a reduction in nest predation when breeding seasons are short and predation rates are low (temperate characteristics). Conversely, selection favors shorter renesting intervals when breeding seasons are long and nest predation rates are high (tropical characteristics). Reducing already low rates provides a proportionately greater increase in annual nesting success than does the same reduction when nest predation rates are higher. In some tropical species, individuals increase reproductive success not by avoiding predation in subsequent nesting attempts, which is largely beyond their control, but rather by reducing renesting intervals. We suggest that the emphasis on nest predation avoidance has biased our perspectives for alternative hypotheses of how birds should respond to nest predation and the consequences of those alternatives for life history theory. Similarly to the need to control for phylogenetics in examining life history strategies, future studies must also control for differences in breeding season lengths and renesting intervals to better understand the influence of nest predation on avian life histories.     

Potential prey make excellent ornithologists: adaptive, flexible responses towards avian predation threat by Arabian Babblers Turdoides squamiceps living at a migratory hotspot

During spring raptor migration, resident, group-living, Arabian Babblers Turdoides squamiceps in Israel are exposed to many avian predators that differ in the level of predation threat that they pose. Accurate information about immediate safety while foraging may be provided by the sentinel, a high-perched non-foraging Babbler that actively scans and warns for predators. We tested whether, at this migratory hotspot, the bewildering number and diversity of potential avian predators (perhaps more than anywhere else in the world) may be more than a reliable sentinel could handle. We monitored ten Babbler groups regularly for a number of hours per day throughout the migration season, and scored whether the Babblers gave alarm calls when different bird species were in sight. Analyses showed that Babblers are very skilled in distinguishing between species of high and low predation threat (interspecific threat-sensitivity). In addition, Babblers distinguished accurately between encounters with dangerous vs. harmless individuals from predator species of potentially high predation threat (intraspecific threat-sensitivity). Overall, Babblers cautiously overestimated predation threat, probably because errors involving alarms for harmless predators have a lower fitness cost than errors involving attacks by dangerous predators that are ignored. However, fewer warnings were given for the more common predators of lower threat, which suggests that Babblers initially overestimated the risk of predation, but subsequently learned to assess the level of threat more accurately as soon as more information was available. Overall, our findings agree closely with theoretical predictions regarding adaptive and sophisticated anti-predation behaviour in a variable world.     

千岛湖栖息地片段化效应对鸟类巢捕食风险的影响

为了解栖息地片段化对鸟类巢捕食风险的影响,我们于2010年4-8月,在千岛湖地区选取16个岛屿,分别在岛屿边缘和内部区域用放置鸡蛋(大型卵)和鹌鹑(Coturnix japonica)蛋(小型卵)的方法进行人工模拟地面巢试验,研究不同体积大小鸟卵的巢捕食率及其差异,分析巢捕食率与岛屿面积、隔离度、形状指数和植物物种丰富...     

Ability to assess nest predation risk in secondary hole-nesting birds: an experimental study

Because nest predation is the major source of nesting mortality in birds, site-specific predation risk may play an important role in determining birds' ability to select nest sites that reduce predation risk. This possibility has not been adequately tested. Here we report on 5-year experiments by which we studied, independently from birds' earlier experience with specific nest boxes, both the selection and predation risk of nest sites in the common goldeneye (Bucephala clangula). New, previously unoccupied nest boxes were erected in two habitat types on three study areas. Experimentally measured predation risk in the nest boxes varied between 0 and 1.0, i.e. goldeneye females could select a nest site along a wide gradient of possible predation-risk values. We did not find a difference in predation risk between occupied and unoccupied nest boxes, nor was the order of nest box occupation associated with predation risk. A power analysis revealed that our test had reasonably high power to reject a false null hypothesis. Our results suggest that common goldeneye females likely have not evolved an ability to assess predation risk of new, previously unoccupied nest sites.     

Informed renesting decisions: the effect of nest predation risk

Animals should cue on information that predicts reproductive success. After failure of an initial reproductive attempt, decisions on whether or not to initiate a second reproductive attempt may be affected by individual experience and social information. If the prospects of breeding success are poor, long-lived animals in particular should not invest in current reproductive success (CRS) in case it generates costs to future reproductive success (FRS). In birds, predation risk experienced during breeding may provide a cue for renesting success. Species having a high FRS potential should be flexible and take predation risk into account in their renesting decisions. We tested this prediction using breeding data of a long-lived wader, the southern dunlin Calidris alpina schinzii. As predicted, dunlin cued on predation risk information acquired from direct experience of nest failure due to predation and ambient nest predation risk. While the overall renesting rate was low (34.5 %), the early season renesting rate was high but declined with season, indicating probable temporal changes in the costs and benefits of renesting. We develop a conceptual cost-benefit model to describe the effects of the phase and the length of breeding season on predation risk responses in renesting. We suggest that species investing in FRS should not continue breeding in short breeding seasons in response to predation risk but without time constraints, their response should be similar to species investing in CRS, e.g. within-season dispersal and increased nest concealment.     

A predator's perspective of nest predation: predation by red squirrels is learned,not incidental

Nest predation has been used to explain aspects of avian ecology ranging from nest site selection to population declines. Many arguments rely on specific assumptions regarding how predators find nests, yet these predatory mechanisms remain largely untested. Here we combine artificial nest experiments with behavioural observations of individual red squirrels Tamiasciurus hudsonicus to differentiate between two common hypotheses: predation is incidental versus learned. Specifically, we tested: 1) whether nest survival could be explained solely by a squirrel's activity patterns or habitat use, as predicted if predation was incidental; or 2) if predation increased as a squirrel gained experience preying on a nest, as predicted if predation was learned. We also monitored squirrel activity after predation to test for evidence of two search mechanisms: area‐restricted searching and use of microhabitat search images. Contrary to incidental predation and in support of learning, squirrels did not find nests faster in areas with high use (e.g. forest edges). Instead, survival of artificial nests was strongly related to a squirrel's prior experience preying on artificial nests. Experience reduced nest survival times by over half and increased predation rates by 150–200%. Squirrels returned to and doubled their activity at the site of a previously preyed on nest. However, neither area‐restricted searching nor microhabitat search images can explain how squirrels located artificial nests more readily with experience. Instead, squirrels likely used cues associated with the nests or eggs themselves. Learning implies that squirrels could be increasingly effective predators as the density or profitability of nests increases. Our results add support to the view that nest predation is complex and broadly influenced (e.g. by predator experience, motivation), and is unlikely to be predicted consistently by simple relationships with predator activity, abundance or habitat.     

Spatial and temporal dynamics of turtle nest predation: edge effects

We determined the effect of distance from ecological edges, both wooded and water edges, on nest predation for 862 painted turtle, Chrysemys picta , nests from 1995 to 1999 at an ∼1.5 ha study site. In three of five years and overall, nests closer to the water edge had a higher probability of predation; and in one year nests closer to the wooded edge had a higher probability of predation. Although more turtles nested closer to the water edge as the nesting season progressed in some years and overall, this behavior does not explain the observed patterns of nest predation. We present a novel application of the cubic spline analysis to address the dynamics of predation across continuous distances from an edge and identify threshold values where the predation rate levels off. Threshold values of ∼25–40 m were detected in 1995, 1998, and with all five years combined. However, even though a significant edge effect was detected in 1997, a threshold was not clear. While an edge effect on predation was not detected in each year, this study provides evidence for a strong effect of distance from the water edge on nest predation over significant ecological time. Focusing on turtle nest predation and smaller spatial scales addresses previous taxonomic and spatial bias in edge effects research, and provides further support for the ecological importance of edge effects.     

Testing ecological and behavioral correlates of nest predation

Variation in nest predation rates among bird species are assumed to reflect differences in risk that are specific to particular nest sites. Theoretical and empirical studies suggest that parental care behaviors can evolve in response to nest predation risk and thereby differ among ecological conditions that vary in inherent risk. However, parental care also can influence predation risk. Separating the effects of nest predation risk inherent to a nest site from the risk imposed by parental strategies is needed to understand the evolution of parental care. Here we identify correlations between risks inherent to nest sites, and risk associated with parental care behaviors, and use an artificial nest experiment to assess site-specific differences in nest predation risk across nesting guilds and between habitats that differed in nest predator abundance. We found a strong correlation between parental care behaviors and inherent differences in nest predation risk, but despite the absence of parental care at artificial nests, patterns of nest predation risk were similar for real and artificial nests both across nesting guilds and between predator treatments. Thus, we show for the first time that inherent risk of nest predation varies with nesting guild and predator abundance independent of parental care.     

Clutch size variation in passerine birds: The nest predation hypothesis

Summary The hypothesis that a negative relationship exists between clutch size and the probability that the nest will be robbed is tested, using data for passerine birds given in the literature. The data for four separate groups of species, viz. hole-nesters, semi hole-nesters and open-nesters nesting above and on the ground, respectively, were examined in relation to geographical gradients and seasonal and annual variation. In general, the data analysis results support the hypothesis, but cannot yet be considered as proven. More data on the riks of nest predation are needed. The most serious discrepancy is that for the Fieldfare, the protection from predation provided by nesting in colonies does not seem to be accompanied by a corresponding increase in clutch size. However, the clutch size of the Brambling, a species with seeks out such colonies for its breeding sites, does tend to increase in these hatitats.The pattern of clutch size variation was similar for the two groups of hole-nesting species, but differed significantly from that found for the two groups of open-nesters. The difference in the clutch size variation of the two groups of open-nesting species predicted from the hypothesis, viz. that, in northern regions, both latitudinal and altitudinal increases in clutch size should more commonly be found for those species which nest on the ground, compared to those nesting above ground level, was confirmed.A seasonal decrease in clutch size, in temperate regions, was found to be typical for species whose nests are subject to relatively little predation, particularly for those species which have a short breeding season. Advantages of laying small clutches, resulting in fewer nestlings than the number which would be possible for the parents to rear successfully, are discussed, and a simple model is presented which does not assume that nest predation is dependent on clutch size.