Provision of supplementary food for wild birds may increase the risk of local nest predation

In countries such as the UK, USA and Australia, approximately half of all households provide supplementary food for wild birds, making this the public's most common form of active engagement with nature. Year‐round supplementary feeding is currently encouraged by major conservation charities in the UK as it is thought to be of benefit to bird conservation. However, little is understood about how the provision of supplementary food affects the behaviour and ecology of target and non‐target species. Given the scale of supplementary feeding, any negative effects may have important implications for conservation. Potential nest predators are abundant in urban areas and some species frequently visit supplementary feeding stations. We assess whether providing supplementary food affects the likelihood of nest predation in the vicinity of the feeder, by acting as a point attractant for potential nest predators. We provided feeding stations (empty, peanut feeder, peanut feeder with guard to exclude potential nest predators) in an area of suburban parkland in the UK and monitored the predation rate of eggs placed in artificial nests located at distances that replicated the size of typical suburban gardens. Nest predators (Magpies Pica pica, Grey Squirrels Sciurus carolinensis) were frequent visitors to filled feeders, and predation caused by Magpies, European Jays Garrulus glandarius and Grey Squirrels was significantly higher when nests were adjacent to filled feeders. The presence of a feeder guard did not significantly reduce nest predation. As supplementary feeding is becoming increasingly common during the breeding season in suburban habitats, we suggest that providing point attractants to nest predators at this time may have previously unconsidered consequences for the breeding success of urban birds.     

Contrast in edge vegetation structure modifies the predation risk of natural ground nests in an agricultural landscape

Nest predation risk generally increases nearer forest-field edges in agricultural landscapes. However, few studies test whether differences in edge contrast (i.e. hard versus soft edges based on vegetation structure and height) affect edge-related predation patterns and if such patterns are related to changes in nest conspicuousness between incubation and nestling feeding. Using data on 923 nesting attempts we analyse factors influencing nest predation risk at different edge types in an agricultural landscape of a ground-cavity breeding bird species, the Northern Wheatear (Oenanthe oenanthe). As for many other bird species, nest predation is a major determinant of reproductive success in this migratory passerine. Nest predation risk was higher closer to woodland and crop field edges, but only when these were hard edges in terms of ground vegetation structure (clear contrast between tall vs short ground vegetation). No such edge effect was observed at soft edges where adjacent habitats had tall ground vegetation (crop, ungrazed grassland). This edge effect on nest predation risk was evident during the incubation stage but not the nestling feeding stage. Since wheatear nests are depredated by ground-living animals our results demonstrate: (i) that edge effects depend on edge contrast, (ii) that edge-related nest predation patterns vary across the breeding period probably resulting from changes in parental activity at the nest between the incubation and nestling feeding stage. Edge effects should be put in the context of the nest predator community as illustrated by the elevated nest predation risk at hard but not soft habitat edges when an edge is defined in terms of ground vegetation. These results thus can potentially explain previously observed variations in edge-related nest predation risk.     

Do trade‐offs between predation pressures on females versus nests drive nest‐site choice in painted turtles?

Predation strongly influences reproductive behaviours because reproducing individuals must balance mortality risks to themselves and to their offspring. In many freshwater turtles, the nest predation risk decreases with nest distance from water, whereas the predation risk to females increases farther from water. To determine whether predation pressure influences the distance from water at which female turtles nest, we measured predation pressure on nesting females and on nests, as well as the distances of nests to water, in two populations of painted turtles. Using models, we found that female survival in both populations was high and did not vary with distance from water. Nest survival was also uncorrelated with nest distance to water, although it was significantly lower than adult survival in both populations and was only 1.2% in one population. Our results suggest that nest sites are not predictably safe from predators. Instead, turtles may hedge their bets by nesting over a wide range of distances from water because any distance is risky for nests and no distance is particularly risky for the nesting female. We suggest that other factors, such as suitable incubation conditions and/or post‐emergence hatchling survival, probably play a larger role than predation in driving nest‐site choice in painted turtles.     

Relative Roles of Grey Squirrels,Supplementary Feeding,and Habitat in Shaping Urban Bird Assemblages

Non-native species are frequently considered to influence urban assemblages. The grey squirrel Sciurus carolinensis is one such species that is widespread in the UK and is starting to spread across Europe; it predates birds’ nests and can compete with birds for supplementary food. Using distance sampling across the urbanisation intensity gradient in Sheffield (UK) we test whether urban grey squirrels influence avian species richness and density through nest predation and competition for supplementary food sources. We also assess how urban bird assemblages respond to supplementary feeding. We find that grey squirrels slightly reduced the abundance of breeding bird species most sensitive to squirrel nest predation by reducing the beneficial impact of woodland cover. There was no evidence that grey squirrel presence altered relationships between supplementary feeding and avian assemblage structure. This may be because, somewhat surprisingly, supplementary feeding was not associated with the richness or density of wintering bird assemblages. These associations were positive during the summer, supporting advocacy to feed birds during the breeding season and not just winter, but explanatory capacity was limited. The amount of green space and its quality, assessed as canopy cover, had a stronger influence on avian species richness and population size than the presence of grey squirrels and supplementary feeding stations. Urban bird populations are thus more likely to benefit from investment in improving the availability of high quality habitats than controlling squirrel populations or increased investment in supplementary feeding.     

Impact of Supplementary Feeding on Reproductive Success of White Storks

European white stork (Ciconia ciconia) populations have been object to several conservation measures such as reintroduction programs, habitat improvement or supplementary feeding in the last decades. Although recent white stork censuses revealed an upward trend of most of the western populations, evaluations of the relative importance of certain conservation measures are still scarce or even lacking. In our study we analyzed the effect of supplementary feeding on the reproductive success of white storks in conjunction with other factors such as weather or nest site characteristics. We present data of 569 breeding events at 80 different nest sites located in variable distances to an artificial feeding site at Affenberg Salem (south-western Germany) collected from 1990–2012. A multilevel Poisson regression revealed that in our study population (1) reproductive success was negatively affected by monthly precipitation in April, May and June, (2) pairs breeding on power poles had a lower reproductive success than pairs breeding on platforms or trees and (3) reproductive success was significantly higher in pairs breeding in close distance to the feeding site. The number of fledglings per nest decreased by 8% per kilometer distance to the feeding site. Our data suggest that supplementary feeding increases fledgling populations which may be a tool to attenuate population losses caused by factors such as habitat deterioration or unfavorable conditions in wintering habitats.     

Different nest predator faunas and nest predation risk on ground and shrub nests at forest ecotones: an experiment and a review

This study examined predator faunas of artificial ground and shrub nests and whether nest predation risk was influenced by nest site, proximity to forest edge, and habitat structure in 38 grassland plots in south-central Sweden. There was a clear separation of predator faunas between shrub and ground nests as identified from marks in plasticine eggs. Corvids accounted for almost all predation on shrub nests whereas mammals mainly depredated ground nests. Nest predation risk was significantly greater for shrub than for ground nests at all distances (i.e. 0, 15 and 30 m) from the forest edge. However, nest predation risk was not significantly related to distance to forest edge, but significantly increased with decreasing distance to the nearest tree. Different corvid species robbed nests at different distances from the forest edge, with jays robbing nests closest to edges. We conclude that the relationship between the predation risk of grassland bird nests and distance to the forest edge mainly depends on the relative importance of different nest predator species and on the structure of the forest edge zone. A review of published articles on artificial shrub and ground nest predation in the temperate zone corroborated the results of our own study, namely that shrub nests experienced higher rates of depredation in open habitats close to the forest edge and that avian predators predominantly robbed shrub nests. Furthermore, the review results showed that predation rates on nests in general are highest <50 m inside the forest and lower in open as well as forest interior habitats (≥50 m from the edge). Received: 16 March 1998 / Accepted: 30 July 1998     

Facultative nest patch shifts in response to nest predation risk in the Brewer’s sparrow: a “win-stay, lose-switch” strategy?

Facultative shifts in nesting habitat selection in response to perceived predation risk may allow animals to increase the survival probability of sessile offspring. Previous studies on this behavioral strategy have primarily focused on single attributes, such as the distance moved or changes in nesting substrate. However, nest site choice often encompasses multiple habitat elements at both the nest site and nest patch scales. We studied the within-season re-nesting strategy of a multi-brooded songbird, the Brewer’s sparrow (Spizella breweri), to determine whether pairs utilized a “win-stay, lose-switch” decision rule with respect to inter-nest distance, nest substrate and/or nest patch characteristics in response to previous nest fate. Pairs moved sequential nest sites slightly farther following nest predation versus success. When inter-nest distance was controlled, however, pairs changed nest patch attributes (shrub height, potential nest shrub density) associated with probability of nest predation to a greater extent following nest predation than success. The strategy appeared to be adaptive; daily nest survival probability for previously depredated pairs increased with greater Euclidian habitat distances between attempts, whereas previously successful pairs were more likely to fledge second attempts when nest sites were similar to those of previous attempts. Our results suggest that nesting birds can use prior information and within-season plasticity in response to nest predation to increase re-nesting success, which may be a critical behavioral strategy within complex nest predator environments. Re-nesting site selection strategies also appeared to integrate multiple habitat components and inter-nest distances. The consideration of such proximate, facultative responses to predation risk may clarify often unexplained variation in habitat preferences and requirements.     

Growth rate variation among passerine species in tropical and temperate sites: an antagonistic interaction between parental food provisioning and nest predation risk

Causes of interspecific variation in growth rates within and among geographic regions remain poorly understood. Passerine birds represent an intriguing case because differing theories yield the possibility of an antagonistic interaction between nest predation risk and food delivery rates on evolution of growth rates. We test this possibility among 64 Passerine species studied on three continents, including tropical and north and south temperate latitudes. Growth rates increased strongly with nestling predation rates within, but not between, sites. The importance of nest predation was further emphasized by revealing hidden allometric scaling effects. Nestling predation risk also was associated with reduced total feeding rates and per-nestling feeding rates within each site. Consequently, faster growth rates were associated with decreased per-nestling food delivery rates across species, both within and among regions. These relationships suggest that Passerines can evolve growth strategies in response to predation risk whereby food resources are not the primary limit on growth rate differences among species. In contrast, reaction norms of growth rate relative to brood size suggest that food may limit growth rates within species in temperate, but not tropical, regions. Results here provide new insight into evolution of growth strategies relative to predation risk and food within and among species.     

Adaptive plasticity in nest-site selection in response to changing predation risk

Birds selecting a nest site have to find the best compromise between the risk of encountering predation, the availability of food near to the nest and microclimatic requirements. As the optimal solution of this problem will vary with changes in predator abundance, we ask whether birds are capable of assessing such changes and of adjusting their nest-site choice accordingly. The reproductive success of dusky warblers Phylloscopus fuscatus , breeding in a mosaic of bushland and tundra habitat in the Russian Far East, varied greatly depending on the abundance of a nest predator, the Siberian chipmunk Tamias sibiricus . Using artificial nests we analysed which strategies dusky warblers should follow to avoid nest predation by chipmunks. We then compared the nest sites which dusky warblers actually had chosen in years with very high chipmunk densities (1998 and 1999) with those chosen in 1997, when chipmunks were almost absent from the study area. We found that safe nest-sites were preferred over those offering other advantages (microclimate, proximity to food) when the risk of predation was high, and we could not detect any confounding factor that might alternatively have caused these striking between-year differences. Our study suggests that even a short-lived passerine may be capable of choosing its nest site according to the actual predation risk. We show that such behavioural plasticity can lead to a paradoxical situation where better-protected nest-sites (selected in years and areas with high risk), on average, suffer greater predation than sites offering low safety. Thus, behavioural plasticity, if undetected, may result in serious misinterpretation of nest-predation patterns. A review of the literature suggests that adaptive plasticity in nest placement may be more widespread than is currently recognized.     

Predation risk and resource availability explain roost locations of Whimbrel Numenius phaeopus in a tropical mangrove delta

Roosts are important sites for shorebirds in non‐breeding areas at night and during high tides. How the spatial configuration of food and risk of predation and disturbance influence roost site use in tropical locations remains poorly known. We analysed the locations of nocturnal roosts of Whimbrel Numenius phaeopus in mangroves of Sanquianga National Park, Colombia, with respect to variation in spatial variables related to food resources and risk of predation and disturbance. We contrasted characteristics of all 13 known nocturnal roost locations with those of all other mangrove islands (n = 209) within the limits of the park. We estimated the distance from roosts and other mangrove islands to foraging sites, and sources of predators and human disturbance. Larger areas of feeding habitat surrounded nocturnal roosts than other mangrove islands, and the average distance to individual feeding patches was shorter. Roosts were also more isolated than other islands, but proximity to sources of human disturbance did not differ. We conclude that Whimbrel roost site use in Sanquianga was best explained by a combination of access to feeding territories and isolation from potential sources of mainland predators, but not by avoidance of human disturbance. Beyond identifying factors influencing roost site selection, the large aggregations of individuals in single locations may suggest that presence of conspecifics itself also plays a role in the formation of Whimbrel roosts. We highlight the interaction of food and risk landscapes with intraspecific attraction on the roost site selection by Whimbrels and the importance of mangroves as roosting sites in tropical regions.     

Nest predation increases with parental activity: separating nest site and parental activity effects.

Alexander Skutch hypothesized that increased parental activity can increase the risk of nest predation. We tested this hypothesis using ten open-nesting bird species in Arizona, USA. Parental activity was greater during the nestling than incubation stage because parents visited the nest frequently to feed their young during the nestling stage. However, nest predation did not generally increase with parental activity between nesting stages across the ten study species. Previous investigators have found similar results. We tested whether nest site effects might yield higher predation during incubation because the most obvious sites are depredated most rapidly. We conducted experiments using nest sites from the previous year to remove parental activity. Our results showed that nest sites have highly repeatable effects on nest predation risk; poor nest sites incurred rapid predation and caused predation rates to be greater during the incubation than nestling stage. This pattern also was exhibited in a bird species with similar (i.e. controlled) parental activity between nesting stages. Once nest site effects are taken into account, nest predation shows a strong proximate increase with parental activity during the nestling stage within and across species. Parental activity and nest sites exert antagonistic influences on current estimates of nest predation between nesting stages and both must be considered in order to understand current patterns of nest predation, which is an important source of natural selection.     

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.     

Predation risk of artificial ground nests in managed floodplain meadows

Nest predation highly determines the reproductive success in birds. In agricultural grasslands, vegetation characteristics and management practices influences the predation risk of ground breeders. Little is known so far on the predation pressure on non-passerine nests in tall swards. Investigations on the interaction of land use with nesting site conditions and the habitat selection of nest predators are crucial to develop effective conservation measures for grassland birds.In this study, we used artificial nests baited with quail and plasticine eggs to identify potential predators of ground nests in floodplain meadows and related predation risk to vegetation structure and grassland management.Mean daily predation rate was 0.01 (±0.012) after an exposure duration of 21 days. 70% of all observed nest predations were caused by mammals (Red Fox and mustelids) and 17.5% by avian predators (corvids). Nest sites close to the meadow edge and those providing low forb cover were faced with a higher daily predation risk. Predation risk also increased later in the season. Land use in the preceding year had a significant effect on predation risk, showing higher predation rates on unmanaged sites than on mown sites. Unused meadows probably attract mammalian predators, because they provide a high abundance of small rodents and a more favourable vegetation structure for foraging, increasing also the risk of incidental nest predations. Although mowing operation is a major threat to ground-nesting birds, our results suggest that an annual removal of vegetation may reduce predation risk in the subsequent year.     

The influence of the food size,distance and food site on food carrying behavior in rats (Rattus norvegicus)

The behavior to foods in rats (Rattus norvegicus) was investigated in a seminatural enclosure and in a straight alley in the laboratory. In both situations, the size of food (0.045-, 1-, and 3-g pellets were used in the enclosure and 0.045-, 0.2-, 1-, and 3-g pellets in the alley) and distance between food sites and nest site (1.8, 3.6, 5.4, and 7.2 m in the enclosure and 2, 4, and 6 m in the alley) were varied. In both situations, when food was presented in only 1 place, the food carrying behavior increased as the size of food increased from 0.045 to 1-g. One and three g pellets were always carried, regardless of the distance to nest. On the other hand, when multiple food sites were presented in the alley, rats were more likely to carry small pellet from the further food site, whereas large pellet was always carried and were not influenced by the distance. The behavior to foods in the enclosure, however, was not influenced in either food site condition. This might be due to the difference in familiarity to the environment and anxiety from perceived predation risk.     

食物源距离对中华姬鼠贮藏策略的影响

在围栏条件下, 检验食物源距离对中华姬鼠贮藏策略的影响,并检验根据快速隔离假说和最优贮藏空间分布模型做出的两个预测：（1）3个释放点无论距离远近,种子均先被大量分散贮藏在各个释放点周围;（2）随着释放点远离巢穴,种子被集中贮藏的比例下降。结果发现: 分散贮藏是中华姬鼠的主要贮藏方式,3个释放点种子被分散贮藏的数量无显著差异,这个结果支持预测（1）;3个种子释放点（1m,5m,13m）被集中贮藏的比例分别为(1m: 6.25%; 5m: 1.88%; 13m: 1.25%),说明被集中贮藏的种子数量随种子释放点与巢穴距离的增大而逐渐减少,这个结果支持预测（2）。本研究表明中华姬鼠应对捕食风险升高的策略是降低取食量和贮藏量。     

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.     

Nesting Success in Crimson Finches: Chance or Choice?

In avian systems, nest predation is one of the most significant influences on reproductive success. Selection for mechanisms and behaviours to minimise predation rates should be favoured. To avoid predation, breeding birds can often deter predators through active nest defence or by modifying behaviours around the nest (e.g. reducing feeding rates and vocalisations). Birds might also benefit from concealing nests or placing them in inaccessible locations. The relative importance of these strategies (behaviour vs. site selection) can be difficult to disentangle and may differ according to life history. Tropical birds are thought to experience higher rates of predation than temperate birds and invest less energy in nest defence. We monitored a population of crimson finches (Neochmia phaeton), in the Australian tropics, over two breeding seasons. We found no relationship between adult nest defence behaviour (towards a model reptile predator) and the likelihood of nest success. However, nest success was strongly related to the visibility of the nest and the structure of the vegetation. We found no evidence that adult nest building decisions were influenced by predation risk; individuals that re‐nested after a predation event did not build their nest in a more concealed location. Therefore, predator avoidance, and hence nest success, appears to be largely due to chance rather than due to the behaviour of the birds or their choice of nesting sites. To escape high predation pressures, multiple nesting attempts both within and between seasons may be necessary to increase reproductive success. Alternatively, birds may be limited in their nest‐site options; that is, high‐quality individuals dominate quality nest sites.     

Fragmentation within and between wetland reserves: the importance of spatial scales for nest predation in reed buntings

Wetlands in many parts of the world are affected by fragmentation at multiple spatial scales. In Switzerland, most wetlands have been destroyed over the past two centuries and management of the remaining wetland reserves has intensified in the recent years leading to increased fragmentation of reed areas within reserves. Using four years of data on the reproductive performance of color-banded reed bunting Emberiza schoeniclus populations, we explored fragmentation effects on nest predation rates at four spatial scales ranging from the nest to the landscape scale. In the egg stage, predation rate was negatively related to vegetation cover, vegetation height and nest height, but positively linked to water cover and depth next to the nest (nest scale). Probability of predation declined with increasing size of reed patches containing the nests and distances of nests to the water and land sided reed edge, as well as with decreasing edge to area ratio (edge scale). There was a weak positive association between degree of fragmentation of reed patches within sites and nest predation rates (site scale). Finally, nest predation probability increased with distance to the nearest wetland (landscape scale). Jointly analyzing variables from different spatial scales revealed that a model combining variables from the nest, edge and landscape scale best explained predation probability in the egg stage. In the nestling stage, the single most important factor influencing nest predation probability was the distance to the nearest wetland (landscape scale), with nest predation decreasing with distance between sites. Our results show that the probability of nest predation in reed buntings is affected by fragmentation within and between wetland reserves and that the effects differ between breeding stages. Future management of wetland reserves should aim at sparing reed patches large and dense enough to provide safe nest sites for birds.     

Compounding effects of habitat fragmentation and predation on bird nests

Habitat fragmentation and invasive species are two of the greatest threats to species diversity worldwide. This is particularly relevant for oceanic islands with vulnerable endemics. Here, we examine how habitat fragmentation influences nest predation by Rattus spp. on cup‐nesting birds in Samoan forests. We determined models for predicting predation rates by Rattus on artificial nests at two scales: (i) the position of the bird's nest within the landscape (e.g. proximity to mixed crop plantations, distance to forest edge); and (ii) the microhabitat in the immediate vicinity of the nest (e.g. nest height, ground cover, slope). Nest cameras showed only one mammal predator, the black rat (Rattus rattus), predating artificial nests. The optimal model predicting nest predation rates by black rats included a landscape variable, proximity to plantations and a local nest site variable, the percentage of low (<15 cm) ground cover surrounding the nest tree. Predation rates were 22 ± 13% higher for nests in forest edges near mixed crop plantations than in edges without plantations. In contrast, predation rates did not vary significantly between edge habitat where the matrix did not contain plantations, and interior forest sites (>1 km from the edge). As ground cover reduced, nest predation rates increased. Waxtags containing either coconut or peanut butter were used as a second method for assessing nest predation. The rates at which these were chewed followed patterns similar to the predation of the artificial nests. Rural development in Samoa will increase the proportion of forest edge near plantations. Our results suggest that this will increase the proportion of forest birds that experience nest predation from black rats. Further research is required to determine if rat control is needed to maintain even interior forest sites populations of predator‐sensitive bird species on South Pacific islands.     

Conspecific nest parasitism is associated with inequality in nest predation risk in the common goldeneye (Bucephala clangula)

Previous studies of the role of nest predation in conspecificnest parasitism have not taken into account the possibilitythat predation risk may not be randomly distributed among nestsites and that breeding individuals may use different cues toassess the risk and adjust their reproductive tactic betweenyears accordingly. Especially in cavity-nesting species, therole of nest predation in conspecific nest parasitism has beendownplayed, while the role of nest site limitation has beenhighlighted. Using both observational and experimental data,I show that in the common goldeneye (Bucephala clangula), acavity-nesting species in which conspecific nest parasitism iscommon, predation risk varies considerably between nest sitesand does not follow a random expectation. The inequality inpredation risk between nest sites also showed up in the occurrenceof parasitized nests in an experimental setup. Nests parasitizedin year t were more frequent in those nest sites that were notdepredated during the previous nesting attempt in year t - nthan in nest sites that were depredated and in control nestsites that had not been used for nesting before. A nest siteaddition experiment revealed that conspecific nest parasitismwas not associated with nest site limitation. My findings givesupport for the hypothesis that nest predation is an importantecological factor explaining conspecific nest parasitism ingoldeneyes.