Changing patterns of nest predation and predator communities along a tropical elevation gradient

Tropical montane communities host the world's highest beta diversity of birds, a phenomenon usually attributed to community turnover caused by changes in biotic and abiotic factors along elevation gradients. Yet, empirical data on most biotic factors are lacking. Nest predation is thought to be especially important because it appears to be common and can change selective pressures underlying life history traits, which can alter competitive interactions. We monitored 2538 nests, 338 of which had known nest predators, to evaluate if nest predation changes along a tropical elevational gradient. We found that nest predation decreased with elevation, reflecting the loss of lowland predators that do not tolerate colder climates. We found different “super” nest predators at each elevation that accounted for a high percentage of events, suggesting that selection pressures exerted by nest predator communities may be less diffuse than has been hypothesized, at least for birds nesting in the understory.     

Multiscale Nest-Site Selection by Black-Capped Vireos

Abstract: We used an information-theoretic approach to investigate nest-site selection by black-capped vireos (Vireo atricapilla) at the landscape and habitat-patch scales on Fort Hood Military Reservation in central Texas, USA, during 2003 and 2004. We used a use-availability sampling design and logistic regression to compare woody cover characteristics at nests to random points in the landscape to determine habitat selection at the landscape scale. At the habitat-patch scale, we used matched case-control logistic regression to compare habitat measures at nests and random non-nest points to evaluate support for hypotheses concerning the influence of woody cover, nest-patch, and nest-site characteristics on black-capped vireo nest-site selection. At the landscape scale, we found strong support (Akaike wt [w_j] = 1.0) for a model with a cubic effect of percent woody cover and woody cover edge density. Sites with the greatest predicted probabilities of use had woody cover values between 30% and 60% and increasing amounts of edge. We found strong support (w_i = 0.93) for the global model at the habitat-patch scale that included characteristics of the nest site, nest patch, and woody cover within 25 m. Based on odds ratios and confidence limits, percent woody cover, cover below 2 m, cover type, and substrate height had the greatest effect on nest-site selection. The predicted probability a site was selected for a nest site increased with foliage cover below 2 m, taller substrates, deciduous cover, and decreased at high levels of percent woody cover (especially >80%). Texas red oak (Quercus buckleyi) was the most used nest substrate (100 of 358 nests), followed by shin oak (Q. sinuata var. breviloba; 86 of 358 nests) and Ashe juniper (Juniperus ashei; 44 of 358 nests). Black-capped vireos used Texas red oak and shin oak in greater proportion to their availability, whereas Ashe juniper was used less in proportion to its availability, suggesting vireos avoided this species. We suggest that managers promote dense deciduous cover for nesting habitat and maximize edge-to-area ratios to maintain spatial and structural heterogeneity.     

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

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Identifying predators clarifies predictors of nest success in a temperate passerine

1.  Nest predation negatively affects most avian populations. Studies of nest predation usually group all nest failures when attempting to determine temporal and parental activities, habitat or landscape predictors of success. Often these studies find few significant predictors and interpret patterns as essentially random.
2.  Relatively little is known about the importance of individual predator species or groups on observed patterns of nest success, and how the ecology of these predators may influence patterns of success and failure.
3.  In 2006 and 2007, time-lapse, infrared video systems were deployed at nests of Swainson's warblers ( Limnothlypis swainsonii Audubon) in east-central Arkansas to identify dominant nest predators and determine whether factors predicting predation differed among these predators.
4.  Analysis of pooled data yielded few predictors of predation risk, whereas separate analyses for the three major predator groups revealed clear, but often conflicting, patterns.
5.  Predation by ratsnakes ( Elaphe obsoleta ) and raptors was more common during the nestling period, whereas predation by brown-headed cowbirds ( Molothrus ater ) occurred more during incubation. Additionally, the risk of predation by raptors and cowbirds decreased throughout the breeding season, whereas ratsnake predation risk increased.
6.  Contrary to expectations, predation by ratsnakes and cowbirds was more common far from edges, whereas raptor predation was more common close to agricultural edges.
7.  Collectively, our results suggest that associating specific predators with the nests they prey on is necessary to understand underlying mechanisms.     

Demographic Differences of Black-Capped Vireos in 2 Habitat Types in Central Texas

Abstract: To understand the effects of habitat selection, we analyzed differences in abundance, age structure, and nesting success of black-capped vireos (Vireo atricapilla) in 2 early successional habitat types found on Fort Hood, a 87,890-ha Military Reservation in central Texas, USA. These habitats were 1) large areas of continuously shrubby vegetation (both natural and mechanically made), referred to as shrubland habitat, and 2) anthropogenically created small patches of shrubby vegetation centered on one or several large trees, known locally as donut habitat. The objectives of our study were to determine whether there were differences in abundance, age structure, and daily nest survival in these 2 habitat types and to determine whether donut habitat is high- or low-quality habitat. Donut habitat had a lower abundance of vireos (half as many as shrubland/point count) and a higher percentage of second-year males, suggesting donut habitat was lower-quality habitat than shrubland. Analyses of daily nest survival indicated that habitat, nest height, and year were all important variables. Nests initiated in 2004, located in shrubland habitats, and higher from the ground were more likely to succeed. Our study provided evidence that habitat is a limiting factor for this federally endangered species. Because habitat is limiting, wildlife biologists at Fort Hood should focus on managing higher quality, contiguous shrubland habitat. Wildlife biologists should also continue to monitor areas of donut habitat to determine whether they represent potential population sinks.     

人工巢箱条件下的大山雀巢捕食

于2004—2008年在次生阔叶林中,采用悬挂巢箱的方法对大山雀的巢捕食作了研究。结果表明:不同年龄巢箱的被捕食率显著不同,新巢箱被捕食率最低,第2年被捕食率最高,第3年下降很大,第4年又略有上升。被捕食巢的窝卵数极显著的低于未被捕食巢的窝卵数。影响巢捕食的主要生境因子为巢箱高度和巢上盖度,其次为灌木的密度和高度。     

Common waxbills use carnivore scat to reduce the risk of nest predation

For many animals, nest construction is a prerequisite for successfulbreeding. The choice of nesting materials is an important componentof nest construction, because material properties can influencenest design and, potentially, reproductive success. Common waxbillsare small African finches that select carnivore scat as a materialto include in, on, and around their nests. I investigated thehypothesis that scat functions to reduce predation risk by documentingits use in a wild population of common waxbills and by conductingan artificial nest experiment. Among natural nests, scat waspresent in every nest that hatched young, and parents continuedto add scat to nests throughout the nestling period. Among artificialnests, those that received experimental additions of carnivorescat survived at a significantly higher rate than did untreatednests, suggesting that scat functions to reduce predation risk.The mechanism by which nests are protected remains unclear,although it is likely that scat acts as an olfactory deterrentand/or camouflage. Researchers have long focused on the implicationsof nest site characteristics for avian life-history evolution.Results of the present study suggest that nest materials, similarto nest sites, may influence life histories of nest-buildinganimals by affecting predation risk.     

基于红外相机技术和人工巢试验分析白冠长尾雉巢潜在捕食者

为了了解保护区内外的白冠长尾雉繁殖生态,2014年3—7月在河南董寨国家级自然保护区和保护区外湖北平靖关村利用人工巢试验(以鸡蛋为诱饵)、红外相机技术和栖息地样方调查搜集巢捕食信息,对其巢捕食率、巢潜在捕食者和影响巢捕食的栖息地因子进行研究.两轮试验分别为繁殖期前期3—4月和繁殖期中期5—6月.试验共放置巢149个,其中红外相机监测62个,累计相机日1315个,拍摄照片7776张,视频6950个.结果表明: 保护区外(平靖关)巢捕食率高于保护区内(董寨),繁殖期前期和繁殖期中期保护区内外差异均极显著.平靖关捕食者种类数(11和6种)在繁殖期前期和中期均高于董寨(7和5种),平靖关捕食者比例较高的是啮齿类和鸦科鸟类,董寨捕食比例较高的是貉.平靖关坡度和乔木盖度对巢捕食影响显著,而董寨的落叶盖度对其影响显著.在红外监测的人工巢中共发现野生白冠长尾雉访问人工巢13巢18次.     

An ecological paradox: More woodland predators and less artificial nest predation in landscapes colonized by noisy miners

Many passerine bird populations, particularly those that have open‐cup nests, are in decline in agricultural landscapes. Current theory suggests that an increase in habitat generalist predators in response to landscape change is partially responsible for these declines. However, empirical tests have failed to reach a consensus on how and through what mechanisms landscape change affects nest predation. We tested one hypothesis, the Additive Predation Model, with an artificial nest experiment in fragmented landscapes in southern Queensland, Australia. We employed structural equation modelling of the influence of the relative density of woodland and habitat generalist predators and landscape features at the nest, site, patch and landscape scales on the probability of nest predation. We found little support for the Additive Predation Model, with no significant influence of the density of woodland predators on the probability of nest predation, although landscape features at different spatial scales were important. Within woodlands fragmented by agriculture in eastern Australia, the presence of noisy miner colonies appears to influence ecological processes important for nest predation such that the Additive Predation Model does not hold. In the absence of colonies of the aggressive native bird, the noisy miner, the influence of woodland predators on the risk of artificial nest predation was low compared with that of habitat generalist predators. Outside noisy miner colonies, we found significant edge effects with greater predation rates for artificial nests within woodland patches located closer to the agricultural matrix. Furthermore, the density of habitat generalist predators increased with the extent of irrigated land‐use, suggesting that in the absence of noisy miner colonies, nest predation increases with land‐use intensity at the landscape scale.     

Genetic monogamy in blue-headed vireos and a comparison with a sympatric vireo with extrapair paternity

Based on the breeding synchrony hypothesis, we predicted, intwo congeners that nest in simiilar habitat but differ in nestingsynchrony, that blue-headed vireos (Vireo solitarius) wouldhave fewer extrapair fertilizations (EPFs) thaii red-eyed vireos(V. olivaceus EPFs were rare in blue-headed vireos (1/37 nestlings),but common in red-eyed vireos (11/19 nestlings). We studiedthe behavior of blue-headed vireos to determine what factorscould promote genetic monogamy. We found no evidence that malesmate guarded to prevent extrapair copulations from occurring.Males did not follow fertile mates closely when mates left thenest (14–25% of female departures) and, during the egg-layingperiod, males were often alone on the nest (22.3 mm/h). Femaleblue-headed vireos, but not red-eyed vireos, obtain direct benefitsfrom social mates such as nest building and incubation (49.1%of the total), and they assess male quality long before becomingfertile. Female blue-headed vireos spent more time incubatingwhen their mates had low incubation effort. Furthermore, maleincubation effort was positively correlated with nest survivalduring incubation. We discuss the evolution of genetic monogamyand sex role convergence in blue-headed vireos in relation toasynchronous breeding.     

Experimental support for the role of nest predation in the evolution of brood parasitism

In 1965, Hamilton and Orians (HO) hypothesized that the starting point for the evolution of obligate interspecific brood parasitism in birds was the facultative laying of physiologically committed eggs in neighbouring active nests of con‐ and heterospecifics, following predation of a bird’s own nest during the laying stage. We tested this prediction of the HO hypothesis by using captive pairs of zebra finches (Taeniopygia guttata), a species with evidence for intraspecific parasitism both in the wild and in captivity. As predicted, in response to experimental nest removal, subjects laid eggs parasitically in simulated active conspecific nests above chance levels. Across subsequent trials, we detected both repeatability and directional change in laying patterns, with some subjects switching from parasitism to depositing eggs in the empty nest. Taken together, these results support the assumptions and predictions of the HO hypothesis, and indicate that the zebra finch is a potential model species for future behavioural and genetic studies in captive brood parasite research.     

Parent birds assess nest predation risk and adjust their reproductive strategies

Avian life history theory has long assumed that nest predation plays a minor role in shaping reproductive strategies. Yet, this assumption remains conspicuously untested by broad experiments that alter environmental risk of nest predation, despite the fact that nest predation is a major source of reproductive failure. Here, we examined whether parents can assess experimentally reduced nest predation risk and alter their reproductive strategies. We experimentally reduced nest predation risk and show that in safer environments parents increased investment in young through increased egg size, clutch mass, and the rate they fed nestlings. Parents also increased investment in female condition by increasing the rates that males fed incubating females at the nest, and decreasing the time that females spent incubating. These results demonstrate that birds can assess nest predation risk at large and that nest predation plays a key role in the expression of avian reproductive strategies.     

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.     

Accuracy of nest fate classification and predator identification from evidence at nests of Least Terns and Piping Plovers

For federally listed species such as Least Tern Sternula antillarum and Piping Plover Charadrius melodus, correct determination of nest fates and causes of nest failure is crucial for understanding population dynamics and improving monitoring programmes. We used video cameras to evaluate nest fate misclassification rate and to identify factors that may cause researchers monitoring nests at different intervals to classify Least Tern and Piping Plover nest fates incorrectly. During the 2013–2015 breeding seasons, we installed miniature surveillance cameras at 65 of 294 Least Tern and 89 of 551 Piping Plover nests under observation on the Missouri River in North Dakota. Nest fates were assigned in the field from remains found at the nest‐site and then again by an independent researcher who reviewed camera footage. We used ordinal logistic regressions to examine whether monitoring interval, clutch age or temporal factors influenced a correct, partially misclassified (probable successful in the field vs. successful by camera) or misclassified nest fate classification. During a 7‐day monitoring interval between visits, 45% of nests were partially and 27.5% were fully misclassified. The percentage of partially (20%) and fully (8.0%) misclassified nests decreased with a more intensive (3‐day) monitoring schedule. Researchers were also less likely to correctly classify nest fates for Least Terns than for Piping Plovers, and as clutch age and monitoring interval increased for both species. Furthermore, causes of failure (e.g. predators, weather) as determined from field evidence vs. video disagreed for 53.5% of nests. The ability to identify accurately nest fate and cause of nest failure will facilitate a better understanding of factors that limit productivity and will lead to better informed management decisions for improving nest survival.     

Simulating northern bobwhite population responses to nest predation,nesting habitat,and weather in south Texas

Nest predation is thought to be one of the major factors limiting northern bobwhite (Colinus virginianus) populations. We examined the relative impact of altering nest-predation rate, nesting habitat, and weather (i.e., temp and precipitation) on northern bobwhite population dynamics in a hypothetical 15,000-ha subtropical-rangeland ecosystem in south Texas using a simulation model. The systems model consisted of a 3-stage (i.e., eggs, juv, and ad) bobwhite population with dynamics influenced by variables affecting production, recruitment, nest predation, and mortality. We based model parameters on data collected from a 3-yr nest-predator study employing infrared-camera technology, from ongoing field research using a radio-marked population of wild bobwhites, and from the literature. The baseline simulated bobwhite population dynamics corresponded closely to empirical data, with no difference between medians of simulated (n = 30 yr) and observed bobwhite age ratios over a 28-yr period. Similarly, a time-series comparison of simulated and observed age ratios showed most (89%) observed values fell within the 5th and 95th percentiles of the simulated data over the 28-yr period. We created simulated population scenarios representing 1) baseline historical conditions, 2) predator control, 3) low precipitation, 4) low precipitation with predator control, 5) high temperature, 6) high temperature with predator control, 7) reduced nest-clump availability, and 8) reduced nest-clump availability with predator control that resulted in considerably different median bobwhite densities over 10 yr. For example, under simulated predator control, populations increased by about 55% from the baseline scenario, whereas under simulated reduced nest-clump availability, populations decreased by about 75% from the baseline scenario. Comparisons of time-series for each scenario showed that reduced nest-clump availability, low precipitation, and high temperature reduced bobwhite densities to a larger degree compared to a natural nest predation rate. Reduced nest-clump availability resulted in the most substantial decline of simulated bobwhite densities. Simulations suggested that management efforts should focus on maintaining adequate nest-clump availability and then possibly consider nest predator control as a secondary priority. © 2010 The Wildlife Society     

Cropland edge,forest succession,and landscape affect shrubland bird nest predation

The effects of habitat edges on nest survival of shrubland birds, many of which have experienced significant declines in the eastern United States, have not been thoroughly studied. In 2007 and 2008, we collected data on nests of 5 shrubland passerine species in 12 early successional forest patches in North Carolina, USA. We used model selection methods to assess the effect of distance to cropland and mature forest edge on nest predation rates and additionally accounted for temporal trends, nest stage, vegetation structure, and landscape context. For nests of all species combined, nest predation decreased with increasing distance to cropland edge, by nearly 50% at 250 m from the cropland edge. Nest predation of all species combined also was higher in patches with taller saplings and less understory vegetation, especially in the second year of our study when trees were 4–6 m tall. Predation of field sparrow (Spizella pusilla) nests was lower in landscapes with higher agricultural landcover. Nest predation risk for shrubland birds appears to be greater near agricultural edges than mature forest edges, and natural forest succession may drive patterns of local extirpation of shrubland birds in early successional forest patches. Thus, we suggest that habitat patches managed for shrubland bird populations should be considerably large or wide (>250 m) when adjacent to crop fields and maintained in structurally diverse early seral stages. © 2011 The Wildlife Society.     

Mothers vigilantly guard nests after partial brood loss: a cue of nest predation risk in a paper wasp

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