Why do grebes cover their nests? Laboratory and field tests of two alternative hypotheses

Egg predation is a common feature influencing the reproductive success of open nesting birds. Evolutionary pressure therefore favours building cryptic, inconspicuous nests. However, these antipredatory pressures may be in conflict with thermoregulatory constraints, which select for dry nest material maintaining optimum temperature inside a nest cup during the absence of incubating parents. Here we examined possible trade-offs between nest crypsis and thermoregulation in Little Grebes (Tachybaptus ruficollis), which lay their eggs in floating nests built from wet plant material. As this species regularly covers its eggs with nest material, we experimentally examined (1) the rates of egg predation on covered and uncovered artificial nests and (2) possible thermoregulatory costs from nest covering by comparing temperature and relative humidity changes inside the nest cup. Results revealed that covering clutches is beneficial in terms of deterring predators, because uncovered eggs were more vulnerable to predation. Moreover, covering clutches also had thermoregulatory benefits because the mean temperature and relative humidity inside nest cups covered by dry or wet materials were significantly higher for covered compared to uncovered treatments. Covering clutches in Little Grebes therefore does not pose thermoregulatory costs.     

Is predation on waterfowl nests density dependent? – Tests at three spatial scales

We tested whether predation on duck nests ( Anas spp.) was density dependent at three spatial scales using artificial and natural nests in the Suisun Marsh, California, USA. At the largest spatial scale, we used 5 years (1998–2002) of data to examine the natural variation in duck nest success and nest densities among 8–16 fields per year, each 5–33 ha in size (n=62 fields). At an intermediate spatial scale, we deployed artificial nests (2000, n=280) within 1-ha plots at three experimental densities (5, 10, and 20 nests ha⁻¹) in a complete randomized block design and examined differences in nest predation. At the smallest spatial scale, we examined nest success in relation to nearest-neighbor fates and distances for artificial (2000, n=280) and natural nests (2000, n=507). We detected no relationship between nest success and the density of natural nests among fields in any year, nor when we pooled data for all years after controlling for year effects. The proportion of artificial nests that survived also did not depend on experimental nest densities within 1-ha plots. Overall, 15.0±12.4%, 15.0±11.0%, and 6.2±4.3% of artificial nests survived the 32-day exposure period in the low, intermediate, and high nest densities, respectively. Additionally, we detected no consistent effect of nearest-neighbor fate or distance on the success of artificial or natural nests. Thus, our results provide no evidence of density-dependent predation on duck nests at any scale of analysis, in contrast to a number of previous studies. Variation among geographical locations in the degree to which predation is density-dependent may reflect the composition of the predator community and the availability of alternate prey.     

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.     

Does land use change influence predation of bird nests?

Worldwide, many areas of agricultural land which were once covered with native vegetation have been converted to tree plantations. Such landscape transformation can influence the dynamics of wildlife populations through, for example, altering rates of predation (e.g. predation of nests of birds). Nest predation can influence reproductive success and, in turn, may alter populations by affecting juvenile recruitment. We quantified predation of bird nests in woodland remnants surrounded by two types of land use, grazing farmland and exotic Radiata pine (Pinus radiata) plantation. We also examined differences in predation rates between artificial and natural nests. We found both artificial and natural nests were more susceptible to nest predation in woodland remnants surrounded by a pine plantation than in woodland remnants located within farmland. Our study suggests that higher levels of nest predation may reduce occupancy of woodland remnants by small‐bodied birds over time, including species of conservation concern. This may have been occurred as a result of the conversion of semi‐cleared grazing land to exotic pine plantation.     

Predation rate on artificial nests increases with human housing density in suburban habitats

Predation causes most nest failure in birds. Predator communities are likely to vary across a gradient of increasing urbanization, so nest predation also is likely to vary across this gradient. Although predation is thought to decline with increasing urbanization, relatively little is known about variation in predation pressure within strata along an urban gradient and how factors known to affect nest success, such as nest location, interact with urban variables, such as human housing density. Native habitats are frequently fragmented and isolated by suburban residential development, thus we quantified predation rates on artificial nests located in natural oak scrub patches within a suburban matrix in south-central Florida. We examined patterns of predation based on nest location relative to habitat edges, artificial nest weathering treatment, nest shrub height, and human housing density. Over two 18-d trials, we placed a total of 240 nests, each containing a single quail egg and a clay sham, along three roadside transects. Nest predation was not influenced by proximity to edge, nest weathering, or trial date, but was highest at high housing density and lowest at low housing density. The proportion of quail eggs removed from nests increased with human housing density. Birds were the most frequent predators of artificial nests, but the relative frequency of predation by birds or mammals did not differ relative to any of our treatments. Higher rates of nest predation with increasing human housing density within suburban habitats may reflect changes in habitat structure and composition that increase the vulnerability of nests to predation or changes in the composition of the predator community. Our results modify the conclusions of previous studies by suggesting that at scales smaller than the entire urban gradient, nest predation may increase with human housing density, one common measure of urbanization.     

Do artificial nests reveal relative nest predation risk for real nests?

Present knowledge of the effects of nest predation on spatial distribution, habitat selection and community structure of birds is to a large extent based on results from experiments with artificial nests. Although nest predation risk is likely to differ between artificial and real nests, most previous studies of nest predation using artificial nests have been lacking a proper control. We investigated whether predation rates on artificial nests predicted those on real nests by simultaneously comparing the fate of real and artificial nests (containing quail Coturnix coturnix and plasticine eggs) in 92 territories of the northern wheatear Oenanthe oenanthe in 1996. We also investigated whether risk for artificial nests was related to relative average risk for real nests in these territories (based on data collected two years before and two years after the experiment). Nest predation on artificial nests did predict relative predation risk for real nests only when quail egg depredation was used as the criterion for artificial nest predation. Despite plasticine egg depredation being the most common type of predation it was not associated with predation risk for real nests. Small mice and vole species dominated among cases with only plasticine egg depredation, while predatory mammals and snakes destroyed most quail eggs in artificial nests and most eggs in real wheatear nests. The results suggest that artificial nests may only predict the risk for real nests when the nest predator species are similar among the two types of nest. Furthermore, our data suggest that small mice and vole species rarely depredate nests of mid-sized passerine birds . Our results cast doubt on many previous conclusions based on experiments with artificial nests, since predation risk for such nests is likely to be uncorrelated with risk for real nests due to nest-type-specific differences in nest preying species.     

An inexpensive method for identifying predators of passerine nests using tethered artificial eggs

We identified nest predators of two European thrush and three European finch species in the central North Island, New Zealand, using artificial clay eggs in active natural nests. The acceptance of the artificial egg by females was 75%, with low rates of female egg ejection (7%) or desertion (7%). Due to high predation rates we could not confirm the acceptance of six (11%) artificial eggs before predation occurred. Of the 57 nests that received an artificial egg 30 were preyed upon. We were able to successfully identify predators from 18 (60%) nests by imprints left in the artificial eggs, with rats (Rattus sp.), Australasian Harriers (Circus approximans) and Australian magpie (Gymnorhina tibicen) accounting for nine, eight and one of the predatory events respectively. In the remaining twelve predatory events imprints were too faint or no marks were left on the clay egg for identification. This study successfully demonstrates the use of an inexpensive, seldom-used method for quantifying and identifying nest predators, with low rates of nest abandonment and high rates of predator identification. We believe this method could add valuable information for future studies of nest predation in New Zealand.     

Nest height, nest concealment, and predator type predict nest predation in superb fairy-wrens (Malurus?cyaneus)

Three factors and their interaction effects are increasingly recognized as important determinants of nest predation: nest concealment, nest height, and predator type. The risk of nest predation is predicted to vary across these variables because of nest detectability and accessibility. In general, however, few studies examine how these three variables interact in relation to nest predation, focusing instead on either nest concealment or nest height (whereby predator identity is usually not known). In this study, we examine the role of nest concealment and nest height for nest survival using both artificial and natural nests in the superb fairy-wren (Malurus cyaneus). We indirectly identified potential predators through marks left on artificial eggs and footprints left on tracking tunnels. Predation level at artificial nests was lower than at natural nests, and this could be due to a failure of some nest predators to locate cryptic nests in the absence of cues provided by parental activity. Our results supported the prediction that exposed and concealed nests have different levels of nest predation, which can be explained by variation in predator type. Visual predators were only detected at exposed nests, and survival from visual predators was lower for high nests that were also exposed. However, olfactory predators were detected irrespective of nest height or nest concealment. Because rodents use olfaction to locate nests, this could explain the lack of association between nest concealment and predation outcome at low nests. In addition, rodent footmarks near nests were significantly associated with rodent tooth marks on eggs.     

Do alien predators pose a particular risk to duck nests in Northern Europe? Results from an artificial nest experiment

Several alien predator species have spread widely in Europe during the last five decades and pose a potential enhanced risk to native nesting ducks and their eggs. Because predation is an important factor limiting Northern Hemisphere duck nest survival, we ask the question, do alien species increase the nest loss risk to ground nesting ducks? We created 418 artificial duck nests in low densities around inland waters in Finland and Denmark during 2017–2019 and monitored them for seven days after construction using wildlife cameras to record whether alien species visit and prey on the nests more often than native species. We sampled various duck breeding habitats from eutrophic agricultural lakes and wetlands to oligotrophic lakes and urban environments. The results differed between habitats and the two countries, which likely reflect the local population densities of the predator species. The raccoon dog (Nyctereutes procyonoides), an alien species, was the most common mammalian nest visitor in all habitats and its occurrence reduced nest survival. Only in wetland habitats was the native red fox (Vulpes vulpes) an equally common nest visitor, where another alien species, the American mink (Neovison vison), also occurred among nest visitors. Although cautious about concluding too much from visitations to artificial nests, these results imply that duck breeding habitats in Northern Europe already support abundant and effective alien nest predators, whose relative frequency of visitation to artificial nests suggest that they potentially add to the nest predation risk to ducks over native predators.

     

Predation on artificial nests in relation to forest type: contrasting the use of quail and plasticine eggs

Previous studies of avian nest predation have focused on how human-induced changes in the landscape influence the frequency of predation However, natural variation in the abundance of predators due to their choice of habitat can also influence predation rate To determine if predation on artificial nests was influenced by forest stand type, we placed ground and shrub nests containing quail and plasticine eggs in contiguous coniferous, mixedwood and deciduous stands in the southern boreal mixedwood forest of central Canada Nest predators were identified using remotely triggered cameras and marks left in plasticine eggs, while the relative abundance of nest predators such as squirrels and corvids were estimated using acoustic-visual surveys Using the fate of quail eggs to calculate predation rate, we found that predation was significantly higher in coniferous (67%) than in deciduous (17%) or mixedwood (25%) forest, with similar predation on ground (37%) and shrub (29%) nests Using plasticine eggs to calculate predation rate, nests in coniferous forest still suffered higher rates of predation, although predation rates were 15–20% higher, and ground nests suffered significantly higher rates of predation than shrub nests Quail eggs seemed to suffer lower rates of predation because small mammals were unable to penetrate the shell, but could leave marks on plasticine eggs The higher predation rate in coniferous forest was likely caused by higher abundance of red squirrels Tamiasciurus hudsonicus , the presence of fishers Martes pennanti and a simplified understory which may have made it easier for predators to find nests relative to the deciduous and mixedwood forest Plasucine eggs provide new insights into nest predation by identifying predation events by smaller predators such as mice that are missed when using quail eggs     

Support for the nest mimicry hypothesis in Yellow-rumped Thornbills Acanthiza chrysorrhoa

The nest of the Yellow-rumped Thornbill Acanthiza chrysorrhoa (endemic to Australia) consists of a lower domed nest where the young are cared for and an upper cup-shaped structure, referred to as the false nest. Numerous hypotheses have been proposed to explain the primary function of the false nest, but their predictions remain largely untested. Here, we test predictions of the nest mimicry hypothesis (with artificial nests) whereby the empty false nest acts as a decoy, deflecting visual nest predators and/or brood parasites away from the true nest below, thereby increasing nesting success. Clear detection of the false nest by adversaries is an intrinsic aspect of nest mimicry; thus we also test whether conspicuous nests (natural) are favoured by natural selection. Specifically, (1) we compare predation level at artificial domed nests with and without a false nest; and (2) we analyse associations between nest site concealment and nesting outcome in natural nests in a wild population. Supporting the nest mimicry hypothesis, predation level was significantly lower for artificial domed nests with false nests, and for natural nests with lower levels of concealment.     

Nest predation in hole-nesting birds in relation to habitat edge: an experiment

We experimentally tested the hypothesis that nest predation rate in hole-nesting birds, especially the common goldeneye Bucephala clangula , was related to the edge between aquatic and terrestrial habitats We also studied whether nest predation rate was related to habitat patch (lake) size In three study areas nest-boxes were erected in pairs one nest-box was erected at the shoreline and the other one inside the forest at a varying distance (range from 14 to 140 m) from the shoreline One chicken egg was placed in each nest-box (dummy nest) Overall predation rate on dummy nests varied from 34 6% to 52 6% depending on the study area The pine marten Martes manes was the primary nest predator m all study areas Daily predation rates did not differ between natural and dummy nests Predation rate on dummy nests was not related to the distance of the nest-box from the shoreline in any study area Predation rate was not related to lake size in two study areas but in one area predation rate on dummy nests was lower around largest lakes However, the result may be accidental and not because of lake size per se because the proportion of forest was exceptionally low in that part of the study area     

Herbaceous ground cover reduces nest predation in olive groves

Capsule Bare ground increases artificial nest predation in olive groves.

Aims To assess the effect of different soil management regimes on nest predation rates in olive groves.

Methods We performed nest predation experiments with artificial nests during the breeding season in 2013, in two areas of southern Spain. Each artificial nest (n?=?300) contained three quail Coturnix eggs, two of which were unmanipulated and the third one was emptied and injected with plaster. Predators were identified by marks on eggs filled with plaster.

Results Ground nests were significantly more depredated, irrespective of the presence of ground cover; tree nests were less depredated in fields with ground cover. There was a clear difference in nest predators of ground and tree nests. Rodents were the most frequent predators of tree nests.

Conclusion Lower predation rates of tree nests in orchards with ground cover are probably linked to a change in the foraging behaviour of rodents, which in these more complex habitats might be restricted by rodents' own risk of predation. This study underscores the important role of agricultural practices in preserving farmland bird communities, particularly tree-nesting species, suggesting that for this group, implementation of ground cover in olive groves might enhance breeding success by reducing nest predation rates.     

Age-specific nest-site preference and success in eiders

Variation in nest concealment is puzzling given the expected strong selection for safe nest sites. Selecting a concealed nest may decrease the risk of clutch predation but hinder parents from escaping predators, providing a possible solution to this paradox. Because the relative value of current versus future reproduction may vary with breeder age or state, nest concealment may also vary as a function of these attributes. We tested four predictions of the female and clutch safety trade-off hypothesis in eiders (Somateria mollissima): (1) nest concealment is negatively related to escape possibilities, (2) our capture rate of females is higher in covered nests, (3) egg predation is higher in open nests, and (4) overall nest success is unrelated to nest habitat. We also analysed nest microhabitat preferences and nest success relative to breeder age and body condition, controlling for nest spatial centrality. As expected, nest concealment and potential escape angle were negatively related, and capture by us, indicating female predation vulnerability, increased with nest cover. Clutch size was smaller in open nests, suggesting higher partial clutch predation, while it was larger among experienced and good-condition breeders. The probability of successful hatching was unrelated to nest habitat, positively associated with breeder experience, and negatively associated with hatching date. Experienced females selected more concealed and centrally located nests without sacrificing potential escape angles. The age-specific spatial distribution of nests on islands was unrelated to nest initiation dates, indicating no apparent competition. The age-specific preference of eiders for concealed nests may reflect declining reproductive value with age or confidence in surviving despite selecting a concealed nest. The apparently positive relationship between female age and survival and fecundity in eiders refutes the former alternative. Individual improvement in choosing safe nest sites, coupled with differential survival of individuals performing well, most likely explains age-specific nest-site preference and success.     

Coating plasticine eggs can eliminate the overestimation of predation on artificial ground nests

Capsule The use of plasticine eggs coated with a thin layer of rubber instead of entirely plasticine eggs in dummy nests reduced the unnaturally high nest predation rate caused by attracted small mammals, while also enabling the identification of nest predators.     

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.     

The contribution of artificial nest experiments to understanding avian reproductive success: a review of methods and conclusions

Artificial nest experiments have been used in approximately 80 recent studies to identify a wide range of factors influencing reproductive success of birds. Despite the same factors being examined in different studies, few consistent patterns have emerged from these experiments. Analysis of reproductive success at artificial and natural nests reveals that reproductive success measured with use of artificial nests is frequently underestimated. In addition, several studies suggest that different species of predators may be differentially attracted to the two types of nest. These results call the utility of artificial nests into question as a tool for elucidating factors influencing the success of natural nests. Analysis of the methodology used in artificial nest experiments reveals that in general there is poor correspondence between the appearance of experimental setups and the natural systems they attempt to model. If future experiments with artificial nests are to contribute to theory, the quality of experiments must improve and the assumptions underlying the artificial approach must be tested. Collecting this information will be relatively simple compared with the time-consuming task of experimenting with natural nests. Until these data are collected, conclusions about nesting ecology derived from the use of artificial nest experiments should be treated as preliminary.     

Using artificial nests to predict nest survival at reintroduction sites

Artificial nests are frequently used to assess factors affecting survival of natural bird nests. We tested the potential for artificial nests to be used in a novel application, the prediction of nest predation rates at potential reintroduction sites where exotic predators are being controlled. We collected artificial nest data from nine sites with different predator control regimes around the North Island of New Zealand, and compared the nest survival rates with those of North Island robin (Petroica longipes) nests at the same sites. Most of the robin populations had been reintroduced in the last 10 years, and were known to vary in nest survival and status (increasing/stable or declining). We derived estimates of robin nest survival for each site based on Stanley estimates of daily survival probabilities and the known incubation and brooding periods of robins. Estimates of artificial nest survival for each site were derived using the known fate model in MARK. We identified the imprints on the clay eggs in the artificial nests, and obtained different estimates of artificial nest survival based on imprints made by different potential predators. We then compared the value of these estimates for predicting natural nest survival, assuming a relationship of the form s = αpβ, where s is natural nest survival and p is artificial nest survival. Artificial nest survival estimates based on imprints made by rats (Rattus spp.) and brushtail possums (Trichosurus vulpecula) were clearly the best predictors (based on AICc), and explained 64% of the variation in robin nest survival among sites. Inclusion of bird imprints in the artificial nest survival estimates substantially reduced their predictive value. We suggest that artificial nests may provide a useful tool for predicting the suitability of potential reintroduction sites for New Zealand forest birds as long as imprints on clay eggs are correctly identified.     

Nest predation in Australian urban environments and the role of the pied currawong,Strepera graculina

Abstract An experiment, involving 2000 members of the public, determined the identity of nest predators in urban environments. Experimental nests of halved tennis balls covered with coconut fibre and wool were manufactured to resemble the nests of willie wagtails, Rhipidura leucophrys. The identity of predators was determined by analysis of imprints left in artificial eggs made of coloured modelling clay. Sixty-four per cent of nests were preyed upon, with most predation being the result of large birds. Direct observations of predation (n = 134) indicated that pied currawongs were the most common large bird, accounting for 52% of all predation. Predation incidence was higher in gardens with more trees and in which kookaburras, Dacelo novaeguineae, were fed frequently. Among nests placed in trees, nest predation was correlated with nest height. Eggs camouflaged by speckling experienced a similar incidence of predation to plain eggs. This study provides evidence to support the contention that pied currawongs are a major threat to the persistence of small birds in Australian urban environments.     

Predators of Greater Sage‐Grouse nests identified by video monitoring

ABSTRACT Nest predation is the primary cause of nest failure for Greater Sage‐Grouse (Centrocercus urophasianus), but the identity of their nest predators is often uncertain. Confirming the identity of these predators may be useful in enhancing management strategies designed to increase nest success. From 2002 to 2005, we monitored 87 Greater Sage‐Grouse nests (camera, N= 55; no camera, N= 32) in northeastern Nevada and south‐central Idaho and identified predators at 17 nests, with Common Ravens (Corvus corax) preying on eggs at 10 nests and American badgers (Taxidea taxis) at seven. Rodents were frequently observed at grouse nests, but did not prey on grouse eggs. Because sign left by ravens and badgers was often indistinguishable following nest predation, identifying nest predators based on egg removal, the presence of egg shells, or other sign was not possible. Most predation occurred when females were on nests. Active nest defense by grouse was rare and always unsuccessful. Continuous video monitoring of Sage‐Grouse nests permitted unambiguous identification of nest predators. Additional monitoring studies could help improve our understanding of the causes of Sage‐Grouse nest failure in the face of land‐use changes in the Intermountain West.