Impacts of nest predators and weather on reproductive success and population limitation in a long‐distance migratory songbird

Although avian nesting success is much studied, little is known about the relative importance of the factors that contribute to annual reproductive success and population limitation, especially for long‐distance migratory songbird species. We combined a field experiment limiting access to nests by mammalian predators with modeling of long‐term field data of American redstarts (Parulidae: Setophaga ruticilla) to assess the effects of multiple environmental variables on breeding success and population limitation. Experimental treatment (baffles placed around tree boles beneath active nests; n = 71) increased nesting success of this single‐brooded species significantly (77 vs 50% in controls; n = 343), demonstrating that scansorial mammals, primarily red squirrels Tamiasciurus hudsonicus and eastern chipmunks Tamias striatus, reduced reproductive success. Based on unbaffled nests (n = 466), daily nest survival varied annually, and was positively influenced by May temperature and negatively by sciurid nest predator abundance. Daily nest survival was also influenced positively by June rainfall, and declined with nest age but not with calendar date. Since nest failure was overwhelmingly caused by nest predation, these significant climate and nest‐age effects in our models are indirect, likely influencing nest predator and/or nesting bird behaviors that in turn influenced nest predation. Redstart population density had no effect on nesting success, after accounting for other factors. Annual reproductive success accounted for 34% of the variability in annual population change in redstarts in our study area. Our findings document 1) breeding season population limitation in this species, 2) a link between tree masting and bird population dynamics via mammal population fluctuations, 3) the independent contributions of summer versus winter population processes in a migratory species, and 4) the potential complexity of climate‐biotic interactions.     

Nest monitoring does not affect nesting success of Whinchats Saxicola rubetra

It is important to assess the effect that research activities may have on animals in the wild, especially when key parameters, such as breeding success, could potentially be influenced by observer activity. For birds, some studies have suggested that nest monitoring can increase the chances of nest failure due to predation, whereas others suggest that human nest visits may actually deter mammalian predators. Nest monitoring visits can also influence breeding success more indirectly by altering parental provisioning behaviour. Here, the influence of monitoring activities on nest success was examined in a ground‐nesting grassland bird, the Whinchat Saxicola rubetra. During the egg phase, a sample of nests were not visited between the initial finding event and the estimated hatching date; instead, the nest status was assessed at a distance. Daily survival rates (DSR) for these nests were compared with that of nests visited every 2 days. During the nestling phase, the effects of observer nest visits on parental provisioning behaviour were determined. Nest visits were found not to affect egg DSR significantly, and parental provisioning was disrupted for a maximum of 20 min (0.52% of the nestling period) following an observer visit. Given the variation in response to nest visits across species, we suggest that consideration should be given to observer impact in all studies where predation risk is high. Here, we illustrate a method for researchers to assess the impact of their nest visits to ensure they are not biasing estimates of breeding success.     

Impact of brood parasitism and predation on nest survival of the fan-tailed gerygone in New Caledonia

Predation and brood parasitism are common reasons for nesting failure in passerine species and the additive impact by invasive species is a major conservation concern, particularly on tropical islands. Recognising the relative contribution of the different components of nesting failure rates is important to understand co-evolutionary interactions within brood parasite–host systems. In the remote archipelago of New Caledonia, the fan-tailed gerygone Gerygone flavolateralis is the exclusive host of the brood-parasitic shining bronze-cuckoo Chalcites lucidus. Additionally, invasive rodents also possibly have an impact on breeding success. To estimate the impact of potential nest predators, we 1) video monitored nests to identify predators, 2) estimated the probability of predation based on nest visibility and predator abundance and 3) tested the possibility that the location of experimental nests and lack of odour cues decrease the predation by rodents. In addition, we estimated nest survival rates using data collected in different habitats over the course of eight breeding seasons. Nesting success of fan-tailed gerygones was relatively low and predation was the main cause of nesting failure. We recorded mainly predation by native birds, including the shining bronze-cuckoo, whereas predation by rats was rare. In open habitats predation by cuckoos was much lower than predation by other avian predators. Neither predator activity around nests nor nest visibility influenced the probability of predation. Experimental nests in more accessible locations and containing an odorous bait were more exposed to rodent predation. Apparently, the fan-tailed gerygone has either never been specifically vulnerable to predation by rats or has developed anti-predator adaptations.     

Nesting success of understory forest birds in central Panama

Greater nest predation in tropical than temperate birds has been hypothesized to be a primary selective force generating latitudinal differences in avian life history traits. Few extensive data sets, however, have been available from tropical forests to compare with data from temperate forests. To increase the amount of empirical information available for addressing issues related to the evolution of life history traits of tropical birds, we measured the nesting success of understory birds in lowland forest of central Panama. We found and monitored the fates of 696 nests of 71 species over two breeding seasons. Daily nest predation rates for the ten species for which we obtained the largest samples ranged from 1.6 to 8.3%, equivalent to a loss of 43 to 92% of nests. These values overlapped extensively the range of daily predation rates experienced by ecologically similar species in North America. Proportion of nests fledging young, estimated with the Mayfield method, was significantly lower in tropical (range: 8 to 57%) than temperate (27 to 60%) species. Nesting success in Panama varied among years, however, being greater in 1996 than 1997. In 1996, nesting success was similar to that of species breeding in forest fragments of midwestern North America. When compared with success of nests in large, contiguous forest tracts of North America, however, tropical avian nesting success was consistently lower by approximately 23%. We conclude that nesting success in central Panama may be poor in most breeding seasons, but also may be punctuated by occasional years of relatively exceptional success, a possibility heretofore unappreciated because of a general paucity of data from the tropics. Furthermore, our results indicate substantial variation in levels of nesting success among species, and almost no variation in clutch size. Such large interspecific variation, as well as potentially large annual variation, in nesting success does not support the hypothesis that uniformly low levels of nesting success select for small tropical clutch sizes.     

Factors Affecting Daily Nest Survival of Burrowing Owls Within Black-Tailed Prairie Dog Colonies

ABSTRACT Identifying environmental parameters that influence probability of nest predation is important for developing and implementing effective management strategies for species of conservation concern. We estimated daily nest survival for a migratory population of burrowing owls (Athene cunicularia) breeding in black-tailed prairie dog (Cynomys ludovicianus) colonies in Wyoming, USA. We compared estimates based on 3 common approaches: apparent nesting success, Mayfield estimates, and a model-based logistic-exposure approach. We also examined whether 8 intrinsic and extrinsic factors affected daily nest survival in burrowing owls. Positive biases in apparent nest survival were low (3–6%), probably because prior knowledge of nest locations and colonial behavior among nesting pairs facilitated discovery of most nests early in the nesting cycle. Daily nest survival increased as the breeding season progressed, was negatively correlated with ambient temperature, was positively correlated with nest-burrow tunnel length, and decreased as the nesting cycle progressed. Environmental features were similar between failed and successful nests based on 95% confidence intervals, but the seasonal midpoint was earlier for failed nests (31 May) compared to successful nests (15 Jun). The large annual variation in nest survival (a 15.3% increase between 2003 and 2004) accentuates the importance of multiyear studies when estimating reproductive parameters and when examining the factors that affect those parameters. Failure to locate and monitor nests throughout the breeding season may yield biased estimates of nesting success in burrowing owls (and possibly other species), and some of the variation in nesting success among years and across study sites may be explained by annual and spatial variation in ambient temperature. Any management actions that result in fewer prairie dogs, shorter burrow lengths, or earlier nesting may adversely affect reproductive success of burrowing owls.     

Absence from the nest due to human disturbance induces higher nest predation risk than natural recesses in Common Eiders Somateria mollissima

Human disturbance of nesting birds may cause reduced breeding success. It is therefore necessary to assess the impact of disturbance to identify steps that minimize negative impacts. We carried out a study of nesting success at two adjacent colonies of Common Eider Somateria mollissima on the islands of Grindøya and Håkøya in northern Norway between 2006 and 2011. Over the study period, nesting success was consistently higher on Håkøya (69–82%) than on Grindøya (35–60%). Between 2009 and 2011 we used camera monitoring of individual nests to identify determinants of nest survival and predation, focusing in particular on the effect of departures from the nest due to human disturbance, which differed between the colonies due to a long‐term research project on Grindøya. Overall, absence of Common Eiders from nests due to disturbance increased the predation risk by a factor of 6.42 for an increase of one additional daily disturbance. In contrast, absence due to natural recesses did not increase nest losses. Under high levels of human disturbance, camera monitoring indicated that the main cause of breeding failure was predation, primarily by Hooded Crows Corvus cornix, but also to some extent Great Black‐backed Gulls Larus marinus. The presence of cameras did not increase the predation risk. Both the presence of researchers and the sight of Common Eider females conspicuously departing from nests are likely to have provided cues to these predators. We suggest management trials to reduce nesting disturbance through the guarding of unoccupied nests to mitigate the effects of human disturbance on reproductive success.     

Factors affecting nesting success of a bird assembly in the central Monte Desert, Argentina

We estimated nesting success of a breeding bird assemblage in the central Monte Desert in four breeding seasons (1995–1999). We analyzed the effect of year, period within the breeding season, nest type, and nest location on daily survival rates (DSR). Averaged nesting success of passeriforms in natural habitats was very low (<20% on average, n=12 species). DSR tended to be lower during the egg-laying stage in Furnariidae, Rhinocryptidae, Mimidae, and Emberizidae. Tyrannidae had the highest DSR for all nesting stages and Emberizidae the lowest. Predation was the main cause of nest failure (>90% on average, n=12 species). DSR differed among years, being higher in the driest year (1995) and lower in the most humid year (1997). Nests initiated in the first half of the breeding season had higher DSR than those initiated in the second half. Closed nests had similar survival rates to open nests. DSR differed according to species of plant that supported the nest. Nest predation in the central Monte Desert may be an important selective pressure in birds' life history, which is in accordance with other results for South American temperate birds.     

No evidence for observer effects on Lark Sparrow nest survival

ABSTRACT Methods for monitoring bird nests might influence rates of nest predation, but the effects of various methods (e.g., visual markers and observer visitation rates) are often separately investigated among disparate avian taxa and geographic regions. Few investigators have explored the potential effects observers might have on nest success of grassland birds, despite concerns regarding population declines of these species in North America. We examined the possible effects of three monitoring techniques on daily nest survival of Lark Sparrows (Chondestes grammacus): (1) presence or absence of visible markers near nests, (2) observer visitation frequency, and (3) presence or absence of data loggers in nests. We monitored 113 Lark Sparrow nests during the 2009 breeding season. Of these nests, 88.5% failed due to predation, abandonment, weather, or unknown causes, yielding an overall nest success estimate of 9.8% based on daily survival estimation. Main effects of each monitoring technique appeared in top (ΔAICc <2) logistic exposure models. However, 95% confidence intervals around parameter estimates for each technique included zero, indicating no significant effects on daily nest survival. Our results suggest that the nest‐monitoring techniques we used had no effect on Lark Sparrow nest success and, if true, nest survival of other songbirds in arid grasslands of the Great Plains may also be unaffected by cautious nest monitoring. However, we cannot rule out the possibility that any effects of the various techniques in our study were masked by locally intense nest predation. Therefore, additional study is needed to determine if there may be observable variation in nest survival among various nest‐monitoring treatments in other areas of the southern Great Plains where nest predation is less frequent.     

Intraseasonal patterns in shorebird nest survival are related to nest age and defence behaviour

Nest survival may vary throughout the breeding season for many bird species, and the nature of this temporal variation can reveal the links between birds, their predators, and other components of the ecosystem. We used program Mark to model patterns in nest survival within the breeding season for shorebirds nesting on arctic tundra. From 2000 to 2007, we monitored 521 nests of five shorebird species and found strong evidence for variation in nest survival within a nesting season. Daily nest survival was lowest in the mid-season in 5 of 8 years, but the timing and magnitude of the lows varied. We found no evidence that this quadratic time effect was driven by seasonal changes in weather or the abundance of predators. Contrary to our prediction, the risk of predation was not greatest when the number of active shorebird nests was highest. Although nest abundance reached a maximum near the middle of the breeding season, a daily index of shorebird nest activity was not supported as a predictor of nest survival in the models. Predators’ access to other diet items, in addition to shorebird nests, may instead determine the temporal patterns of nest predation. Nest survival also displayed a positive, linear relationship with nest age; however, this effect was most pronounced among species with biparental incubation. Among biparental species, parents defended older nests with greater intensity. We did not detect a similar relationship among uniparental species, and conclude that the stronger relationship between nest age and both nest defence and nest survival for biparental species reflects that their nest defence is more effective.     

Coastal saltpans are a good alternative breeding habitat for Kentish plover Charadrius alexandrinus when umbrella species are present

The loss and degradation of natural habitats in coastal areas worldwide has adversely affected many waterbird species, changing their breeding distribution and reducing their productivity. Anthropogenic habitats such as saltpans can provide alternative or complementary habitats for waterbirds and mitigate the increasing human impact on natural coastal habitats. Unvegetated linear paths between salt ponds are used by ground‐nesting waterbird species to breed but their linear structure may facilitate the detection of nests by predators. This negative effect may, however, be counterbalanced by the advantages of breeding in mixed colonies. To evaluate the importance and the risks of breeding in saltpans we used the Kentish plover Charadrius alexandrinus breeding in coastal saltpans of southern Portugal as a model species. Specifically, we assessed the role of nest‐site characteristics, predation and nesting proximity to species with aggressive antipredator behaviour (black‐winged stilt Himantopus himantopus) on their breeding success. Kentish plovers selected nest‐sites on the edges of paths, with about 20% of water around the nest, and a mean visibility of more than 72%; however nest‐site characteristics were not correlated with nesting success. Predation was the main cause of nest loss in saltpans (42%); carrion crows Corvus corone were responsible for most daylight nest predation (58%) and red fox (Vulpes vulpes; 73%) for night predation. An 8‐yr monitoring plan of a Kentish plover population showed a linear increase in their breeding success as their breeding season increasingly overlapped with that of the black‐winged stilt. An experiment with artificial nests showed a significant increase in the number of exposure days (7 to 12) when these nests were within close distance of black‐winged stilt nests. Overall, our results showed that saltpans are an important alternative breeding habitat for the Kentish plover, especially for the maintenance of mixed species colonies.     

Effects of nest-site characteristics and parental activity on cowbird parasitism and nest predation in Brown-and-yellow Marshbirds

ABSTRACT Nest‐site selection and nest defense are strategies for reducing the costs of brood parasitism and nest predation, two selective forces that can influence avian nesting success and fitness. During 2001–2002, we analyzed the effect of nest‐site characteristics, nesting pattern, and parental activity on nest predation and brood parasitism by cowbirds (Molothrus spp.) in a population of Brown‐and‐yellow Marshbirds (Pseudoleistes virescens) in the Buenos Aires province, Argentina. We examined the possible effects of nest detectability, nest accessibility, and nest defense on rates of parasitism and nest predation. We also compared rates of parasitism and nest predation and nest survival time of marshbird nests during the egg stage (active nests) with those of the same nests artificially baited with passerine eggs after young fledged or nests failed (experimental nests). Most nests (45 of 48, or 94%) found during the building or laying stages were parasitized, and 79% suffered at least one egg‐predation event. Cowbirds were responsible for most egg predation, with 82 of 107 (77%) egg‐predation events corresponding to eggs punctured by cowbirds. Nests built in thistles had higher rates of parasitism and egg predation than nests in other plant, probably because cowbirds were most active in the area where thistles were almost the only available nesting substrate. Parasitism rates also tended to increase as the distance to conspecific nests increased, possibly due to cooperative mobbing and parental defense by marshbirds. The proportion of nests discovered by cowbirds was higher for active (95%) than for experimental (29%) nests, suggesting that cowbirds used host parental activity to locate nests. Despite active nest defense, parental activity did not affect either predation rates or nest‐survival time. Thus, although nest defense by Brown‐and‐yellow Marshbirds appears to be based on cooperative group defense, such behavior did not reduce the impact of brood parasites and predators.     

Nesting success of native and introduced forest birds on the island of Kaua‘i

Forests of the Hawaiian archipelago are a global hotspot for conserving avian diversity and contain among the world's most imperiled species. Demographic studies are necessary to determine primary causes of Hawaiian forest bird population declines. We conducted research on the nesting success of multiple bird families on the island of Kaua‘i, allowing us to investigate the importance of factors related to breeding biology on forest bird declines at a community scale. Our study included two Hawaiian honeycreepers, ‘anianiau Magumma parva and ‘apapane Himatione sanguinea, a native monarch flycatcher, Kaua‘i ‘elepaio Chasiempis sclateri, and one introduced species, Japanese white‐eye Zosterops japonicus. Data from 123 nests showed that nesting success ± SE, estimated using program MARK, was low for ‘apapane (0.23 ± 0.10), but did not vary substantially among our other study species (‘anianiau = 0.56 ± 0.09, Kaua‘i ‘elepaio = 0.63 ± 0.08, Japanese white‐eye = 0.52 ± 0.11). Causes of nest loss for 51 nest failures included nest predation (43%), unknown (25%), empty after termination with no signs of nest predation (e.g. eggshell or chick remains in nest, disheveled nest) (24%), and abandoned clutch or brood (4% each). Kaua‘i ‘elepaio suffered more than twice as many nest losses to predation compared to our other study species, but also had the highest nesting success; and, ‘apapane suffered least to nest predation, but had the lowest nesting success. Further, rates of nesting success derived in our study were relatively high compared to multi‐species studies in mainland tropics. Therefore, although nest predation accounted for the greatest proportion of nest failures, it may not be a cause of forest bird population declines in our system. We suggest that future demographic studies focus on post‐fledgling, juvenile, and adult survival, in addition to the importance of double‐brooding and renesting attempts on annual reproductive success.     

Investigator activities reduce nest predation in blackbirds Turdus merula

The effect of investigator perturbance has been traditionally considered detrimental for avian nesting success in terms of enhanced nest predation. This conclusion was however based on a taxonomically biased group of species and, thus, the study of that effect in additional species was essential for reaching a more firm conclusion. Furthermore, although it has been suggested that the effect of nest visiting could also depend on nest predator community, no study has so far tested this hypothesis yet. Trying to detect possible influence of nest‐visiting rates and predator community on nesting success we visited European blackbird Turdus merula nests at two different experimental rates in two populations that considerably differ in the composition of their nest predator communities and natural nest predation rates. Contrary to the traditional ideas, our results not only show that investigator disturbance significantly reduces nest predation, but also that this reduction is maintained in both populations despite the difference in the community of nest predators. We discuss these findings and suggest that predators, especially mammals, might avoid places disturbed by investigators.     

Sources of variation in the nesting success of understory tropical birds

Survival of offspring is a key fitness component and, for birds, the threat of predation on nests is especially influential. Data on rates of nest success from tropical regions are comparatively few, conservation‐relevant, and essential for assessing the validity of models comparing the life histories and behavior or birds across latitudinal gradients. We monitored over 2 000 nests in the lowland forests of central Panama and, using the logistic exposure to model the fate of nests, explored the importance of variation in rate of nest success according to type of nest, height of nests, among years, in early versus late nests, and at different stages of the nest cycle. Analyses of over 1 400 nests for 18 species revealed considerable variation among species in the daily survival rate of nests (range among 18 species=0.91 to 0.98), but nest type and stage of the nesting cycle were generally influential on the probability of nest success. Cavity or enclosed nesters experienced greater nest success than open cup nesters and rates of nest loss were generally greatest in the nestling stage. We found limited evidence that height of nests affected probability of success, but no indication that timing of nesting effort was influential. Despite the occurrence of a severe ENSO event during our sampling, annual variation in nest success was not consistent among species. Interspecific variation in the rates and patterns of nest predation in our study, coupled with reports of high rates of nest loss at temperate latitudes, lead us to question long standing assumptions about latitudinal trends in rates of nest loss. We urge further work to understand the implications of nest predation on the evolutionary ecology of tropical birds.     

Nesting success in Redshank Tringa totanus breeding on coastal meadows and the importance of habitat features used as perches by avian predators

Capsule Nest survival rates could not be explained by distance to habitat edges or other features used by predators.

Aims To investigate if predation on Redshank nests was affected by habitat characteristics at a local scale.

Methods We examined survival rates of Redshank nests on coastal meadows on the Baltic island of Gotland, Sweden, over two breeding seasons. We analysed nest survival rates in relation to several habitat characteristics that may benefit predators searching for nests. We examined existing studies concerning predation rates on wader nests in relation to edges and habitat features potentially used by avian predators.

Results We found no significant effects of distance to habitat edge or to nearest potential lookout for avian predators or to shoreline. Abundance of Lapwings Vanellus vanellus, an aggressive species with active nest-defence, did not have any significant effect on nest survival rate, nor did vegetation concealment of nests. Nest survival rates were significantly different between years and lower later in the season.

Conclusions There is only weak support for general effects on wader nest predation rates of proximity to edges and features used by avian predators. Simple mechanical management actions such as removal of trees and bushes on coastal meadows may not directly, and by itself, result in higher reproductive success of waders. Further understanding is needed of the behaviour of predators and the composition of the predator community in different landscapes in order to increase the efficiency of management actions to remove threats to vulnerable species on coastal meadows.     

Are goose nesting success and lemming cycles linked? Interplay between nest density and predators

The suggested link between lemming cycles and reproductive success of arctic birds is caused by potential effects of varying predation pressure (the Alternative Prey Hypothesis, APH) and protective association with birds of prey (the Nesting Association Hypothesis, NAH). We used data collected over two complete lemming cycles to investigate how fluctuations in lemming density were associated with nesting success of greater snow geese ( Anser caerulescens atlanticus ) in the Canadian High Arctic. We tested predictions of the APH and NAH for geese breeding at low and high densities. Goose nesting success varied from 22% to 91% between years and the main egg predator was the arctic fox ( Alopex lagopus ). Nesting associations with snowy owls ( Nyctea scandiaca ) were observed but only during peak lemming years for geese nesting at low density. Goose nesting success declined as distance from owls increased and reached a plateau at 550 m. Artificial nest experiments indicated that owls can exclude predators from the vicinity of their nests and thus reduce goose egg predation rate. Annual nest failure rate was negatively associated with rodent abundance and was generally highest in low lemming years. This relationship was present even after excluding goose nests under the protective influence of owls. However, nest failure was inversely density-dependent at high breeding density. Thus, annual variations in nest density influenced the synchrony between lemming cycles and oscillations in nesting success. Our results suggest that APH is the main mechanism linking lemming cycles and goose nesting success and that nesting associations during peak lemming years (NAH) can enhance this positive link at the local level. The study also shows that breeding strategies used by birds (the alternative prey) could affect the synchrony between oscillations in avian reproductive success and rodent cycles.     

Interactive effects of time and vegetation on reproduction of redshanks (Tringa totanus) breeding in Wadden Sea salt marshes

As shown for various species, nesting waders are non-randomly distributed on wetlands and preferentially select riparian nest-sites adjacent to limnic or marine waterbodies. Studying the redshank Tringa totanus, we tested the hypotheses that, in a coastal wader species which conceals its clutch in vegetation, predation and hatching success are affected by vegetation zonation, and that breeding in lower salt marsh areas has negative consequences for reproduction. We further predicted effects of timing of breeding and breeding experience/age of adults potentially reflected by egg biometrics both on nest-site selection and reproduction. Effects of vegetation, space, time and individual quality on hatching success of redshanks were studied in the German part of the Wadden Sea. Dominant plant species, vertical vegetation structure and nest concealment varied significantly between nests. Variation in nest concealment was relatively low: about 90% of clutches were classified as being well concealed. This variation was explainable by vegetation structure but not by vegetation composition at the nest-site, distance to shoreline, and time of clutch initiation. Vertical vegetation structure varied by dominant plant species but not by distance to shoreline and time of clutch initiation. Hatching success of clutches was low (10.6%) due to high predation (daily predation rate: 7.4%). Hatching success and duration of clutch survival were negatively and predation positively related to the date of clutch initiation. Furthermore, negative relationships were found between egg size and predation and duration of survival, respectively. We assume that concealed nests, early breeding and breeding experience diminish predation in salt marsh breeding redshanks. Thus, redshank reproduction appears to be affected by interactive effects of timing of breeding and vegetation facilitating early breeding. In contrast to open-nesting species, breeding in riparian habitats next to waterbodies may be disadvantageous for species breeding concealed in vegetation if these are covered by less structured vegetation.     

Spatiotemporal patterns of duck nest density and predation risk: a multi‐scale analysis of 18 years and more than 10 000 nests

Many avian species are behaviorally‐plastic in selecting nest sites, and may shift to new locations or habitats following an unsuccessful breeding attempt. If there is predictable spatial variation in predation risk, the process of many individuals using prior experience to adaptively change nest sites may scale up to create shifting patterns of nest density at a population level. We used 18 years of waterfowl nesting data to assess whether there were areas of consistently high or low predation risk, and whether low‐risk areas increased, and high‐risk areas decreased in nest density the following year. We created kernel density maps of successful and unsuccessful nests in consecutive years and found no correlation in predation risk and no evidence for adaptive shifts, although nest density was correlated between years. We also examined between‐year correlations in nest density and nest success at three smaller spatial scales: individual nesting fields (10–28 ha), 16‐ha grid cells and 4‐ha grid cells. Here, results were similar across all scales: we found no evidence for year‐to‐year correlation in nest success but found strong evidence that nest density was correlated between years, and areas of high nest success increased in nest density the following year. Prior research in this system has demonstrated that areas of high nest density have higher nest success, and taken together, our results suggest that ducks may adaptively select nest sites based on the local density of conspecifics, rather than the physical location of last year's nest. In unpredictable environments, current cues, such as the presence of active conspecific nests, may be especially useful in selecting nest sites. The cues birds use to select breeding locations and successfully avoid predators deserve continued attention, especially in systems of conservation concern.     

The influence of nest-site characteristics on the nesting success of the Karoo Prinia (Prinia maculosa)

Choice of nest site has important consequences for nest survival. We examined nest-site characteristics relative to nest success in Karoo Prinias breeding in coastal dwarf shrubland, where high nest predation is the main cause of nest failure. Initially, we compared nests that failed during the building, laying, incubation and nestling stages and those from which young were successfully raised, to test whether nests that survived to progressive stages in the nesting cycle differed in their nest-site characteristics. Subsequently, we compared the characteristics of successful nests with those of unsuccessful nests. The nest-site characteristics considered included nest height, nest-plant height, nest-plant species, distance from lateral foliage edge, nest concealment, nest-patch heterogeneity and vegetation cover at four different heights. We were unable to distinguish between the nest-site characteristics of nests that failed during the various stages of the nesting cycle. Concealment was the main nest-site characteristic that differentiated successful nests from unsuccessful nests, with successful nests being located in more concealed sites. The other variables that contributed to the discrimination between successful and unsuccessful nests by discriminant function analysis included nest-plant type and distance from edge, which are also directly related to concealment. This suggests that nest concealment is the most important variable influencing nesting success at this site, which has a preponderance of visually-oriented predators.     

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.