Effects of Mesopredators on Nest Survival of Shrub-Nesting Songbirds

ABSTRACT Although nest predation is often the single largest source of mortality in avian populations, manipulative studies to determine predator impacts on nest survival are rare, particularly studies that examine impacts of mid-size mammalian predators (hereafter, mesopredators) on nest survival of shrub-nesting birds. We quantified nest survival and identified nest predators of shrub-nesting songbirds within 4 large (approx. 40-ha) exclosures and 4 control sites within a longleaf pine (Pinus palustris) ecosystem. During 2003–2006, we located and monitored 535 shrub nests (222 with videography) for 4,804 nest-days to quantify daily nest survival and document predation events. We found no support for a treatment effect, suggesting mesopredators had little impact on daily nest survival (0.9303 in controls and 0.9260 in exclosures) of shrub-nesting songbirds. For the 5 most commonly monitored species, daily nest survival within species was constant. Our analysis suggested that shrub nests were most vulnerable during the nestling stage and presence of cameras on nests increased survival with the increase in survival being more pronounced during the incubation stage. We filmed 107 nest predation events, identifying predators at 88 nests. Of these 88 nests, snakes caused 33%, red imported fire ants (hereafter fire ants, Solenopsis invicta) 28%, raptors 17%, corvids 8%, mesopredators 6%, and small mammals 8% of nest predations. Cause-specific nest predation in controls and exclosures did not differ from expectation, providing evidence that compensatory predation did not occur. Nest predators differed from expectation with regard to nest stage; fire ants and raptors only depredated nests during the nestling stage. Presence of cameras had no effect on nest abandonment. Fire ants were the most prevalent nest predator, and nest predation by fire ants was only observed on nestlings, potentially reducing likelihood of renesting. Magnitude and timing of fire ant predation suggests that fire ants may be the most influential nest predator of shrub-nesting birds within the longleaf pine ecosystem. Our data suggest that controlling mesopredators will have no effect on nest success of shrub-nesting birds within longleaf pine forests.     

Nest‐site selection by golden plover: why do shorebirds avoid nesting on slopes?

It is widely known that many upland-breeding shorebirds tend to nest on plateaus but to date no studies have put forward explanations for this phenomenon. We examined the effect of slope and habitat on the distribution of ground-nesting golden plover Pluvialis apricaria at two study sites in County Durham, U.K. Golden plovers showed strong selection for nesting on flat ground. Habitat significantly affected nest-site location on one study site (heather burnt within the past 2 years was favoured and older stands of heather were avoided) but not on the other. Fifty-nine per cent of all nests failed. We attributed 95% of all losses to predation. Seventy-five per cent of nests, in which the predator was identified, were taken by ground predators, mainly stoats Mustela erminea . Nests on flat ground had significantly higher rates of survival than those on slopes. Nest survival did not vary significantly with habitat type nor with vegetation height or density around the nest. Neither habitat type, vegetation height nor vegetation density around nests differed between nests on slopes and on flat ground. In addition, individuals that nested on flat ground tended to have less black on their underparts (a suggested indicator of dominance). We suggest that birds nesting on slopes are less efficient at avoiding nest predation than those nesting on the flat. The nature or degree of a bird's response to a predator may be related to the efficiency of individual anti-predator responses and/or to visibility from the nest. This study cannot differentiate between these two explanations. We encourage further work to investigate differences in behaviour between individuals nesting on slopes and those on flat ground.     

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.     

Relationship Between Nest Predation Suffered by Hosts and Brown-Headed Cowbird Parasitism: A Comparative Study

There are at least four main hypotheses that may explain how the evolution of host selection by avian brood parasites could be linked to nest predation among their potential hosts. First, selection may have favoured parasite phenotypes discriminating among hosts on the basis of expected nest failure. Second, parasitized nests may be more easily detected by predators and extra costs of parasitism may accelerate the evolution of host defences. Third, selection may have favoured predator phenotypes avoiding parasitized nests because parasitism enhances nest defence. Fourth, female brood parasites may directly or indirectly induce host nesting failures in order to enhance future laying opportunities. We collected data on brood parasitism and nest failure due to predation to test these hypotheses in a comparative approach using North American passerines and their brood parasite, the brown-headed cowbird Molothrus ater. Under the hypotheses 1 or 3 we predicted brood parasitism to be negatively associated with nest predation across species, whereas this relation is expected to be positive if hypotheses 2 or 4 are true. We demonstrate that independent of host suitability, nest location, habitat type, length of the nestling period, body mass and similarity among species due to common ancestry, species experiencing relatively high levels of nest predation suffered lower levels of cowbird parasitism. Our results suggest a previously ignored role for nest predation suffered by hosts on the dynamics of the coevolutionary relationships between hosts and avian brood parasites. Co-ordinating editor: Dr. F. Stuefer     

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

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

Strategies for Nest-Site Selection by King Eiders

ABSTRACT Nest site selection is a critical component of reproduction and has presumably evolved in relation to predation, local resources, and microclimate. We investigated nest-site choice by king eiders (Somateria spectabilis) on the coastal plain of northern Alaska, USA, 2003–2005. We hypothesized that nest-site selection is driven by predator avoidance and that a variety of strategies including concealment, seclusion, and conspecific or inter-specific nest defense might lead to improved nesting success. We systematically searched wetland basins for king eider nests and measured habitat and social variables at nests (n = 212) and random locations (n = 493). King eiders made use of both secluded and concealed breeding strategies; logistic regression models revealed that females selected nests close to water, on islands, and in areas with high willow (Salix spp.) cover but did not select sites near conspecific or glaucous gull (Larus hyperboreus) nests. The most effective nest-placement strategy may vary depending on density and types of nest predators; seclusion is likely a mammalian-predator avoidance tactic whereas concealment may provide protection from avian predators. We recommend that managers in northern Alaska attempt to maintain wetland basins with islands and complex shorelines to provide potential nest sites in the vicinity of water.     

Does cowbird parasitism increase predation risk to American redstart nests?

Brood parasitism and nest predation are major causes of reproductive failure for many bird species nesting in fragmented landscapes. While brood parasites and predators may act independently, they could also interact if brood parasites increase the likelihood that predators detect nests. In this study, we examined the interaction between cowbird parasitism and nest predation in a 10 year study on 466 American redstart Setophaga ruticilla nests in central Alberta, Canada. We used advanced nest survival models to examine the support for three mechanisms that might lead to a positive correlation between brood parasitism and nest predation: 1) the presence of a cowbird nestling might increase the detection of the nest by predators, 2) nests with lower cover are more likely to be detected by both cowbirds and predators, and 3) cowbirds and predators may co-occur in landscapes of similar structure. Twelve percent of nests were parasitized and those nests had a 16–19% higher rate of failure due to predators compared to unparasitized nests. Daily nest predation rates increased during the nestling stage for both groups, but more strongly for parasitized nests. Loud begging by the cowbird nestling and/or higher parental feeding rates for the cowbird may have increased nest detectability to predators. Brood parasitism and nest predation were also positively related to forest cover, indicating landscape level effects were influential. Most nest predators were forest species and we suspect cowbirds responded positively to forest cover because of the increased abundance of songbird hosts. Nest-site features had less of an impact on nest predation or brood parasitism, although nests with higher overhead cover were less susceptible to predators. Our study shows how multiple mechanisms, particularly the behavioral effects of the brood parasite nestling and landscape structure, can lead to a positive relationship between nest predation and brood parasitism.     

Nest predators of Lance-tailed Manakins on Isla Boca Brava, Panamá

ABSTRACT.   Nest predation is often the primary cause of nest failure for passerines. Despite this, little is known about predation rates and the nest predators of birds in the tropics. I used video cameras to monitor seven Lance-tailed Manakin ( Chiroxiphia lanceolata ) nests on Isla Boca Brava, Panamá. One nest fledged young and six nests failed due to predation. I recorded five predation events involving four avian predators and one mammalian predator. Crested Oropendolas ( Psarocolius decumanus ) predated two nests and a Roadside Hawk ( Buteo magnirostris ) and a Black-chested Jay ( Cyanocorax affinis ) each predated one. The mammalian predator was a common opossum ( Didelphis marsupialis ). All avian predation was diurnal; the mammalian predation was nocturnal. My results suggest that tropical birds are subject to a diverse suite of nest predators, and that avian predators may be an important cause of nest failure at my study site.     

Nest predation in forest birds: influence of predator type and predator's habitat quality

We used the introduction of a generalist nest predator, the red squirrel Tamiasciurus hudsonicus, and of a large herbivore, the Sitka black-tailed deer Odocoileus hemionus sitkensis, to the islands of Haida Gwaii (Queen Charlotte Islands, British Columbia, Canada) to study how predator assemblage and habitat quality and structure influenced nest predation in forest birds. We compared losses of natural nests to predators on islands with and without squirrels. We selected nine islands with or without squirrel or deer and used 506 artificial nests put on the ground or in shrubs to further analyse variation of nest predation with predator assemblage and habitat quality for the predators. For both natural and artificial nests predation risk was higher in presence of squirrels. But predation risk varied within island categories. In presence of squirrels it was highest in stands with mature conifers where it fluctuated from year to year, in response to fluctuations in squirrel abundance. Vegetation cover around the nest had little effect on nest predation by squirrels. Where squirrels were absent, nest predation concentrated near predictable food sources for corvids, the main native predators, and increased with decreasing vegetation cover, suggesting that removal of the vegetation by deer increased the risk of predation by native avian nest predators that use visual cues. Predation risk in these forests therefore varies in space and time with predator composition and with quality of the habitat from the predators' perspective. This temporal and spatial variation in predation risk should promote trade-offs in the response of birds to nest predation, rather than fine-tuned adaptations to a given predation pattern.     

Black-capped vireo nest predator assemblage and predictors for nest predation

Nest predation is a major limiting factor for songbird productivity, including the federally endangered black-capped vireo (Vireo atricapilla). However, nest predator information is limited across the range of the black-capped vireo in central and southwest Texas. We monitored nests in 3 counties within the breeding range of black-capped vireos in Texas in 2008 and 2009 and used continuous recording digital video cameras to record predation events. We video-monitored 115 nests and documented 39 predation events by at least 9 predator species. Overall, we observed avian species (51%, n = 39), specifically brown-headed cowbirds (Molothrus ater; n = 12), and snakes (26%, n = 39) as the most frequent nest predators. The estimated daily nest survival rate during the laying and incubation stage was 0.985 (95% CI = 0.967–0.993) and 0.944 (95% CI = 0.921–0.961) during the nestling stage. In addition, we analyzed models of predator-specific nest predation using multinomial logistic regression. Effect of nest height on predation rate was significant for snakes; nest stage was significant for nests depredated by avian predators. By identifying and increasing our knowledge of nest predators and vegetation characteristics associated with greater risk of predation in multiple locations within the black-capped vireo's range, we can effectively manage habitat to benefit recovery efforts of the species. © 2012 The Wildlife Society.     

Nest predators and the evolution of avian reproductive strategies: a comparison of Australian and New Zealand birds

Summary Nest predation has been considered an important factor in the evolution of avian life histories: smaller clutches and shorter incubation and nestling periods are expected where nest predation has significant effects on reproductive success. Unlike the Australian avifauna, terrestrial New Zealand birds have evolved in the absence of reptilian and mammalian predators. Here we compare the reproductive strategies of terrestrial native New Zealand birds with those of their Australian sister taxa. In 11 of 14 comparisons, New Zealand birds were larger than their Australian relatives, but we did not find any significant differences in reproductive tactics between the two regions, a result inconsistent with the nest predation hypothesis. We discuss several reasons why this may be so. One possibility is that selection imposed on avian life history tactics by mammalian predators following the arrival of humans in New Zealand has led to strategies similar to those adopted in Australia.     

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.     

Foraging behaviour of nest predators at open-cup nests of woodland passerines

Nest predation patterns and processes cannot be understood without studying the behaviour of predators. I videotaped the behaviour of 22 species of predators at 171 depredated nests of 13 passerine species, in woodland in the Czech Republic. About 32% (60/187) of all events occurred during the night; mammals accounted for 95% (57/60) and 22% (28/127) of nocturnal and diurnal predation, respectively. About 67% (57/85) of mammalian predation, but only 3% (3/102) of avian predation, occurred during night. Multiple predations by the same species were detected in at least 7% (6/82) and 42% (37/88) of nests depredated by mammals and birds, respectively. Martens Martes martes/foina took nest content mostly all at once; birds (mainly Jay Garrulus glandarius) revisited partially depredated nest during 1–4 consecutive days. Martens stayed at the depredated nest about five times longer than Jays. Martens spent similar time at nests with eggs and nestling, while Jays stayed about twice longer at nests with eggs. Mammals consumed eggs always at the nest (23/23), but took nestlings away in at least 48% (31/64) cases. Birds took the eggs and nestling away in at least 31% (18/58) and 76% (71/94) cases, respectively. Predator visits to active nests without taking the content, repeated partial predation and revisitation of previously depredated nests suggest an effect of memory on predator’s foraging behaviour.     

Nest-site preference and maternal effects on offspring growth

Maternal preferences for oviposition sites are assumed to beadaptive, but offspring fitness is not always higher at preferredsites and, thus, further study of the selection pressures thatinfluence oviposition behavior is warranted. Among birds, predationis regarded as the primary agent of selection on nest-site microhabitatpreferences, but alternatives are rarely considered. We testedthe hypothesis that avian nest-site preferences are an adaptiveresponse to fitness costs imposed by variation in nest-sitemicroclimate. We documented that Chestnut-collared Longspurs(Calcarius ornatus) strongly preferred to orient nests towardsthe southeast and showed that this preference influenced microclimate:nests facing southeast had the highest midday temperatures.Yet, preferences were not adaptive because nestlings in nestswith the preferred orientation gained mass at a slower rate,had retarded skeletal growth, and reached a smaller final size.We experimentally tested this result by altering orientationof nests and confirmed, for the first time, that variation innestling growth was causally linked to variation in nest microclimatearising from nest-orientation preferences. Adults respondedto the high temperatures at preferred southeast-facing nestsby spending more time shading young from the sun, apparentlyattempting to ameliorate heat costs. This response, however,resulted in parents spending less time feeding young, potentiallyexplaining slower growth in these nests. Direct effects of highertemperatures may also play a role in slower growth. Althoughwe lack an explanation for this apparently maladaptive preference,these results demonstrate that nest-site choices of birds canyield fitness costs imposed by variation in nest microclimate.     

Coping with Shifting Nest Predation Refuges by European Reed Warblers Acrocephalus scirpaceus

Predation, the most important source of nest mortality in altricial birds, has been a subject of numerous studies during past decades. However, the temporal dynamics between changing predation pressures and parental responses remain poorly understood. We analysed characteristics of 524 nests of European reed warblers monitored during six consecutive breeding seasons in the same area, and found some support for the shifting nest predation refuge hypothesis. Nest site characteristics were correlated with nest fate, but a nest with the same nest-site attributes could be relatively safe in one season and vulnerable to predation in another. Thus nest predation refuges were ephemeral and there was no between-season consistency in nest predation patterns. Reed warblers that lost their first nests in a given season did not disperse farther for the subsequent reproductive attempt, compared to successful individuals, but they introduced more changes to their second nest sites. In subsequent nests, predation risk remained constant for birds that changed nest-site characteristics, but increased for those that did not. At the between-season temporal scale, individual birds did not perform better with age in terms of reducing nest predation risk. We conclude that the experience acquired in previous years may not be useful, given that nest predation refuges are not stable.     

Testing sources of variation in nestling‐stage nest success of Florida Scrub‐Jays in suburban and wildland habitats

Human modification of habitats can reduce reproductive success by providing novel cues to which birds may respond with behaviors that are actually maladaptive in those environments. Ad libitum human‐provided foods may provide the perception that urban habitats are food‐rich even as natural food availability decreases. Similarly, human activity may increase the perception that predation risk is high even as natural predators may decrease in abundance. In response, birds may reduce parental care with a subsequent cost to successful reproduction. Florida Scrub‐Jays (Aphelocoma coerulescens) in suburban areas have lower nest success during the nestling period than do wildland jays, possibly the result of such maladaptive responses, but maybe because of ecological differences with wildlands. We manipulated adult perception of predation risk and the availability of nestling foods in suburban and wildland areas to determine if these factors influenced parental care and nestling begging, and if the behavioral responses of adults influence nest survival during the nestling stage. Experimentally increasing perception of predation risk reduced parental care by both suburban and wildland females, but did not influence care by males. Increasing food availability, but not predation risk, had little influence on parental care, but resulted in decreased nestling begging rates and an increase in the frequency (pitch) of begging calls in both habitats. However, neither parental care nor food availability influenced nest survival during the nestling stage. Instead, the presence of helpers was the most important variable in nest survival analyses, suggesting that habitat‐specific differences in nest survival during the nestling stage were not simply the result of maladaptive parental behavior or shortage of nestling food resources in the suburban habitat. The lack of helpers combined with ecological differences, such as the abundance of nest predators, may be why fewer nests of Florida Scrub‐Jays survive during this stage in suburban areas.     

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.     

Nest position and type affect predation rates of artificial avian nests in the tropical lowland forest on Mount Cameroon

Nest predation is the leading cause of reproductive failure in birds and thus it shapes their life history strategies. Intensities of nest predation appear to differ among nest locations and types in both temperate and tropical regions. However, there is limited knowledge of factors influencing susceptibility of avian nests to predation in Africa. The aim of our study was to investigate artificial nest predation rates of different ground and shrub nests located at different heights in the rainforest undergrowth. We placed artificial avian nests within a homogeneous lowland forest interior with sparse forest undergrowth in the Mount Cameroon National Park, Cameroon. We exposed three sets of nests: 50 bare-ground, 50 cup-ground and 50 cup-shrub nests, for 10 d. Predation was higher for cup-ground nests compared to cup-shrub nests, and bare-ground nests were more depredated than cup-ground nests. We concluded that the presence of a cup as well as higher nest position significantly increased probability of artificial nest survival. The results of this study suggest a potential selection pressure on nest type and placement in lowland forest birds for a poorly known tropical region.     

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.     

Anti-predator adaptations in nesting Marsh Tits Parus palustris: the role of nest-site security

A proposal that nest predation is the most important selective pressure shaping nest-site use of Marsh Tits Parus palustris was checked in a long-term study (almost 500 nests) carried out in the Białowieża National Park (eastern Poland) in primeval conditions, where the birds breed in natural holes and their nests are at risk from a variety of predators. It was predicted that predation rates would depend on hole attributes, so that Marsh Tits should use the most secure holes. Predation was responsible for 70% of total nest losses. Predation risk depended on hole attributes. Nests in dead wood were predated significantly more often than those in live wood. Nests in old woodpecker holes were predated more frequently than nests in holes of other origin, and nests situated closer to the entrance were more at risk than ones further from it. The entrance size did not influence overall predation risk, but small entrance size was important in preventing access by larger predators. These differences can account for the very rare use of holes in dead wood or of woodpecker holes, the fact that they nest in holes with small entrances and relatively far from the entrance. It is concluded that the patterns of nest-site use found in this species are best explained as anti-predator adaptations, which have evolved and are maintained by the pressure of nest predators. This study also indicates the possible limits of, and constraints on, these adaptations.