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.     

Nesting Success of Costa Rican Lowland Rain Forest Birds in Response to Edge and Isolation Effects

Although open-cup nesting birds generally face increased risk of nest depredation from forest edge predators and brood parasites in fragmented temperate landscapes, little information exists to assess such risks in tropical birds. We compared nesting success of real birds' nests in large and small forest fragments to a control site in Caribbean lowland wet forest of Costa Rica. Pooling across species, nesting success was significantly greater in unfragmented forest than in either small, isolated fragments or the La Selva Biological Reserve, which is at the tip of a forest 'peninsula' embedded in a largely deforested landscape. Nesting success in isolated fragments did not vary according to distance from edge, suggesting that predators in fragments act throughout these forest patches. The case for increased nest predation as a plausible mechanism to explain the documented decline of forest interior bird populations in this fragmented tropical landscape is enhanced by a simple demographic model that suggests nesting success is likely too low to maintain populations at La Selva and in the fragments. The fact that the large (> 1000 ha) La Selva forest reserve is experiencing nest predation rates similar to those in much smaller fragments is cause for concern. Our results make a strong case for additional studies to document the identities of nest predators in both fragmented and unfragmented forests in such tropical forest landscapes.     

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     

Large forests enhance songbird nesting success in agricultural‐ dominated landscapes of the Midwestern US

We studied the effects of habitat composition and distance from edges on nesting success and brood parasitism of forest birds in the Kaskaskia River Bottoms, one of the largest remaining tracts of floodplain forest in the agricultural Midwestern United States. Our goal was to help the private landowners, who have maintained this region in forest cover, enhance the value of these forests for nesting birds. We measured nest predation rates and levels of brood parasitism of four species, the indigo bunting Passerina cyanea, Acadian flycatcher Empidonax virescens, northern cardinal Cardinalis cardinalis and prothonotary warbler Protonotaria citrea in relation to distances from natural and anthropogenic edges and proportion of natural and anthropogenic habitats within fixed radii around nests. We predicted that nesting success would increase with increasing distance from anthropogenic habitats and with increasing land cover in natural habitats. Our results showed no strong effect of any of these variables on avian nesting success, although parasitism levels increased slightly with increasing proportion of agricultural land around nests for two of the species. Nevertheless, nesting success for at least three of these species was much higher than in more fragmented forest tracts elsewhere in the agricultural Midwest where most forest tracts appear to be population sinks for most species. These results suggest that forest tracts in the Kaskaskia may be saturated with nest predators and brood parasites, but are not super‐saturated in a way that would cause these tracts to become ‘black hole’ population sinks. Our data further suggest that, as long as landowners maintain their private landholdings in forest cover, the details of how they manage their land may have little effect on songbird nesting success. These results also suggest that reforestation efforts in areas with many openings may still benefit forest birds.     

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.     

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.     

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.     

Opportunistic Predation of Wild Turkey Nests by Wild Pigs

Wild pigs (Sus scrofa; i.e., feral hogs, feral swine) are considered an invasive species in the United States. Where they occur, they damage agricultural crops and wildlife habitat. Wild pigs also depredate native wildlife, particularly ground-nesting bird species during nesting season. In areas inhabited by wild turkeys (Meleagris gallopavo), nest destruction caused by wild pigs may affect recruitment. There is debate whether wild pigs actively seek ground-nesting bird nests or depredate them opportunistically. To address this debate, in 2016 we examined the movements of wild pigs relative to artificial wild turkey nests (i.e., control [no artificial nests], moderate density [12.5–25 nests/km²], and high density [25–50 nests/km²]) throughout the nesting season (i.e., early, peak, and late) in south-central Texas, USA. We found no evidence that wild pigs learned to seek and depredate wild turkey nests relative to nest density or nesting periods. Despite wild pigs being important nest predators, depredation was not a functional response to a pulsed food resource and can only be associated with overlapping densities of wild pigs and nests. Protecting reproductive success of wild turkeys will require reducing wild pig densities in nesting habitat prior to nesting season. © 2019 The Wildlife Society.     

Effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens

We used presentations of models to determine the effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens against a nest predator (Blue Jay Cyanocitta cristata ) and a brood parasite (Brown-headed Cowbird Molothrus ater ). Principal components analysis (PCA) of four component variables of nest defence (call rate, swoop rate, closest approach and number of adults) generated a measure of overall nest defence (aggression). We determined effectiveness of defence by looking for correlations between measures of defence and measures of nest success (nest predation and brood parasitism). We also determined whether nest defence increased with clutch size, nestling age and time in the breeding season. Defence against model Brown-headed Cowbirds did not correlate with levels of parasitism, clutch size, age of young or time of breeding. There was, however, a strong, but insignificant, trend for nests with high levels of all measures of defence to suffer less from brood parasitism. Aggression, vocalization rate, closest approach and number of adults defending against models of predatory Blue Jays correlated positively with nesting success during the egg stage but not the nestling stage of the nesting cycle. Aggression, vocalization rate, closest approach correlated with clutch size and age of the brood. These results suggest that nest defence can effectively deter nest predators, but may be less effective against brood parasites. Different behavioural components of nest defence may work at different stages of the nest cycle and against different nest predators. The components of nest defence that correlated with nest success also correlated with clutch value, a result consistent with hypotheses on the evolution of nest defence.     

The role of reed management and habitat quality on brood parasitism and chick survival of the brood parasitic Common Cuckoo

Despite efforts on ecosystem restoration and management, biodiversity loss remains one of the major environmental concerns of our time. Beyond the focus on threatened species, animals that indicate regional biodiversity hotspots and population trends, such as brood parasites, should also be targeted by conservation actions. We studied how reed habitat quality and management influence brood parasitism rate and offspring survival in Common Cuckoos Cuculus canorus parasitizing nests of Great Reed Warblers Acrocephalus arundinaceus in six reed habitats in an intensive agricultural landscape. Data collected from 45 sites over 13 years showed that the brood parasitism rate was highest on large canals and was positively influenced by the availability of potential perches (Cuckoo vantage points) and the height where host nests were built. Cuckoo chick survival decreased with water depth and was not affected by other factors. Our results suggest that the habitat-dependent detectability of host nests was central in brood parasitism rate and that water level was central in Cuckoo chick survival. Our study shows that a maintenance of intermediate water levels is the most optimal for maintaining Cuckoo populations in intensive agricultural landscapes. Because brood parasites are excellent bioindicators as their presence predicts regional hotspots of taxonomic and functional diversity as well as population trends in bird communities, knowledge on their habitat requirements is relevant in management targeting diverse bird communities.     

Nest height is affected by lamppost lighting proximity in addition to nestbox size in urban great tits

Both natural and artificial light have proximate influences on many aspects of avian biology, physiology and behaviour. To date artificial light at night is mostly considered as being a nuisance disrupting for instance sleep and reproduction of diurnal species. Here, we investigate if lamppost night lighting affects cavity‐nesting bird species inside their breeding cavity. Nest height in secondary cavity‐nesting species is the result of trade‐offs between several selective forces. Predation is the prevailing force leading birds to build thin nests to increase the distance towards the entrance hole. A thin nest may also limit artificial light exposure at night. Yet, a minimum level of daylight inside nesting cavities is necessary for adequate visual communication and/or offspring development. Against this background, we hypothesised that avian nest‐building behaviour varies in response to a change in night lighting. We monitored nest height of urban great tits Parus major during six years and found that it varied with artificial light proximity. The birds built thinner nests inside nestboxes of various sizes in response to increasing lamppost night light availability at the nest. In large nestboxes, the nests were also thinner when a lamppost was present in the territory. Whether this relationship between artificial night lighting and nest height reflects a positive or negative effect of urbanisation is discussed in the light of recent experimental studies conducted in rural populations by other research groups.     

Impacts of Argentine ants on avian nesting success

Biological invasions can have severe and widespread impacts on ecological communities. A few species of ants have become particularly damaging invaders but quantitative data of their impacts on many taxa is still lacking. We provide experimental evidence using artificial nests baited with quail eggs that the invasive Argentine ant (Linepithema humile) can be a significant avian nest predator – Argentine ants recruited to more nests and in higher abundance than the native ant species they displace. However, at a site invaded by Argentine ants, we monitored over 400 nests of a ground-nesting species, the Dark-eyed Junco (Junco hyemalis), and found that less than 2% of nests failed as a result of Argentine ant predation/infestation. A review of the literature also suggests that Argentine ants may not be a serious threat to bird nests relative to other predators or parasites. However, invasive ants with the capability of overwhelming prey though stinging (specifically the red-imported fire ant, Solenopsis invicta), may have a higher impact on avian nesting success. Received 14 January 2005; revised 28 April 2005; accepted 12 May 2005.     

Of fleas and geese: the impact of an increasing nest ectoparasite on reproductive success

Past studies on the relationship between nest ectoparasites and avian fitness have been primarily limited to altricial hosts. Life history strategies of precocial and altricial birds vary considerably, limiting our ability to infer the effect of nest parasites on fitness of precocial species. Ross's Chen rossii and lesser snow goose Chen caerulescens caerulescens populations have been growing at unprecedented high rates. New limiting factors on vital rates of these precocial birds may arise after populations have been released from previously regulating factors. The flea Ceratophyllus vagabundus vagabundus is an apparently newly emerging nest parasite in the arctic goose colony at Karrak Lake, Nunavut, Canada. We examined the relationship between flea abundance (measured by the proportion of goose eggs covered by blood in each nest) and goose reproductive success from 2001–2004. In three of four years of study, nest success was inversely related to flea abundance in nests. Despite the potential for high costs to individuals, the overall effects of fleas on goose nesting success have thus far been small. We demonstrated that nest parasites negatively influence reproductive success of precocial bird hosts despite host life history strategy of leaving the nest quickly after hatch, which results in minimal exposure to nest parasites compared to altricial birds that raise their young in the nest.     

Snake behavior and seasonal variation in nest survival of northern cardinals Cardinalis cardinalis

Seasonal variation in nest success is well documented for many bird species. Predator behavior has been suggested as a mechanism behind these seasonal patterns, but this hypothesis has received little attention. Here we test the hypothesis that predator behavior produces seasonal patterns of nest success by relating nest success of northern cardinals Cardinalis cardinalis to the activity of Texas rat snakes Elaphe obsoleta. Cardinal nest survival varied over the season and was lower when rat snakes were more active. The probability that a nest survived was associated both with when cardinals nested and with nest height, indicating that both temporal and habitat factors affected predation risk. The increased success of higher nests could be associated with some aspect of rat snakes’ climbing ability. In combination with results for two other species studied previously at the same location, our results for cardinals suggest that the specific seasonal pattern of nest success expected for a given bird species will depend on how its nesting season coincides with predator activity. Determining the generality of seasonal variation in predator behavior as a mechanism for producing seasonal patterns of avian nest success will require additional studies that investigate birds and their nest predators simultaneously.     

Cattle grazing in CRP grasslands during the nesting season: effects on avian reproduction

Bird populations in grasslands have experienced declines coinciding with loss and fragmentation of prairies. The United States Department of Agriculture (USDA)-administered Conservation Reserve Program (CRP) is the most extensive grassland restoration program in North America and it has especially benefitted grassland birds. Grazing by domestic cattle has been restricted in CRP during avian nesting seasons despite the potential improvements in structuring habitat for a greater diversity of grassland bird species. Potential negative consequences of grazing in CRP grasslands include trampling of nests by cattle, reductions in nest concealment from predators, and attraction of brood-parasitic brown-headed cowbirds (Molothrus ater). We designed an experiment to test for effects of cattle grazing in CRP fields during the nesting season on nest survival and brood parasitism of 5 bird species that commonly nest in CRP grasslands: mourning dove (Zenaida macroura), grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana), and eastern (Sturnella magna) and western (S. neglecta) meadowlarks. Grazing was implemented during summers 2017 and 2018 on 17 of 36 fields followed by a year of rest on all fields in 2019. Of the 879 nests on grazed fields, only 4 were likely trampled by cattle (vs. 54% of all nests estimated as failing because of depredation). Experimental grazing (grazed vs. ungrazed fields) had variable effects on nest survival and cowbird parasitism among the bird species analyzed. Negative effects of grazing on daily nest survival of dickcissel and meadowlarks were apparent, at least in some years. We found no direct effects of grazing on nest survival of mourning dove or grasshopper sparrow. Probability and intensity (cowbird offspring/nest) of cowbird parasitism in dickcissel nests was higher on grazed versus ungrazed sites but only in conservation practice (CP) CP2 (vs. CP25 fields). Parasitism probability of grasshopper sparrow nests by cowbirds was higher on grazed fields in the 2 years after introduction of cattle in 2017. Greater vegetative concealment around nest sites was associated with reduced cowbird parasitism of meadowlark and grasshopper sparrow nests and higher nest survival for grasshopper sparrows. Reductions in vegetative height caused by longer-term or high-intensity grazing might therefore have negative consequences for some grassland birds by increasing nest site visibility and exposure to cowbird parasitism. Our results indicate that cattle grazing in CRP fields during the nesting season might have some negative effects on reproductive success of some grassland bird species, at least in the short term; however, the potential improvements of structuring habitat to accommodate more grassland bird species and increasing landowner participation in the CRP are considerable.     

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.     

Do Hosts Discriminate between Sexually Dichromatic Male and Female Brown‐headed Cowbirds?

Female brood parasites are recognized as threats to reproductive success by many host species. Male brood parasites may accompany females while they search for nests to parasitize and males depredate nests throughout the nesting cycle. Hence, selection may also favour recognition of males. We examined whether two common host species perceive male brown-headed cowbirds ( Molothrus ater ) as brood parasites, as nest predators, or neither. We quantified visits of male cowbirds to nests of yellow warblers ( Dendroica petechia ) and red-winged blackbirds ( Ageliaus phoeniceus ) to assess the frequency with which these host species interact with male cowbirds. Males were observed near nests during hosts' laying and incubating stages, although less frequently than female cowbirds. No visits by cowbirds occurred while parents cared for nestlings. We then presented models of male and female cowbirds plus a non-threatening control to yellow warblers and red-winged blackbirds during laying and nestling periods. If hosts perceive males and females similarly, they should respond more intensely to the cowbird models during the laying period, when nests are most likely to be parasitized. Both species responded similarly to male and female cowbird models during laying, which suggests that hosts view cowbirds of both sexes as threats. The responses of yellow warblers with nestlings to male cowbirds were strongly influenced by the order of model presentation. Warblers first presented with the male cowbird gave much reduced anti-parasite responses than those that first interacted with the female then the male cowbird. These results suggest that yellow warblers recognized male vs. female cowbirds, but that discrimination was not expressed during laying. By contrast, red-winged blackbirds did not discriminate between male and female cowbirds at either nesting stage.     

Nesting Ecology of Boreal Forest Birds Following a Massive Outbreak of Spruce Beetles

Abstract: We studied breeding dark-eyed juncos (Junco hyemalis), yellow-rumped warblers (Dendroica coronata), and spruce-nesting birds from 1997 to 1998 among forests with different levels of spruce (Picea spp.) mortality following an outbreak of spruce beetles (Dendroctonus rufipennis) in Alaska, USA. We identified species using live and beetle-killed spruce for nest sites and monitored nests to determine how the outbreak influenced avian habitat selection and reproduction. We tested predictions that 1) nesting success of ground-nesting juncos would increase with spruce mortality due to proliferation of understory vegetation available to conceal nests from predators, 2) nesting success of canopy-nesting warblers would decrease with spruce mortality due to fewer live spruce in which to conceal nests, and 3) both species would alter nest-site selection in response to disturbance. Juncos did not benefit from changes in understory vegetation; nesting success in highly disturbed stands (46%) was comparable to that in undisturbed habitats throughout their range. In stands with low spruce mortality, nesting success of juncos was low (5%) and corresponded with high densities of red squirrels (Tamiasciurus hudsonicus). Yellow-rumped warblers nested exclusively in spruce, but success did not vary with spruce mortality. As disturbance increased, nesting warblers switched from selecting forest patches with high densities of live white spruce (Picea glauca) to patches with beetle-killed spruce. Warblers also placed nests in large-diameter live or beetle-killed spruce, depending on which was more abundant in the stand, with no differences in nesting success. Five of the 12 other species of spruce-nesting birds also used beetle-killed spruce as nest sites. Because beetle-killed spruce can remain standing for >50 years, even highly disturbed stands provide an important breeding resource for boreal forest birds. We recommend that boreal forest managers preserve uncut blocks of infested forest within managed forest landscapes and practice partial harvest of beetle-killed spruce rather than commercial clear-cutting of infested stands in order to sustain breeding bird populations until natural reforestation occurs. Because breeding densities do not always reflect fitness, assessing impacts of a massive natural disturbance should include measuring impacts of changes in vegetation on both reproductive success and predator-prey dynamics.     

Predation risk alters interactions among species: competition and facilitation between ants and nesting birds in a boreal forest

Although interactions between species are often assumed to be fixed, theory and empirical evidence suggest that they may be quite variable, changing in the presence of other species or environmental conditions. The interaction between ants and nesting birds exhibits such variability, ants sometimes being predators of bird nests and other times protectors of them. Hypothesizing that predation risk might be a critical factor in altering the interaction of ants with birds, I investigated the interaction of wood ants Formica aquilonia with nesting birds under different levels of predation risk. In a controlled field experiment, I allowed tits ( Parus major, P. caeruleus, P. ater ) and pied flycatchers ( Ficedula hypoleuca ) to select nest boxes in trees with ants (ant trees) or trees without ants. I found that birds usually nested in trees without ants, apparently to avoid the danger of injury from encounters with ants. Nesting in ant trees occurred mainly in the habitat where risk of predation was highest (along the forest edge), and with the bird taxa that lost nests most frequently in trees without ants (tits). Tits nesting on the forest edge achieved significantly greater nesting success, and fledged significantly more young, in ant trees compared with trees without ants. As the season progressed, ant traffic increased in trees without nesting birds, but decreased in trees with nesting birds, indicating that the outcome of interference competition between ants and nesting birds was reversed under increased predation risk. These results support the idea that predation risk can shift species interactions from predominately competitive processes to predominately facilitative processes.     

The nest defence by the red‐backed shrike Lanius collurio – support for the vulnerability hypothesis

The majority of altricial bird species defend their brood against predators more intensively in nestlings rather than eggs stage. Several hypotheses have been proposed to explain this difference. The majority of existing experimental studies have recorded a gradually increasing intensity of nest defence supporting the reproductive value hypothesis. We have compared nest defence in two nesting stages of the red‐backed shrike against two predators of adult birds and against two predators of nests. While the nests with nestlings were defended by parents against three out of four predators, nests with eggs were almost not defended at all. This rapid change in parent nest defence supports rather the vulnerability hypothesis, predicting that the threat to nests with nestlings increases rapidly after hatching, as they became more conspicuous due to their begging and parental provisioning. Unlike most of the species tested previously, the red‐backed shrike uses very vigorous mobbing towards predators. We suggest that the occurrence of this active mobbing (strikes, including physical contact) is a good proxy of the current threat to the nest.