Selection of Nest Trees by Cavity‐nesting Birds in the Neotropical Atlantic Forest

One of the five most important global biodiversity hotspots, the Neotropical Atlantic forest supports a diverse community of birds that nest in tree cavities. Cavity‐nesting birds may be particularly sensitive to forestry and agricultural practices that remove potential nest trees; however, there have been few efforts to determine what constitutes a potential nest tree in Neotropical forests. We aimed to determine the characteristics of trees and cavities used in nesting by excavators (species that excavate their own nest cavity) and secondary cavity‐nesters (species that rely on existing cavities), and to identify the characteristics of trees most likely to contain suitable cavities in the Atlantic forest of Argentina. We used univariate analyses and conditional logistic regression models to compare characteristics of nest trees paired with unused trees found over three breeding seasons (2006–2008). Excavators selected dead or unhealthy trees. Secondary cavity‐nesters primarily selected cavities that were deep and high on the tree, using live and dead cavity‐bearing trees in proportion to their availability. Nonexcavated cavities suitable for birds occurred primarily in live trees. They were most likely to develop in large‐diameter trees, especially grapia Apuleia leiocarpa and trees in co‐dominant or suppressed crown classes. To conserve cavity‐nesting birds of the Atlantic forest, we recommend a combination of policies, economic assistance, environmental education, and technical support for forest managers and small‐scale farmers, to maintain large healthy and unhealthy trees in commercial logging operations and on farms.     

Enriching small trees with artificial nest boxes cannot mimic the value of large trees for hollow‐nesting birds

Large trees support unique habitat structures (e.g. hollows) that form over centuries and cannot be provided by small trees. Large trees are also declining in human‐modified landscapes worldwide. One restoration strategy gaining popularity involves adding nest boxes to smaller trees to replicate natural hollows. However, limited empirical research has tested how hollow‐nesting fauna responds to the presence of nest boxes. We asked: can the addition of nest boxes increase tree visitation by hollow‐nesting birds? We conducted a before‐after control‐impact (BACI) experiment using 144 nest boxes and 96 sample trees comprised of three sizes (small [20–50 cm dbh], medium [51–80 cm], and large [>80 cm]) and located in four landscape contexts (reserves, pasture, urban parklands, and urban built‐up areas). We recorded a significant increase in hollow‐nesting bird abundance and richness at large trees after nest box additions. However, the same response was not observed at medium, small, or control trees. We also recorded nonsignificant increases in hollow‐nesting bird abundance and richness at trees in modified landscapes after nest box additions compared to trees in reserves and control trees. Our results suggest that adding nest boxes to smaller‐sized trees may not attract hollow‐nesting birds. Therefore, nest box management strategies may require re‐evaluation as it is often assumed that hollow supplementation will attract hollow‐using fauna and sufficiently ameliorate the loss of large, hollow‐bearing trees. We advocate that large tree retention remains crucial and should be prioritized. Large trees could be effective target structures for habitat restoration, especially in modified landscapes.     

Nest‐site selection by Slender‐billed Parakeets in a Chilean agricultural‐forest mosaic

ABSTRACT Species in the family Psittacidae may be particularly vulnerable to anthropogenic habitat transformations that reduce availability of suitable breeding sites at different spatial scales. In southern Chile, loss of native forest cover due to agricultural conversion may impact populations of Slender‐billed Parakeets (Enicognathus leptorhynchus), endemic secondary cavity‐nesting psittacids. Our objective was to assess nest‐site selection by Slender‐billed Parakeets in an agricultural‐forest mosaic of southern Chile at two spatial scales: nest trees and the habitat surrounding those trees. During the 2008–2009 breeding seasons, we identified nest sites (N= 31) by observing parakeet behavior and using information provided by local residents. Most (29/31) nests were in mature Nothofagus obliqua trees. By comparing trees used for nesting with randomly selected, unused trees, we found that the probability of a tree being selected as a nest site was positively related to the number of cavity entrances, less dead crown, and more basal injuries (e.g., fire scars). At the nesting‐habitat scale, nest site selection was positively associated with the extent of basal injuries and number of cavity entrances in trees within 50 m of nest trees. These variables are likely important because they allow nesting parakeets to minimize cavity search times in potential nesting areas, thereby reducing energetic demands and potential exposure to predators. Slender‐billed Parakeets may thus use a hierarchical process to select nest sites; after a habitat patch is chosen, parakeets may then inspect individual trees in search of a suitable nest site. Effective strategies to ensure persistence of Slender‐billed Parakeets in agricultural‐forest mosaics should include preservation of both individual and groups of scattered mature trees.     

Characteristics of nest cavities of Barrow's Goldeneyes in eastern Canada

ABSTRACT Barrow's Goldeneyes (Bucephala islandica) are secondary cavity nesters found in western North America and, to a lesser extent, in eastern North America. The eastern North American population is concentrated in the province of Québec and totals about 2000 pairs. Characteristics of nest cavities used by Barrow's Goldeneyes have been described in western North America, but no nest cavities have been found in eastern North America. From 2004 to 2008, we searched for nest cavities in the species’ core breeding area in the boreal forests north of the St. Lawrence River. We captured 12 adult females on their breeding grounds and fitted them with transmitters, but none apparently nested so we conducted ground searches in areas near lakes where paired birds were observed. We found 11 cavities, with 10 in dead, decaying trees and one in the dead part of a dying tree. Nine cavities were in white birch (Betula papyrifera) trees. Mean cavity height was 3.5 ± 1.6 (SD) m (range = 1.2–6.6 m) and mean diameter at breast height (DBH) of cavity trees was 37.8 ± 4.7 cm (range = 32.2–47.5 cm). In contrast to the population in western North America, Barrow's Goldeneyes in eastern North America appear to rely on the availability of natural cavities formed in large, decaying trees for nesting. Current forestry regulations in Québec do not promote the retention of either large trees or older forests, reducing the availability of potential nest cavities for Barrow's Goldeneyes and likely threatening their long‐term conservation. Therefore, we recommend that guidelines be developed to promote silvicultural practices aimed at preserving the long‐term availability of large (DBH ≥ 30 cm) decaying trees across the breeding range of Barrow's Goldeneyes.     

The role of body size in nest-site selection by secondary cavity-nesting birds in a subtropical Chaco forest

Most bird species that nest in tree cavities globally occur in diverse assemblages in little-studied tropical and subtropical forests which have high rates of habitat loss. Conservation of these communities will require an understanding of how species traits, such as body size, influence nest-site selection. We examined patterns of nest-site selection of secondary cavity-nesting birds at the nest patch, tree and cavity scale, and investigated how these patterns are influenced by body size. Using conditional logistic regression, we compared characteristics of 155 nest tree cavities paired with 155 unused tree cavities in quebracho Schinopsis balansae forests in Chaco National Park, Argentina (2016–2018). The odds of a cavity being used for nesting increased with its depth and height above ground, decreased with entrance size, and were greater for dead trees than live. Small-bodied (13–90 g) species used floor diameters in proportion to availability, but medium- (150–200 g) and large-bodied (400–700 g) species selected cavities with larger floors. Model selection indicated that characteristics at the nest patch scale (canopy cover, tree density) had little effect on nest-site selection when cavity-scale variables were included. Cavity floor diameter, entrance size, cavity height and tree diameter (but not cavity depth) increased with body mass, and larger bird species more often used live trees. Two tree species proved to be key for the community: large and medium-sized birds used almost exclusively large live Schinopsis balansae, whereas small birds used live and dead Prosopis spp. in a proportion greater than its availability. Small birds could be differentiated according to species-specific cavity characteristics, but medium and large species overlapped considerably with one another. Although body mass explained much of the overall variation in tree and cavity characteristics between small and medium/large species, several small-bodied species consistently used cavities outside of the expected characteristics for their body size, suggesting that other natural history traits may play important roles in nest-site selection by small-bodied birds. To retain the full suite of secondary cavity-nesters in species-rich tropical and subtropical forests, it is necessary to conserve a diversity of trees and cavities that meet the full range of nesting requirements of these trait-diverse communities.     

Nest sites as limiting resources for cavity‐nesting birds in mature forest ecosystems: a review of the evidence

ABSTRACT Assumptions that populations of cavity‐nesting birds are limited by access to nest sites have largely been based on anecdotal reports or correlative data. Nest‐box‐addition experiments or tree‐cavity‐blocking experiments are potentially rigorous ways to investigate how densities of breeding birds are affected by access to nest cavities. Experimental evidence indicates that natural tree holes are limited in human‐altered landscapes, but the possibility that cavity nests are limited in old growth (unmanaged) forests is less clear. I reviewed 31 nest‐cavity‐removal or addition experiments conducted with 20 species of cavity‐nesting birds in mature forests. Of these 31 experiments conducted with a variety of different species of birds, only 19% reported statistically significant changes in breeding densities. However, none of these studies included data about the reproductive history of individuals colonizing the boxes (i.e., whether birds using the boxes would have otherwise been floaters or that birds excluded from blocked cavities on the plots did not simply move elsewhere), so they provided no strong evidence that the number of breeding pairs was limited by availability of nest sites at the population scale. Although some studies indicate that nest sites are limited at local (plot) scales in old growth forests, there is still little empirical evidence for nest‐site limitation at the population‐ and landscape‐level in mature, unmanaged forests. I review the challenges in designing and interpreting box‐addition experiments and highlight the main gaps in knowledge that should be targeted in the future.     

Nest habitat selection by the Austral parakeet in north‐western Patagonia

Identifying habitat or nesting microhabitat variables associated with high levels of nest success is important to understand nest site preferences and bird–habitat relationships. Little is known about cavity availability and nest site requirements of cavity nesters in southern hemisphere temperate forests, although nest site limitation is suggested. Here we ask which characteristics are selected by the Austral parakeet (Enicognathus ferrugineus) for nesting in Araucaria araucana–Nothofagus pumilio forest in Argentine Patagonia. We compared nest plot and tree characteristics with unused plots and trees among areas of different A. araucana–N. pumilio density. We also examine whether nest plot and tree use and selection, and the associated consequences for fitness of Austral parakeets are spatially related to forest composition. Austral parakeets showed selectivity for nests at different spatial scales, consistently choosing isolated live and large trees with particular nest features in a non‐random way from available cavities. Mixed A. araucana–N. pumilio forests are ideal habitat for the Austral parakeets of northern Patagonia, offering numerous potential cavities, mainly in N. pumilio. We argue that Austral parakeet reproduction and fitness is currently very unlikely to be limited by cavity availability, although this situation may be rapidly changing. Natural and human disturbances are modifying south temperate forests with even‐aged mid‐successional stands replacing old growth forests. Cavity nesting species use and need old growth forests, due to the abundance of cavities in large trees and the abundance of larvae in old wood. Neither of the latter resources is sufficiently abundant in mid‐successional forests, increasing the vulnerability and threatening the survival of the Austral.     

Cavity-nesting birds are limited by nesting habitat in Neotropical agricultural landscapes

Most studies comparing biodiversity between natural and human-modified landscapes focus on patterns in species occurrence or abundance, but do not consider how different habitat types meet species' breeding requirements. Organisms that use or nest in tree cavities may be especially threatened by habitat conversion due to the loss of their nesting sites. Although cavity-nesting bird diversity is highest in the tropics, little is known about how tropical birds use cavities, how agriculture affects their reproductive biology, and how effective nest boxes could be as a conservation strategy in tropical agriculture. Here, we explored how habitat conversion from tropical forests to pasture affects the abundance, nesting habitat availability, and nest success of cavity-nesting birds in Northwest Ecuador. We conducted bird surveys and measured natural cavity availability and use in forest and agriculture. We also added artificial nest boxes to forest and agriculture to see whether cavity limitation in agriculture would elicit higher use of artificial nest boxes. We found evidence of cavity limitation in agriculture—there were many more natural cavities in forest than in agriculture, as well as more avian use of nest boxes placed in agriculture as compared to forest. Our results suggest that it is important to retain remnant trees in tropical agriculture to provide critical nesting habitat for birds. In addition, adding nest boxes to tropical agricultural systems could be a good conservation strategy for certain species, including insectivores that could provide pest-control services to farmers. Abstract in Spanish is available with online material.     

Dark tree cavities – a challenge for hole nesting birds?

Holes provide the safest nest sites for birds, but they are an underutilized resource; in natural forests there are usually more holes than birds that could use them. Some bird species could be prevented from nesting in holes because of their inability to operate in the low light conditions which occur in cavities. As no visual system can operate in complete darkness some nest cavities could be too dark to be useable even by hole‐nesters. Thus, the light conditions within tree cavities could constrain both the evolution of the hole nesting habit, and the nest site choice of the hole‐nesting birds. These ideas cannot be tested because little is known about the light conditions in cavities. We took an opportunity provided by ongoing studies of marsh tits Poecile palustris and great tits Parus major breeding in a primeval forest (Bia?owie?a National Park, Poland) to measure illumination inside their nest cavities. We measured illuminance in cavities at daybreak, which is just after the parents commenced feeding nestlings. Only ca 1% of incoming light reached the level of the nest. Illuminance at nests of both species (median = 0.1–0.2 lx) fell within mesopic‐scotopic range, where colour vision is impaired. Measurements in model cavities showed strong declines in illumination with distance from the entrance, with light levels typically as low as 0.01 lx at 40 cm from the cavity entrance. Thus cavities can be very dark, often too dark for the use of colour vision, and we suggest that ‘lighting’ requirements can affect the adoption of specific nest sites by hole nesting birds. We discuss implications of the findings for understanding the adaptations for hole‐breeding in birds.     

Nest site availability and niche differentiation between two cavity‐nesting birds in time and space

Niche differentiation is a key concept in the field of ecology and refers to the process by which competing species within an ecological community partition utilization of environmental resources to achieve coexistence. The existence of niche differentiation is uniquely difficult to prove on account of the fact that historical interaction among species, which plays a key role in elucidating the current state of coexistence among species, is not well known. We created continuous niche gradients in nest‐site resources between two sympatric secondary cavity‐nesting birds, the green‐backed tit (Parus monticolus) and the russet sparrow (Passer cinnamomeus), and investigated whether nesting site is a factor contributing to limiting breeding overlap by regular inspection and 388,160 min of film recording. Our results indicate that although we manipulated nest site availability to be uniformly high along the habitat gradient, the two bird species have little overlap in nest sites and rarely compete for them. Furthermore, the green‐backed tit possessed a wide range of fundamental niche that covered that of the russet sparrow, while their reproductive time was largely segregated. The sparrow was more aggressive and outcompeted the tit in their overlapped range. These results suggest that even though nesting sites are crucial to the reproduction of cavity‐nesting birds, some other factor plays a more important role in limiting niche overlap between sparrows and tits in space and time. Given that these two cavity‐nesting birds continued to use different habitats and breed in segregated time after our manipulation, their relationship is better explained by the ghost of competition past theory.     

Nest site selection of a primary hole‐nesting passerine reveals means to developing sustainable forestry

Anthropogenic habitat loss and fragmentation affect populations worldwide. For example, many bird populations of boreal forests have declined due to intensive forestry. To target conservation actions for such species, determining the key factors that affect their habitat selection is essential. Remote sensing methods provide highly potential means to measure habitat variables over large areas. We aim at identifying the key‐features of habitats by utilizing remote sensing data. As a case example, we study the nest site selection of a primary hole‐nesting passerine, the willow tit Poecile montanus, in a managed forest landscape. Using presence–absence data, we determine the most important habitat characteristics of the nest sites for three spatial scales by generalized linear mixed effect models. Our results highlight the importance of the availability of nesting sites – standing decaying deciduous trees – in the nest site selection of P. montanus. It seems to prefer moist habitats with high densities of deciduous trees and to avoid open areas, but does not require mature or intact habitats. Most of the nest site selection seems to occur within small scales. In this case, remote sensing data alone was insufficient for producing reliable models, but adding information of an ecologically important feature from direct field surveys greatly improved model performances. For the conservation and maintenance of dead wood dependent species, changes in forestry practices are necessary to keep the key characteristics of the habitat. Most importantly, continuous availability of standing decaying wood should be secured.     

The conservation value of tree decay processes as a key driver structuring tree cavity nest webs in South American temperate rainforests

South American temperate rainforests, a global biodiversity hotspot, have been reduced to nearly 30% of their original extent and most remaining stands are being degraded. Cavity-nesting vertebrate communities are dependent on cavity-bearing trees and hierarchically structured within nest webs. Evaluating the actual degree of cavity dependence (obligate, non-obligate) and the preferred attributes of trees by cavity nesters is critical to design conservation strategies in areas undergoing habitat loss. During three breeding seasons (2010–2013), we studied the cavity-nesting bird community in temperate rainforests of Chile. We found the highest reported proportion of tree cavity nesters (n = 29 species; 57%) compared to non-cavity-using birds for any forest system. Four species were excavators and 25 were secondary cavity nesters (SCNs). Among SCNs, ten species were obligate and 15 were non-obligate cavity nesters. Seventy-five percent of nests of SCNs were located in cavities produced by tree decay processes and the remaining 25% were in cavities excavated mainly by Pygarrhichas albogularis and Campephilus magellanicus. Nest web structure had a low dominance and evenness, with most network interactions occurring between SCNs and large decaying trees. Tree diameter at breast height (DBH) was larger in nest-trees (57.3 cm) than in available trees (26.1 cm). Cavity nesters showed a strong preference for dead trees, both standing and fallen (58% of nests). Our results stress that retaining large decaying and standing dead trees (DBH > 57 cm), and large fallen trees, should be a priority for retention in forest management plans in this globally threatened ecosystem.     

Aggregated recruitment patterns under adult crowns in Photinia glabra,a bird‐dispersed tree species

To clarify recruitment patterns of Photinia glabra, which is an evergreen, broad‐leaved, bird‐dispersed tree species, we analyzed spatial distribution in P. glabra recruits at each growth stage and demography of current‐year seedlings with respect to distributions of adults in a warm‐temperate secondary forest, western Japan. Although individuals ≥ 5 cm diameter at breast height (DBH) that had nearly produced fruits showed a random distribution, seedlings (≥ 1 year old, < 10‐cm stem length [SL]), small saplings (10 ≤ SL < 30 cm) and large saplings (≥ 30‐cm SL, < 5‐cm DBH) were clumped and associated with reproductive adults at approximately 2–3‐m scales, nearly equal to their average crown radius. Based on monitoring the demography of current‐year seedlings, emerged seedling density profoundly decreased, and no seedlings survived at longer than an adult's crown scales, with distance‐dependent mortality as a result of disease and herbivory not greatly affecting the current‐year seedling mortality. Thus, aggregated seed dispersal under the crown of adult P. glabra would directly influence the distribution of recruits for P. glabra in this forest. Of the bird‐dispersed tree species in this forest, P. glabra produced the highest amount of fruits during large crop years, and their fruits ripened during the late seasonal period (early January), suggesting that birds might be strongly attracted to these species, in turn leading to seeds being deposited mostly under the tree crowns. We propose that dispersal limitation would occur, even in a bird‐dispersed tree species such as P. glabra, owing to plant–bird interactions in the forest.     

Nest survival rates of Red‐crested Cardinals increase with nest age in south‐temperate forests of Argentina

ABSTRACT The main cause of nest mortality for most bird species is predation and nest survival rates often vary in relation to time‐specific variables. Few investigators have examined time‐specific patterns of nest survival in Neotropical birds, and most such studies have focused on tropical and subtropical species. To better understand age‐related patterns of nest survival, we studied nest survival of Red‐crested Cardinals (Paroaria coronata, Thraupidae) in a south‐temperate forest in Argentina. We modeled daily nest survival rates (DSR) using program MARK. We examined the relationship between nest age and nest survival rate, controlling for the effects of physical characteristics of nest sites and progression of the breeding season. We monitored 367 nests for a total of 4018 exposure days. We found that DSR increased with nest age and was higher in small isolated patches than in large continuous patches of forests. The increase of DSR with nest age could be a consequence of more vulnerable nests being predated early in the nesting cycle or a result of parents defending nests more vigorously as nestlings age because of their increasing reproductive value. Open areas of grassland that surrounded the small isolated patches of forests in our study may have been a barrier to predator movements, possibly explaining the lower predation rates. Nest survival rates in our study were lower than those reported for tropical or Nearctic temperate birds, but similar to those reported in other studies of Neotropical temperate birds. Reasons for the low nest survival rates of Neotropical temperate birds remain unclear, and additional studies of predator communities are needed to help elucidate this topic.     

A new method for catching cavity‐nesting birds during egg laying and incubation

ABSTRACT The physiological condition of female birds during the egg‐laying and incubation periods is of considerable interest and yet is relatively understudied in wild birds, primarily due to the difficulty of catching birds during this period without causing nest desertion. We therefore developed a box‐net to capture cavity‐nesting birds using sections of a mist‐net placed around a metal cubic frame. We captured female Great Tits (Parus major) as they left nest boxes during the egg‐laying and incubation periods and measured desertion rates. Using box‐nets, we captured 108 of 119 (90%) females during egg laying and 10 of 12 (83%) during incubation. Our recapture rate over two consecutive days during incubation was 50% (5 of 10). Females not captured left nest boxes before we attempted to capture them, escaped through a hole in the mist‐net, or remained in nest boxes for more than 2 h, after which we ended capture attempts. Overall, 22% of egg‐laying females deserted, with desertion rates highest early in the egg‐laying period. Desertion rates of females captured using box‐nets did not differ from those of undisturbed females. One of 10 females captured in a box‐net deserted during the incubation period. Box‐nets are portable, can be set up and taken down quickly and easily, and could potentially be used with nest boxes or natural cavities at any height. Box‐nets are easy to construct and adaptable for use with an array of cavity‐nesting birds, and can be an important tool for studying female physiology during egg laying and incubation.     

Nest site competition between cavity nesting passerines and golden paper wasps Polistes fuscatus

Nest boxes provide sheltered nesting sites for both passerines and paper wasps. Although neither wasps nor birds appear to evict the other once one is fully established, it is unclear which is the dominant competitor at the onset of the breeding season. Using wire mesh, we excluded birds but not golden paper wasps Polistesfuscatus from alternating boxes along a transect through edge habitat in North Carolina from 2006 – 2008. If wasps dominate Carolina chickadees Poecile carolinensis and eastern bluebirds Sialia sialis during the early spring (all have similar nest initiation dates), we would expect wasps to settle in both box types at equal frequencies. However, if birds dominate wasps, we would expect wasp nests to be concentrated in “bird‐proof” boxes. We found wasps in bird‐proof boxes significantly more often than in bird‐accessible boxes, indicating that secondary‐cavity nesting birds are able to exclude wasps from available nest sites. Additionally, we found that during the period of nest initiation, birds usurp wasps more often than vice versa.     

Size matters: nest colonization patterns for twig‐nesting ants

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Nest cavity orientation in black‐capped chickadees Poecile atricapillus: do the acoustic properties of cavities influence sound reception in the nest and extra‐pair matings?

Birds that nest in cavities may regulate nest microclimate by orienting their nest entrance relative to the sun or prevailing winds. Alternatively, birds may orient their nest entrance relative to conspecific individuals around them, especially if the acoustic properties of cavities permit nesting birds to better hear individuals in front of their nest. We measured the cavity entrance orientation of 132 nests and 234 excavations in a colour‐banded population of black‐capped chickadees Poecile atricapillus for which the reproductive behaviour of nesting females was known. Most chickadees excavated cavities in rotten birch Betula papyrifera, aspen Populus tremuloides and maple Acer saccharum. Nest cavities showed random compass orientation around 360° demonstrating that chickadees do not orient their cavities relative to the sun or prevailing winds. We also presented chickadees with nest boxes arranged in groups of four, oriented at 90° intervals around the same tree. Nests constructed in these nest box quartets also showed random compass orientation. To test the acoustic properties of nest cavities, we conducted a sound transmission experiment using a microphone mounted inside a chickadee nest. Re‐recorded songs demonstrate that chickadee nest cavities have directional acoustic properties; songs recorded with the cavity entrance oriented towards the loudspeaker were louder than songs recorded with the cavity entrance oriented away from the loudspeaker. Thus, female chickadees, who roost inside their nest cavity in the early morning during their fertile period, should be better able to hear males singing the dawn chorus in front of their nest cavity. Using GIS analyses we tested for angular‐angular correlation between actual nest cavity orientation and the azimuth from the nest tree to the territories and nest cavities of nearby males. In general, nest cavity entrances showed no angular‐angular correlation with neighbourhood territory features. However, among birds who followed a mixed reproductive strategy and nested in the soft wood of birch and aspen trees, nest cavity entrances were oriented towards their extra‐pair partners. We conclude that nest cavity orientation in birds may be influenced by both ecological and social factors.     

Safety first: terrestrial predators drive selection of highly specific nesting sites in colonial‐breeding birds

Nesting is a critical yet hazardous life stage for many birds. For colonial‐breeding birds, the conspicuousness of the colony to predators suggests immense pressure to select optimal colonial nesting sites. But what drives selection of those sites? As with solitary nesting birds, reducing access by predators may be the single most important factor. If so, knowledge of the predators involved and the attributes of different potential colony sites can allow us to predict the features that make a site especially safe. We examined the attributes of trees used by breeding colonies of metallic starlings Aplonis metallica in tropical Australia, and experimentally tested if those attributes prevented nest access by predatory snakes. Our surveys confirmed that tree choice by starling nesting colonies is highly non‐random, with all colonies located in tall trees in rainforest clearings, with no low branches and smooth bark. Experimental tests demonstrated that the climbing ability of predatory snakes depends upon bark rugosity, and that colony access by snakes depends on tree attributes such as bark rugosity and canopy connectivity. Our study confirms that colonial‐nesting starlings select colony sites that provide a safe refuge from predation. Intense predation pressure may have driven the evolution of stringent breeding habitat criteria in many other species of colonial‐breeding birds.     

Nest site selection in middle and great spotted woodpeckers <Emphasis Type="Italic">Dendrocopos medius</Emphasis> &; <Emphasis Type="Italic">D. major</Emphasis>: implications for forest management and conservation

Success of species conservation depends to a large extent on comprehensive management that considers all critical aspects of a species’ niche. Many studies have examined habitat factors in relation to occurrence, abundance or foraging behaviour of European woodpecker species, while relatively little is known about nest site selection. I compared habitat structures used for nesting by middle and great spotted woodpeckers Dendrocopos medius and D. major with available structures in an oak forest in the Swiss lowlands. I first tested if nest trees were randomly selected among available trees by focusing on species, condition and diameter of nest trees, and on the presence of the fruiting body (hereafter sporophore) of polypores (wood-decomposing fungi). Second, I examined if the nesting niches of the two species were differentiated. Both species showed strong preferences for oaks, large trees, dead trees and for trees with sporophores. Nest sites of the two species differed most strongly with respect to the presence of sporophores, cavity age and tree condition, pointing towards interspecific competition for nest sites. Old living or dead trees with sporophores are central components of the nesting niche of middle and great spotted woodpeckers. Conservation plans for the threatened middle spotted woodpecker have so far mostly focused on the needs in terms of distribution and foraging; future conservation strategies and forest management must take into account the preference for dead and decaying trees with sporophores as another vital resource. This will also provide benefits for other woodpecker species as well as for the community of secondary cavity nesters.