Direct and indirect effects of an insect outbreak increase the reproductive output for an avian insectivore and nest‐cavity excavator,the red‐breasted nuthatch Sitta canadensis

Community‐wide food pulses may ameliorate food constraints but may also result in increased competition for other resources and predation rates. In cavity‐nesting vertebrate communities, where the availability of tree cavities can limit reproduction and the reuse of cavities can increase nest predation by squirrels, excavators may maximize their fecundity by creating new cavities in competitor‐ and predator‐rich habitats that undergo food pulses. The reproductive cost associated with excavation (i.e. increased energy allocation early in the breeding season that often delays laying and thereby reduces clutch size), may be reduced if food pulses allow for a longer breeding season and larger clutches. A large‐scale mountain pine beetle Dendroctonus ponderosae outbreak that occurred during our long‐term study (1995–2009) provided a natural food supplementation experiment across 27 sites in British Columbia, Canada. We examined the effects of a reduction in food constraints accompanied with increases in excavation rates, conspecific density and nest predation risk on the fecundity of a facultative excavator, the red‐breasted nuthatch Sitta canadensis. We found a total of 420 nests in tree cavities. Nuthatch clutch sizes ranged from two to nine eggs, and broods from one to nine fledglings per nest. Later clutches were larger at sites and in years with high beetle abundance (mean clutch size of six eggs did not decline later in the season), second broods were produced in outbreak years (usually only one nesting attempt/normal year), and the number of fledglings per successful nest increased with increasing beetle abundance and nuthatch densities, but declined with increased squirrel densities. Since fecundity did not differ between new and reused cavities, the costs and benefits of excavation versus cavity reuse may be neutralized for nuthatches during strong resource pulses. Overall, the beetle outbreak reduced food constraints for nuthatches and provided alternate food for nest predators, allowing increased annual fecundity.     

On the evolution of clutch size and nest size in passerine birds

Summary I examined the hypothesis that the clutch size of some altricial birds may be limited by over-crowding of the nestlings in the nest, by comparing data on different species of European passerines. Large-sized birds build, relative to the body, larger nests than small-sized birds, both as regards the inner and the outer nest widths and as regards edge breadth; only inner nestcup depth did not change relatively to body size. Nest size also varied in relation to nesting place. Birds with open nests built off the ground had a rather narrow nestcup, whereas those with a domed nest, or which nest in a cavity, had a wide nestcup. When only open-nesters were compared, birds nesting on, or close to, the ground tended to have a wider nestcup than birds nesting above the ground. Inner nestcup width was correlated with the amount of mosses and lichens used in building the nest; the more of such materials the narrower the nestcup. The three variables: standardised body size, nesting place, and type of nesting material used accounted for 92% of the overall variation observed in inner nestcup width. When controlling for adult body size, clutch size was positively correlated with the size of the nestcup. A multiple regression analysis showed that relative nestcup depth, nest site, and type of nesting materials used, accounted for 64% of the overall variation in clutch size.     

Competition, predation and nest niche shifts among tropical cavity nesters: phylogeny and natural history evolution of parrots (Psittaciformes) and trogons (Trogoniformes)

Nest site selection by birds is a critically important life history trait as competition for suitable sites can be intense, and because birds are at their most vulnerable to predators during nesting. Previous studies show that the clutch size and nestling period evolve in response to competition for nest sites and nest predation, respectively. This provides the opportunity to study the relative contribution of competition and predation to the evolution of nesting niche. Using previously published phylogenies for parrots and trogons, I found evidence for at least 13 independent evolutionary transitions from tree cavities to alternative nesting niches (including termitaria, cliffs, and burrows). I analyzed variations in clutch size, incubation period and nestling period for 16 phylogenetically controlled pairs of species to test the relative roles of competition for tree cavities and nest predation, in favoring evolutionary switches to alternative nest sites. Tree cavity nesting species did not have larger clutch sizes as predicted if competition for tree cavities leads birds to invest heavily in nesting once they obtain a nest site (the limited breeding opportunities hypothesis). Instead I found that shifts to alternative nesting niches were accompanied by an increase in nestling period. As nestling period is a surrogate measure for long-term nest predation rates, this finding suggests that nest predation has been more important than competition in niche diversification among cavity nesting parrots and trogons. The timing of events in South America suggests that the explosive radiation of mammalian nest predators during the Upper-Oligocene, Lower-Miocene (20–30 million years ago) corresponded with the radiation of parrot and trogon taxa that exploit novel nesting niches.     

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.     

Projected Availability of Natural Cavities for Wood Ducks in Southern Illinois

Abstract: Wood ducks (Aix sponsa) and other species use tree cavities in forested wetlands and adjacent upland forests for nest sites and cover. The availability of tree cavities suitable for nesting is important to the population dynamics of hole-nesting species, but there is little quantitative information on how forest succession and maturation affect densities of suitable nest sites in eastern deciduous forests. Several studies have measured availability of tree cavities for nesting wood ducks, but data on cavity formation and persistence rates are needed to model changes in cavity abundance. We measured abundance and persistence of tree cavities suitable for nesting wood ducks in southern Illinois, USA, during 1993-2002. We simulated changes in abundance of nest cavities in the Mississippi River floodplain and adjacent upland forests using estimates of tree cavity densities by tree-diameter size classes and 10-year cavity persistence rates by tree species. Cavities were disproportionately common in the largest size classes, but tree species varied in their propensity to form cavities. Beech (Fagus grandifolia; 0.41 cavities/tree) and sycamore (Plantanus occidentalis; 0.50 cavities/tree) were prolific cavity producers, whereas a small proportion (0.05 cavities/tree) of cottonwoods (Populus deltoides) contained cavities. Kaplan-Meier estimates of annual and 10-year cavity persistence averaged 0.95 and 0.64, respectively. Cavity persistence also differed among species (P = 0.02): cottonwoods had the lowest (0.54) and sycamores had the highest (0.89) 10-year tree cavity persistence rates. Tree fall (50.0%), cavity floor deterioration (37.5%), and narrowing of the cavity entrance (12.5%) were the most prevalent causes of tree cavity loss. Forest stand projections indicated that cavity abundance will increase up to 34% over recent levels during the first 10 years and by 44% after 50 years. Most of this increase will be contributed by tree species that are not commonly used by wood ducks, but cavities will increase in oaks (Quercus spp.) and beeches as the forest matures into cavity-bearing size classes. Sycamores will steadily contribute cavities, but cottonwood is predicted to provide fewer cavities due to low survival of cavity-bearing size classes. Our results suggest that availability of nest and den sites for cavity-dependent wildlife will increase as eastern deciduous forests mature over the next half century. Cost-effectiveness of artificial nest box programs should be reevaluated in light of projected changes in tree cavity availability as deciduous forests mature in the eastern United States.     

Natural cavity restoration as an alternative to nest box supplementation

Nest box supplementation is widely used to increase nest‐site availability for cavity nesting animals but the analysis of its effects on individuals breeding in natural cavities is often neglected. This study offers a novel restoration technique to revert abandonment of natural breeding sites by a secondary cavity avian bird, the European roller (Coracias garrulus), and other ecologically similar species. We found that, after a program of nest box supplementation with ensuing monitoring, rollers gradually abandon nesting in natural and seminatural cavities in favor of nest boxes because the latter are of higher quality. We examine whether reducing the entrance size of natural and seminatural cavities improves their suitability for rollers. A 6‐year program reduced the diameter of the entrance of sandstone cavities and cavities in bridges. This led to a high occupancy (59%) of manipulated nest‐sites. Manipulated sites were most frequently occupied by rollers and little owls (Athene noctua) (31 and 18% of sites, respectively). Manipulation did not affect clutch size or fledgling success. We suggest that nest‐site diversity and nesting in natural cavities should be preserved to reduce nest box dependence. Our study illustrates the value of nest boxes when used alongside restoration of natural breeding sites and provides insights for the management of natural cavities.     

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.     

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.     

Nesting ecology of a naturalized population of Mallards Anas platyrhynchos in New Zealand

Investigating the reproductive ecology of naturalized species provides insights into the role of the source population's characteristics vs. post‐release adaptation that influence the success of introduction programmes. Introduced and naturalized Mallards Anas platyrhynchos are widely established in New Zealand (NZ), but little is known regarding their reproductive ecology. We evaluated the nesting ecology of female Mallards at two study sites in NZ (Southland and Waikato) in 2014–15. We radiotagged 241 pre‐breeding females with abdominal‐implant transmitters and measured breeding incidence, nesting chronology and re‐nesting propensity. We monitored 271 nests to evaluate nest survival, clutch and egg size, egg hatchability and partial clutch depredation. Breeding incidence averaged (mean ± se) 0.91 ± 0.03, clutch size averaged 9.9 ± 0.1 eggs, 94 ± 2% of eggs hatched in successful nests, partial depredation affected 6 ± 1% of eggs in clutches that were not fully destroyed by predators, and re‐nesting propensity following failure of nests or broods was 0.50 ± 0.003. Nesting season (first nest initiated to last nest hatched) lasted 4.5 months and mean initiation date of first detected nest attempts was 28 August ± 3.3 days. Smaller females were less likely to nest, but older, larger or better condition females nested earlier, re‐nested more often and laid larger clutches than did younger, smaller or poorer condition females. Younger females in Southland had higher nest survival; cumulative nest survival ranged from 0.25 ± 0.007 for adult females in Waikato to 0.50 ± 0.007 for yearling females in Southland. Compared with Mallards in their native range, the nesting season in NZ was longer, clutches and eggs were larger, and nest survival was generally greater. Different predators and climate, introgression with native heterospecifics and/or the sedentary nature of Mallards in NZ may have contributed to these differences.     

利用天然树洞繁殖的五种鸟的巢位特征及繁殖成功率

对吉林省左家自然保护区次生阔叶林中的大山雀 ( Parusmajor)、沼泽山雀 ( Paruspalustris)、普通跓( Sitta europacea)、白眉姬? ( Ficedula zanthopygia)和灰椋鸟 ( Sturnus cineraceus) 5种利用天然树洞繁殖的次级洞巢鸟进行了巢位选择和繁殖成功率研究。本研究中共发现 1 41巢。五种鸟对树洞类型的选择存在种间差异 ,普通跓不利用裂洞 ,沼泽山雀不利用啄洞 ,其它 3种鸟对 3种洞均有利用 ,但有一定的倾向性。对 5种鸟 9个巢位变量的比较中 ,只有洞口方向差异不显著 ( p >0 .0 5 ) ,其它 8个变量均差异显著 ( p<0 .0 5 ) ,该结果说明 5种次级洞巢鸟对巢位的选择具有其各自的需求。洞口横径、洞口纵径、洞处树直径、洞内径、巢距地高是巢位选择重要变量 ,它们决定不同种类对树洞的利用。巢损失多数出现在产卵之前和孵化阶段 ,44个繁殖失败的巢中有 35个在这两个阶段损失。大山雀的巢成功率最低 ,灰椋鸟的巢成功率最高。 5种鸟的孵化率都超过 90 %。人为破坏和动物捕食是繁殖失败的主要原因 ,占总数的 61 .4%。洞巢鸟巢位选择中的重要变量影响繁殖成功。普通跓繁殖是否成功受洞口横径和巢高影响 ,沼泽山雀受洞口纵径、树胸径和洞内径影响 ,大山雀受洞口横径、巢高和洞内径影响 ,灰椋鸟受洞内径和洞深影     

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.     

Determinants of reproductive success and offspring sex in a turtle with environmental sex determination

Despite the importance of maternal effects in evolution, and knowledge of links among nest site choice, timing of nesting, offspring sex, and reproductive success in animals with environmental sex determination, these attributes have not been rigorously studied in a combined and natural context. To address this need we studied the relationships between three maternal traits (nest site choice, lay date, and nest depth) and two fitness‐related attributes of offspring (hatchling sex and embryonic survival) in the riverine turtle Carettochelys insculpta, a species with temperature‐dependent sex determination, for four years. Predation and flooding were the major sources of embryonic mortality in 191 nests. Embryonic survival was influenced by both lay date and nest site choice: in one year when nesting began later than average, nests laid later and at lower elevations were destroyed by early wet season river rises. In other years early nesting precluded flood mortality. However, turtles did not nest at the highest available elevations, and a field experiment confirmed that turtles were constrained to nest at lower elevations where they could construct a nest chamber. The principal determinant of hatchling sex in 140 nests was lay date, which in turn was apparently related to the magnitude of the previous wet season(s). Clutches laid earlier in the season (a female's first clutch) produced mainly males, while later clutches (her second clutch) yielded mostly females, due to seasonal increases in air temperatures. Accordingly, later nesting produced female‐biased hatchling sex ratios in 1996, while earlier nesting resulted in sex ratios near unity in the other years. However, all‐female nests were more likely to be flooded than mixed‐sex or all‐male nests in years when nesting was late. In conclusion, we found evidence that the position of two maternal trait distributions (elevation of the nest site and lay date), associated with the reproductive strategy of C. insculpta, reflect a combination of natural selection, physical constraints, and phenotypic plasticity. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 1–16.     

Reproductive success of Tree Pipits Anthus trivialis in relation to habitat selection in conifer plantations

Aspects of the reproductive success of Tree Pipits Anthus trivialis were examined in relation to broad‐scale habitat and nest‐site selection in Thetford Forest, a coniferous plantation forest in eastern England. Three habitat classes were defined corresponding to previously reported densities of Tree Pipits: clearfell and recently planted stands (habitat class A: low density), stands 2–5 years old (B: high density) and stands 6 years or older (C: low density). The preference for 2–5‐year‐old stands indicated by higher densities was supported by the timing of territory settlement. Tree Pipits also showed distinct preferences for nest‐site characteristics that were relatively consistent across habitat classes and throughout the breeding season. At the ‘habitat scale’, results were consistent with the predictions of the ideal despotic distribution model. First clutches were laid significantly earlier in the preferred habitat class B. Overall nesting success (i.e. the proportion of nests producing fledglings), but not clutch size, also varied between habitats, being greater in habitat classes B and C than in habitat class A. The variation in overall nesting success between habitats was primarily driven by low nest survival rates during the laying/incubation period in clearfell and recently planted stands. Nest survival rates during the nestling period were lower in the preferred 2–5‐year‐old (and older) stands and declined over the course of the study. Preferences for nest‐site characteristics (at least for those that were measured) provided no apparent benefit to nest survival rates. Overall nesting success thus appeared to be determined at the habitat scale, perhaps because the broad differences in cover between habitats affected the likelihood of nest predation (the main cause of nest failure). It is suggested that the very low nesting success experienced by Tree Pipits in clearfell and new stands may be one factor in the species’ relative avoidance of this habitat and preference for 2–5‐year‐old stands.     

Cavity use and breeding biology of endangered Bahama Swallows (Tachycineta cyaneoviridis): implications for conservation

Bird populations, especially on islands, have declined or gone extinct due to overhunting, habitat loss and fragmentation, and adverse effects of the introduction of non-native species. Bahama Swallows (Tachycineta cyaneoviridis), endangered secondary cavity nesters that breed on only three islands in the northern Bahamas, are an island species with a declining population, but the causes of this decline are unknown. During four breeding seasons on Great Abaco Island (2014–2017), we identified cavity-nesting resources used by breeding swallows in native pine forest and other habitats, and estimated phenology and breeding parameters from a subset of nests. Bahama Swallows nest in cavities in a variety of structures, but rely most on woodpecker-excavated cavities in pine snags and utility poles. Swallows nesting in cavities in pine snags had higher fledging success (92%) than those nesting in cavities in utility poles (50–62%), which were concentrated in non-pine habitat that may expose swallows to predation and increased competition for nest cavities from other species. The high reproductive success of Bahama Swallows nesting in the pine forest indicates that the decline in population cannot be attributed to poor productivity on southern Great Abaco. However, our results suggest that the dependence of Bahama Swallows on cavities excavated by Hairy Woodpeckers (Dryobates villosus) for nesting sites may be a factor in their decline, and highlight the potential importance of the protection and management of pine forests in future efforts to ensure the survival of Bahama Swallows.     

Interspecific competition affects evolutionary links between cavity nesting,migration and clutch size in Old World flycatchers (Muscicapdae)

The ecology of cavity nesting in passerine birds has been studied extensively, yet there are no phylogenetic comparative studies that quantify differences in life history traits between cavity‐ and open‐nesting birds within a passerine family. We test existing hypotheses regarding the evolutionary significance of cavity nesting in the Old World flycatchers (Muscicapidae). We used a multi‐locus phylogeny of 252 species to reconstruct the evolutionary history of cavity nesting and to quantify correlations between nest types and life history traits. Within a phylogenetic generalized linear model framework, we found that cavity‐nesting species are larger than open‐nesting species and that maximum clutch sizes are larger in cavity‐nesting lineages. In addition to differences in life history traits between nest types, species that breed at higher latitudes have larger average and maximum clutch sizes and begin to breed later in the year. Gains and losses of migratory behaviour have occurred far more often in cavity‐nesting lineages than in open‐nesting taxa, suggesting that cavity nesting may have played a crucial role in the evolution of migratory behaviour. These findings identify important macro‐evolutionary links between the evolution of cavity nesting, clutch size, interspecific competition and migratory behaviour in a large clade of Old World songbirds.     

Nesting behavior and nest site selection in monk parakeets (Myiopsitta monachus) in the Pantanal of Brazil

We examined nesting behavior in monk parakeets (Myiopsitta monachus) in their native habitat in the Brazilian Pantanal. Unique among parrots, monk parakeets build communal nest structures that contain many cavities, each belonging to an individual pair. We studied 41 parakeet colonies that had 104 nest structures. We hypothesized that nest structures would be located in trees providing the greatest support and protection from predators and inclement weather, and that nest sites and nest cavities would differ from random locations with respect to tree characteristics, location of houses, and presence of jabiru stork (Jabiru mycteria) nests, as suggested anecdotally by other authors. Fewer than half of the colonies were close (<350 m) to houses. There was a strong association with jabiru storks; 21 of 23 stork nests had monk parakeet nest cavities attached, accounting for 51% of parakeet colonies. Of the 21 jabiru-associated colonies, 6 had additional parakeet structures and 15 had only the jabiru-attached parakeet structure. Monk parakeet colonies associated with jabiru nests had significantly more nesting cavities than did monospecific monk parakeet colonies, due mainly to those attached directly to the jabiru nest. In jabiru-associated colonies, parakeet nest structures were located higher and in taller trees than in monospecific colonies. There was no difference in trunk diameters of parakeet nesting trees with or without jabirus. Although we tabulated 24 tree species as nest trees, nearest-neighbor trees, or matched-point trees, only 6 species were used for nesting. When compared to matched points, monk parakeet structures were preferentially located in piuva (Tabebuia spp.) and mandovi trees (Sterculia apetela). Parakeet structures were in taller trees with thicker trunk diameters than matched points. Most nest cavities (71%) faced in a northerly direction (northwest to northeast), away from cold southerly winds. Choice of a nest site and orientation appears to reflect structural, weather, and predator constraints. Thick, tall trees with stout branches provided stable sites for their large nest structures, which are known to collapse because of their own weight and strong winds. Nesting with jabirus confers structural advantages (they could attach many nests to the bottom of the jabiru nest, potentially gaining benefits from social facilitation), early warning, and predator defense.     

Breeding biology of Orange‐breasted (Harpactes oreskios) and Red‐headed (H. erythrocephalus) trogons in Khao Yai National Park,Thailand

ABSTRACT As tropical habitats continue to be cleared or degraded, obtaining basic information about the ecology of birds in intact habitats is essential for understanding their life histories. We studied the breeding biology of Orange‐breasted Trogons (Harpactes oreskios) and Red‐headed Trogons (H. erythrocephalus) in Khao Yai National Park in Thailand from 2003 to 2009. Nests were in excavated cavities in well‐rotted stumps or other tree parts. Mean cavity heights were 2.1 m (N= 19) for Orange‐breasted Trogons and 2.0 m (N= 49) for Red‐headed Trogons. Eggs were laid every other day. For Orange‐breasted Trogons, the mean clutch size was 2.4 ± 0.1 (SE) eggs (N= 17); incubation periods for two nests were 17 and 18 d, respectively, and the nestling period ranged from 12 to at least 14 d (N= 4). For Red‐headed Trogons, the mean clutch size was 2.6 ± 0.1 eggs (N= 48), the mean incubation period was 18 d (N= 9), and the mean nestling period was 13.4 d (N= 5). In both species, both males and females excavated nest sites, incubated eggs, and brooded and provisioned nestlings. Only females incubated and brooded at night, and males provisioned nestlings more than females. Breeding seasons lasted from January to March for Orange‐breasted Trogons, and from late February to July for Red‐headed Trogons. Mayfield estimates of nest success were 8% and 9% for Orange‐breasted and Red‐headed trogons, respectively. Unusual for cavity nesters, nest failure due to predation was high and nestling periods short. The low nesting success is typical of many other tropical species, but considerably lower than reported for some Neotropical trogons, possibly due to the unenclosed structure of the nests of these Asian trogons.     

Honey‐Making Bee Colony Abundance and Predation by Apes and Humans in a Uganda Forest Reserve1

Honey‐making bee colonies in Bwindi Impenetrable National Park were investigated with Batwa Pygmies locating 228 nests of Apis and five stingless bees (Meliponini). The relative importance of predation, food supply, nesting site, and elevation affecting abundance were studied for meliponines in particular. Nest predation and overall nest abundance had no correlation with elevation along a 1400 m gradient, nor did flowering phenology or pollen collection. Many suitable, large trees were unoccupied by bee nests. In 174 ha of forest plots, 2 Meliponula lendliana, 13 M. nebulata, 16 M. ferruginea, 16 M. bocandei, and 20 Apis mellifera adansonii nests occurred, suggesting a habitat‐wide density of 39 nests/km². Compared to other studies, Ugandan Meliponini were uncommon (0.27 colonies/ha, tropical mean = 1.9/ha), while Apis mellifera was numerous (0.12 nests/ha, tropical mean = 0.06/ha), despite park policy allowing humans to exploit Apis. Meliponine colony mortality from predators averaged 12 percent/yr and those near ground were most affected. Tool‐using humans and chimpanzees caused 82 percent of stingless bee nest predation. Selective factors affecting nest heights and habit may include auditory hunting by predators for buzzing bees, and indirect mutualists such as termites that leave potential nesting cavities. Mobility and free‐nesting by honey bee colonies should enable rapid community recovery after mortality, especially in parks where human honey hunting is frequent, compared to sedentary and nest‐site‐bound Meliponini.     

Variation in incubation effort during egg laying in the Mountain Bluebird and its association with hatching asynchrony

Females in many bird species reportedly begin incubation prior to clutch completion, but the nature of such incubation and the degree to which it varies among females remains undescribed for almost all species. We used continuous recording of nest‐cup temperatures to document incubation effort during egg laying at 57 Mountain Bluebird (Sialia currucoides) nests in a high‐elevation Wyoming population. We then asked whether such effort predicted the degree to which eggs hatch asynchronously. Although substantial egg heating could begin abruptly late in laying (previously reported as the norm for this species) or even after clutch completion, we found that most (>90%) females began incubation gradually, engaging in a few (usually 1–8), brief (<10 min) bouts of heating on the day they laid their first or second egg. Thereafter, females varied markedly in when they increased incubation effort and by how much. The onset of nocturnal incubation also varied, with females beginning to incubate at night after laying their prepenultimate, penultimate, or last egg and not always initially incubating through the night. As an index of the total amount of heat applied to eggs during laying, we calculated the cumulative number of degrees by which nest‐cup temperatures exceeded the threshold temperature required for embryonic development. This value varied by more than 150‐fold between nests and explained >50% of the variation in hatching asynchrony. Our results thus provide strong support for the widely held, but rarely tested, assumption that parent birds can have substantial control over the degree of hatching asynchrony by varying the amount of incubation done prior to clutch completion.