Egg concealment is an antipredatory strategy in a cavity‐nesting bird

Birds have developed different behavioural strategies to reduce the risk of predation during the breeding period. Bird species that nest in the open often cover their eggs to decrease the risk of predators detecting the clutches. However, in cavity nesters, the potential functions of egg covering have not been explored despite some bird species that nest in cavities also covering their eggs as open nesters do. We analysed whether egg covering is an antipredatory behaviour in the blue tit (Cyanistes caeruleus). We simulated an increase in the perceived risk of predation at experimental nests by adding predator scent inside the nest boxes during the egg‐laying period, whilst adding lemon essence or water to control nest boxes. Birds exposed to predator chemical cues in the nest of experimental pairs more frequently covered their eggs than birds exposed to an odorous control. These results suggest that egg covering may have evolved as an antipredatory behaviour also in cavity nesters to reduce the risk of egg predation and thus increase reproductive success in birds.     

Nest temperature fluctuations in a cavity nester,the southern ground-hornbill

Southern ground-hornbills Bucorvus leadbeateri inhabit savanna and bushveld regions of South Africa. They nest in the austral summer, which coincides with the wet season and hottest daytime temperatures in the region. They are secondary cavity nesters and typically nest in large cavities in trees, cliffs and earth banks, but readily use artificial nest boxes. Southern ground-hornbills are listed as Endangered in South Africa, with reintroductions into suitable areas highlighted as a viable conservation intervention for the species. Nest microclimate, and the possible implications this may have for the breeding biology of southern ground-hornbills, have never been investigated. We used temperature dataloggers to record nest cavity temperature and ambient temperature for one artificial and 11 natural southern ground-hornbill tree cavity nests combined, spanning two breeding seasons. Mean hourly nest temperature, as well as mean minimum and mean maximum nest temperature, differed significantly between southern ground-hornbill nests in both breeding seasons. Mean nest temperature also differed significantly from mean ambient temperature for both seasons. Natural nest cavities provided a buffer against the ambient temperature fluctuations. The artificial nest provided little insulation against temperature extremes, being warmer and cooler than the maximum and minimum local ambient temperatures, respectively. Nest cavity temperature was not found to have an influence on the breeding success of the southern ground-hornbill groups investigated in this study. These results have potentially important implications for southern ground-hornbill conservation and artificial nest design, as they suggest that the birds can tolerate greater nest cavity temperature extremes than previously thought.     

Nestbox provisioning in a rural population of Eurasian Kestrels: breeding performance,nest predation and parasitism

The breeding biology of the Eurasian Kestrel Falco tinnunculusin nestboxes in farmland was studied to test for differences between artificial and natural sites. We report on the direct effect of nestbox provisioning on some life-history traits and how nestbox use affects nest predation and parasitism. Five types of nest-sites were available: nestboxes on poles and trees (artificial sites), stick nests on trees, stick nests on pylons and holes in buildings (‘natural’ sites). The Kestrel population increased from 23 pairs in 1993 (prior to nestbox installation) to 55 in 1998 as nestboxes were provided. In general, pairs breeding in trees started to lay later than those nesting in nestboxes on poles or in building holes, but this difference was probably associated with habitat quality rather than nest type. Differences in clutch size were found between nest-sites in some years, and were associated with laying date and, probably, with variation in territory quality. Using only data from successful nests, pairs breeding in nestboxes produce more fledglings than those in building holes or pylons. The frequency of nest predation was higher in natural sites than in nestboxes. The number of fledglings from pairs breeding in nestboxes was higher than from those breeding in old stick nests in trees when all nests were considered. Nestbox provisioning had no effect on the occurrence of the ectoparasite Carnus hemapterus, but chicks from nestboxes showed higher intensity of infection. Our results suggest that nestbox provisioning increases reproductive success and the frequency of nest predation or intensity of parasite infestation in Kestrels.     

潜在洞巢资源差异对次级洞巢鸟及繁殖鸟类群落的影响

为了解次生林中潜在洞巢资源(包括各种啄木鸟的啄洞和人工巢箱)的多寡对次级洞巢鸟集团及繁殖鸟类群落结构的影响, 2007年11月至2008年7月, 我们在吉林省吉林市大岗林场选择洞巢密度不同的样地, 对其次级洞巢鸟及鸟类群落结构进行了比较研究。根据洞巢资源密度我们将9块样地分为3组, 即巢箱区(啄洞密度最低, 悬挂人工巢箱使其潜在洞巢资源总密度大幅提高)、低密度区(啄洞密度较低, 无巢箱)和高密度区(啄洞密度较高, 无巢箱), 调查了3组样地内鸟类的组成和密度、潜在洞巢资源的利用情况等。3组样地中均调查到4种初级洞巢鸟, 其种类组成略有不同; 4种次级洞巢鸟在3组样地广泛分布, 分别为白眉姬鹟(Ficedula zanthopygia)、大山雀(Parus major)、沼泽山雀(P. palustris)和普通鳾(Sitta europaea)。巢箱区和高密度区的次级洞巢鸟总密度显著高于低密度区。巢箱区同高密度区一样, 大山雀和白眉姬鹟的密度显著高于低密度区, 这是由于大山雀和白眉姬鹟是人工巢箱的主要利用鸟种, 而沼泽山雀和普通鳾的密度在三组样地间差异不显著。初级洞巢鸟总密度与啄洞密度、次级洞巢鸟总密度与潜在洞巢资源总密度都呈显著正相关关系。潜在洞巢资源丰富的样地中鸟类群落多样性指数显著高于潜在洞巢资源贫乏样地中的鸟类群落多样性指数, 人为增加洞巢资源可以改变鸟类群落组成并显著提高群落的多样性指数。三组样地中鸟类群落的均匀性、丰富度指数和种间相遇率没有显著差异, 群落相似性指数也相近。高密度区和低密度区鸟类群落集团结构相似。次级洞巢鸟密度的增加短时期内未对群落内其他主要鸟种的密度产生显著影响。研究结果显示, 初级洞巢鸟的密度决定了啄洞的丰富程度, 而洞巢资源的差异会对次级洞巢鸟集团的分布模式产生影响, 进而影响整个繁殖鸟类群落的结构。     

Competition, predation and nest niche shifts among tropical cavity nesters: ecological evidence

I studied cavity-nesting birds in an undisturbed site in lowland Peru to determine the relative roles of competition and predation in favoring termitarium nesting over tree cavity nesting. Occupancy rates of both nest boxes and natural tree cavities near 2% suggest that competition for tree cavities is not favoring the use of termitaria. Artificial nests and bird nests in termitaria suffer significantly lower predation rates than similar nests in old tree cavities showing that predation is favoring the use of termitaria over old tree cavities. Bird nests in newly excavated tree cavities also show lower predation rates than older cavities suggesting that cavity age is more important than substrate (tree or termitaria) per se . This study suggests that nest predation has a greater influence than nest competition on life history evolution for many cavity-nesting birds.     

Effect of nestbox construction and colour on the occupancy and breeding success of nesting tits Parus spp.

Capsule Breeding performance was not affected, although variation in nestbox occupancy may result from perceived differences in protection from predators and insulation properties.

Aim To assess if nestboxes of different construction and colour were occupied differently by breeding tits and affected breeding success.

Methods A total of 292 nestboxes of different construction and colour were placed in a range of habitats and their occupancy, and the clutch size, brood size and number of young fledged of the birds that used them were measured.

Results Overall 272 tit nests were included in the analysis. Although there were species-related differences in the occupancy of the different types of nestboxes, more tits nested in the boxes constructed from woodcrete (64% of nests) compared to the wooden boxes (36%). More tits nested in green (72%) rather than brown boxes (28%) and in boxes with entrance holes (68%) rather than wedges (32%). The clutch size, brood size and number of young fledged did not vary in relation to nestbox type.

Conclusion Within the parameters of the experimental design of this study, tits are more likely to nest in woodcrete boxes than in wooden boxes, and in wooden boxes that are green and have holes rather than brown boxes with wedge entrances. It is likely that the smaller internal volume, internal darkness, insulating properties and perceived protection from predation were the reasons for these differences.     

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.     

Effects of Shiny Cowbird Molothrus bonariensis parasitism on different components of House Wren Troglodytes aedon reproductive success

Avian brood parasites, including cuckoos and cowbirds, have multiple negative effects on their hosts. We analysed the effects of Shiny Cowbird Molothrus bonariensis parasitism on different components (e.g. egg losses, hatching success, chick survival and nest abandonment) of House Wren Troglodytes aedon reproductive success. We also conducted an experiment to discriminate between two mechanisms that may reduce hatching success in parasitized clutches: lower efficiency of incubation due to the increase in clutch volume and disruption of host incubation by the early hatching of Cowbirds. Egg puncturing by Shiny Cowbirds reduced host clutch size at hatching by 10–20%, and parasitized nests had a decrease in hatching success of 40–80%. Egg losses and hatching failures were positively associated with the intensity of parasitism. Brood reduction was greater in parasitized nests, but the growth rate of the chicks that fledged was similar to that in unparasitized nests. The combined effects of egg losses, hatching failures and brood reduction decreased the number of fledged chicks by 80%. In addition, egg puncturing increased the likelihood of nest abandonment by Wrens. Experimental data showed that hatching failures occurred when there was a combination of: (1) an increase in the volume of the clutch by the addition of the Cowbird egg without removal of host eggs, and (2) the addition of the Cowbird egg before the onset of incubation. This was relatively common in House Wren nests, as Cowbirds generally parasitize before the onset of incubation. Our results indicate that Shiny Cowbird parasitism imposes a major impact on House Wrens, as it affects all components of the Wren's reproductive success.     

Building multiple nests is associated with reduced breeding performance in a south temperate population of Grass Wrens Cistothorus platensis platensis

Grass Wrens Cistothorus platensis build two types of non-breeding nest structures: platforms and dummy nests. Platforms are rudimentary accumulations of grasses concealed between vegetation. Dummy and breeding nests are dome-shaped with a similar structural layer. We used a nest-removal experiment and observational data to evaluate several hypotheses regarding the adaptive significance of building multiple nests in a south temperate population of Grass Wrens. Building non-breeding nests was not a strategy of males to attract additional females, as most of these nests were built after pair formation and both sexes collaborated during building. Building non-breeding nests was not a post-pairing display as the presence of multiple nests did not increase female investment in the breeding attempt: clutch size and female provisioning to nestlings did not differ between experimental and control territories where no non-breeding nests were removed. Similarly, in non-manipulated territories, clutch size and female provisioning were not correlated with the number of non-breeding nests or with males’ nest-building effort. Contrary to this hypothesis, the number of non-breeding nests was associated with delayed clutch initiation and reduced hatching success. The presence of non-breeding nests did not reduce nest predation and brood parasitism, which did not differ between experimental and control territories. We did not detect differences in concealment between non-breeding and breeding nests, suggesting that non-breeding nests were not the result of abandonment before egg-laying to reduce subsequent nest predation. Dummy nests did not provide shelter; they were not used frequently for roosting over the breeding season and were not maintained during the non-breeding season. We suggest that building non-breeding nests may be an attempt by males to manipulate the decision of females to breed with a mate they might otherwise reject or to start reproduction earlier than optimal for the females.     

Lack of predator‐odor detection and avoidance by a songbird,the House Wren

ABSTRACT Recent studies suggest that songbirds have a better sense of smell than initially suspected. Work is now focused on determining how birds use their sense of smell to enhance survival and reproduction. One question is whether birds use smell to detect and avoid predators. We examined the reaction of House Wrens (Troglodytes aedon) to the odor of a potential predator in their nests. On two different days of the nestling stage, we placed papers infused with the urine and anal scent gland material of a mustelid (American mink, Neovison vison), a neutral odoriferous substance (cologne, garlic, or vinegar), or water in nest boxes. The proportion of individuals that hesitated to enter nest boxes after first arrival did not differ significantly between treatments on either day. We also found no significant differences on either day in the time it took wrens to first enter nest boxes, time spent in nest boxes after first entry, the propensity to stay in nest boxes and brood young, or latency to return to nest boxes after first exposure to treatments. Our results suggest that House Wrens either did not detect or did not respond to foreign odors in nest cavities, including the odor of a mustelid predator. In a similar study ( Amo et al. 2008 . Functional Ecology 22: 289–293), adult Blue Tits (Cyanistes caeruleus) were less likely to enter nest boxes that contained the odor of a mustelid. One explanation for this difference is that Blue Tits may have a better sense of smell than House Wrens. Alternatively, or additionally, Blue Tits may be inherently (or as result of experience) more prone than House Wrens to avoid unusual odors or, specifically, the odor of mustelid predators in their nests. Additional studies are needed to determine the extent to which songbirds use their sense of smell to detect and avoid nest predators.     

Factors affecting the probability of double brooding by Southern House Wrens

Seasonal fecundity of birds is influenced by clutch sizes and the number of successful breeding attempts during a breeding season. As such, understanding the factors that determine the decision to initiate multiple broods within a season and the consequences of this reproductive tactic is important. We examined the frequency of double brooding by Southern House Wrens (Troglodytes aedon musculus) in eastern Argentina. We analyzed inter‐ and intraseasonal variation in double brooding and evaluated the effect of weather conditions and laying date on the frequency and occurrence of this behavior. Finally, we assessed the effect of double brooding on the seasonal and lifetime productivity of female Southern House Wrens. During our 8‐year study, we found that ～43% (range = 17–83% each year) of breeding pairs attempted a second brood after successfully raising a first brood. The probability of females having a second brood was affected by the laying date of the first nesting attempt, but was independent of the number of young fledged. About 65% of females that started laying eggs before the first quarter of each breeding season produced a second brood, and this percentage decreased to ～40% after this period. In addition, variation in double‐brooding frequency among years was related to weather conditions, with the proportion of pairs double brooding increasing with increased precipitation early in the breeding season. More precipitation likely contributed to an increase in insect abundance. Although double brooding increased the seasonal and lifetime productivity of female Southern House Wrens, additional study of the survival and fate of fledglings from first and second broods is needed to assess the importance of multi‐brooding in the reproductive success of these wrens.     

Effects of site fidelity and breeding performance on mate retention in a short‐lived passerine,the tree swallow Thachycineta bicolor

We studied the relationship among re‐mating, site fidelity and breeding performance in the tree swallow Tachycineta bicolor using 16 y of data on reproductive biology in a population breeding in nest boxes near Ithaca, New York. Of 217 pairs for which both members survived the non‐breeding season, 76% mated with a new partner and 24% reunited with their previous mate. Pairs did not increase their breeding success by breeding together for more than one breeding season. Males produced fewer fledglings after breeding with a new partner, but females neither increased nor decreased their success when breeding with a new mate. Females who bred with a new partner were younger than females that reunited with their previous mates, and they were more likely to move to a different nest box. Males that bred with a new mate were of similar age to males that reunited, and they did not move more often. The probability of breeding with a new partner was better predicted by female age than by previous breeding success, suggesting that re‐mating was not strongly affected by past breeding performance. Because younger females change breeding sites more frequently than do older females and females that mated with a new partner were younger than females that reunited with their previous mates, we suggest that the tendency of tree swallows to change partners between years is a by‐product of lower site fidelity of younger females rather than a strategy for increasing breeding success.     

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.     

The relationship between introduced European Starlings and the reproductive activities of Mountain Bluebirds and Tree Swallows in British Columbia,Canada

The European Starling Sturnus vulgaris is an introduced species in North America and is an aggressive competitor for tree cavity nest‐sites. Starlings are commonly considered to influence nest‐site selection and reproductive success of native cavity‐nesting species negatively. We examined the relationship between Starling nest density and the fecundity of two native secondary cavity‐using passerines, Mountain Bluebird Sialia currucoides and Tree Swallow Tachycineta bicolor. We monitored a total of 622 nests (approximately equal numbers for each of the three species) in woodpecker‐excavated and naturally occurring cavities in 29 small forest groves in central British Columbia, Canada, between 2000 and 2009. The dimensions of cavities used and the timing of nest initiation overlapped for all species, although Starlings initiated clutches earliest. Mixed‐effects models were used to assess whether nest abundance, clutch size or nest success were affected directly by Starling nest abundance, or indirectly via a shift in cavity selection or timing of breeding. Starlings and Mountain Bluebirds showed inverse trends in nest abundance. Mountain Bluebird clutch sizes were smaller if they were initiated later in the breeding season. There was weak evidence that Tree Swallow clutch size decreased with cavity depth when Starling nests were abundant, and increased with cavity depth where there were few Starling nests. We conclude that despite the aggressive nature of this exotic cavity‐nester, the influence of Starlings on native secondary cavity‐nesting passerines is modest where cavities are abundant.     

Intraspecific competition and the maintenance of monogamy in tree swallows

Intraspecific competition for access to breeding resources maylimit male mating success typically monogamous birds. We examinedthe potential for intraspecific competition to limit polygynyin tree swallows at Beaverhill Lake, Alberta, Canada. In thispopulation, polygynous males raisedmore fledglings than monogamousmales, and there was little or no cost to females from nestingpolygynously. Under these conditions one might expect polygynyto be more common than that observed(8% of males). We foundthat females were most aggressive toward conspecific intrudersearly in the breeding season. This aggression was associatedwith (1) females settling farther apart than expected underrandom settlement, (2) later settlement by secondary than bymonogamous females, and (3) no relationship between female settlementdate and male territory size instead of the negative correlationexpected if females settled randomly without competition. Earlyin the season, males also settled farther apart than expectedif they had settled randomly, and among males with two or morenest boxes on their territory, males with widely separated nestboxes were more likely to be polygynous. Monogamy is probablythe most common pairing association in this population becauseintraspecific competition for nest sites prevents most malesfrom gaining a territory with nest sites far enough apart topermit two females to breed without one female excluding theother. Females appeared to be defending an area surroundingtheir nest box to limit nest usurpation or intraspecific broodparasitism, rather than to limit any loss of male parental carefrom polygyny.     

Mate and territory availability affect breeding dispersal and divorce in a resident Southern House Wren Troglodytes aedon musculus population

Breeding dispersal is the movement of an individual between breeding attempts and is usually associated with the disruption of the social pair bond, although mates may disperse together as a social unit. In monogamous territorial species, the decision to disperse may be affected by individual attributes such as sex, age and condition of the disperser. However, environmental and social contexts may also play a crucial role in the decision to disperse. We analysed capture‐resighting data collected over 9 years to study breeding dispersal and divorce rates of a Southern House Wren Troglodytes aedon musculus population in South Temperate Argentina. Between‐season dispersal was more frequent than within‐season dispersal, with females dispersing more often than males, both between and within seasons. Both within‐season and between‐season breeding dispersal probability was affected by territory availability, but not by previous breeding success. When the adult sex ratio (ASR) was more skewed towards males, male between‐season dispersal was also affected by mating status, with widowed and single males dispersing more often than paired males. Within‐season divorce increased the reproductive success of females but not males, and was affected by the availability of social partners (with increasingly male‐skewed ASR). Our results suggest that territorial vacancies and mating opportunities affect dispersal and divorce rates in resident Southern House Wrens, highlighting the importance of social and environmental contexts for dispersal behaviour and the stability of social pair bonds.     

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.     

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.     

The effect of nestboxes on breeding populations of birds in broadleaved temperate woodlands

Removal of nestboxes from a semi-mature, broadleaved woodland caused a decrease in the number of breeding pairs of Great Tits, but did not significantly reduce breeding densities of Blue Tits. Breeding densities of warblers were unaffected by changes in the population of Great Tits. In the absence of nestboxes, intraspecific competition for natural nestholes was more intense amongst Great Tits than Blue Tits. Great Tits tended to occupy larger holes on or near the trunk, while Blue Tits used smaller holes on side branches. Nesting success for both species was lower in natural cavities than nestboxes, due to a higher percentage of total nest failures. The reintroduction of nestboxes did not produce an immediate rise in the breeding population of Great Tits; possible reasons for this are discussed.     

The effect of nestboxes on breeding populations of birds in broadleaved temperate woodlands

Removal of nestboxes from a semi-mature, broadleaved woodland caused a decrease in the number of breeding pairs of Great Tits, but did not significantly reduce breeding densities of Blue Tits. Breeding densities of warblers were unaffected by changes in the population of Great Tits. In the absence of nestboxes, intraspecific competition for natural nestholes was more intense amongst Great Tits than Blue Tits. Great Tits tended to occupy larger holes on or near the trunk, while Blue Tits used smaller holes on side branches. Nesting success for both species was lower in natural cavities than nestboxes, due to a higher percentage of total nest failures. The reintroduction of nestboxes did not produce an immediate rise in the breeding population of Great Tits; possible reasons for this are discussed.