Possible functions of alternative nests in raptors: the case of Bonelli’s Eagle

Four non-exclusive hypotheses have been proposed to explain the alternative nest-building behaviour of raptors: (1) nests as an advertising signal in territories, (2) frustration nests, (3) competition avoidance by nest-site and (4) reduction of nest ectoparasites. We report here data collected during an observational study of a population of Bonelli’s Eagle Hieraaetus fasciatus in southeastern Spain. Our data does not support the first two hypotheses based on the closeness of nests within territories, the lack of correlation between the number of nests and the distance to neighbours and the construction of secondary nests at similar frequencies after breeding failures as after successes. In contrast, the construction of alternative nests to avoid competition with other cliff-nesting raptor species breeding nearby was clearly important since 30% of the pairs obtained some direct benefit from the existence of alternative nests within their territories. We also found abundant Coleoptera (Dermestidae) and Diptera (Carnidae, Calliphoridae and Phoridae) as ectoparasites in nests, and the alternative use of nests, synergetically with the presence of greenery as mechanisms for avoiding ectoparasites, was an important factor for the breeding success of the eagle. Our data suggest that competition avoidance and, in particular, the reduction of nest ectoparasite hypotheses are the more plausible explanations for the maintenance of alternative nests in raptors.     

Natural expansion versus translocation in a previously human‐persecuted bird of prey

Many threatened species in Europe have been expanding their distributions during recent decades owing to protection measures that overcome historical human activity that has limited their distributions. Range expansion has come about via two processes, natural expansion from existing range and reintroductions to new ranges. Reintroductions may prove to be a better way to establish populations because individuals are less subject to competitive relationships lowering breeding success than individuals expanding from existing populations. Whether this is true, however, remains uncertain. We compared success of breeding pairs of an expanding and a reintroduced population of spanish imperial eagles monitored for over 15 years in the south of Spain. We found significant differences in productivity between breeding pairs of each population. Newly established territories in reintroduction areas were almost three times more productive than new territories established as individuals expanded out from an existing population. We conclude that among these eagle populations reintroduced to new areas may fare as well or better than individuals expanding out form existing populations.     

Assessing the importance of artificial nest‐sites in the population dynamics of endangered Northern Aplomado Falcons Falco femoralis septentrionalis in South Texas using stochastic simulation models

Habitat availability might be the most important determinant of success for a species reintroduction programme, making investigation of the quality and quantity of habitat needed to produce self‐sustaining populations a research priority for reintroduction ecologists. We used a stochastic model of population dynamics to predict whether attempts to improve existing breeding territories using artificial nest platforms improved the population growth rate and persistence of a reintroduced population of Northern Aplomado Falcons Falco femoralis septentrionalis in South Texas. We further assessed whether the creation of new territories, i.e. conversion of entire areas to suitable habitat and not simply the erection of nest platforms, would lead to a subsequent increase in the nesting population. Our model was able to reproduce several characteristics of the wild population and predicted the number of breeding pairs per year strikingly well (R² = 0.97). Simulations revealed that the addition of nest platforms improved productivity such that the population would decline to extinction without them but is stable since their installation. Moreover, the model predicted that the increase in productivity due to nest platforms would cause the population to saturate available breeding territories, at which point the population would contain a moderate proportion of non‐territorial birds that could occupy territories if new ones become available. Population size would therefore be proportional to the increase in available territories. Our study demonstrates that artificial nest‐sites can be an effective tool for the management of reintroduced species.     

Reproduction of the white-tailed sea eagle Haliaeetus albicilla in Sweden

Sixty-eight white-tailed sea eagle territories were studied on the Swedish Baltic coast in 1964–1982 and 22 territories in Swedish Lapland in 1976–1982. Older data were assembled for comparison. The breeding frequency of adult pairs was high in both study areas (ca. 90%). Median egg-laying dates were 17 March (Baltic) and 7 April (Lapland). Average clutch size and egg volume did not differ between samples from the Baltic taken between 1856 and 1982. Average clutch size was significantly smaller in Lapland (1.6 eggs) than on the Baltic coast (2.1 eggs). Clutch size in Lapland was positively related to carrion availability during March. Nestling survival was significantly lower in Lapland, due to starvation in broods of more than one young. Brood loss was significantly more frequent in Lapland. At the Baltic, nest success decreased from 75 to 22%, and brood size from 1.8 to 1.2, from the early 1950's to the mid 1960's. Nest success in 1976-1982 was significantly lower at the Baltic (26%) than in Lapland (45%), with a much larger proportion of consistently failing pairs. The proportion of eggs laid that resulted in fledged young was estimated at 16% at the Baltic and 39% in Lapland. It is concluded that egg production and nestling survival were suppressed by limited food supply and nest success by human interference in Lapland, and that the reproductive impairment of the Baltic population was due primarily to contamination with toxic substances, mainly DDE.     

Physical, environmental and human factors influencing productivity in Bonelli's eagle Hieraaetus fasciatus in Granada (SE Spain)

We analysed the productivity of the Bonelli's eagle Hieraaetus fasciatus in relation to 26 variables that account for physiography, level of human presence, land-use, climate, laying date, and nest orientation, in the province of Granada (SE Spain), where a stable population exhibits the maximum productivity value for the species in Europe. Productivity positively correlated with cliff availability within territories and closeness of nest site to SE orientation. Cliff availability permitted some pairs to choose the best nest site orientation, thereby favouring high productivity. Nests in the preferred orientation (SE) had 35% higher productivity than those towards non-preferred orientations. Higher productivity in the best orientation nests might be related to warmer conditions. High productivity in the study area could offset the main problem for Bonelli's eagle conservation in Europe, that is, the high preadult and adult mortality. Actions required to ensure the recovery of neighbouring populations must focus on the preservation of high cliff availability and the diminution of preadult mortality in southeastern Spain. This would favour restocking of the decreased northern populations by the juvenile dispersion.     

Population size, distribution and habitat selection of the white-tailed eagle Haliaeetus albicilla in the alluvial wetlands of Croatia

From 2003–2006, research on the breeding distribution of the white-tailed eagle (Haliaeetus albicilla) was conducted in Croatia in order to assess the size of the national population. In 125 locations, clear signs of breeding activity were found. An additional 10 presumably active territories were detected but it was not possible to locate the exact position of the nests and confirm the breeding. Based on this, it is concluded that the national breeding population is not less than 135 breeding pairs. The present distribution can be compared with previous reports with the exception of the area along the Ilova and Lonja rivers that have never been reported as an important breeding site. Analysis of the characteristics of 138 nest positions as well as preferences/avoidance of specific structural features were performed. The results showed that white-tailed eagles prefer to build their nests on pedunculate oaks, narrow-leafed ash and white poplars with the greatest preference for mature trees with a diameter above 92.5 cm. The minimal distance between two active pairs was 348 meters. More than 50% of the national population breed less than two km from a large water area and 95% of the population less than four km. More than 95% of the population breed at altitudes lower than 140 m above sea level and are further than one km away from the nearest human settlement, regardless of the availability of forests. According to several parameters (distance to a large water area, elevation, forest presence, distance to the nearest settlement, distance to highways and railways) geographic information system (GIS) helped to determine potential white-tailed eagle breeding areas.     

Nonmotorized recreation and motorized recreation in shrub‐steppe habitats affects behavior and reproduction of golden eagles (Aquila chrysaetos)

Different forms of outdoor recreation have different spatiotemporal activity patterns that may have interactive or cumulative effects on wildlife through human disturbance, physical habitat change, or both. In western North America, shrub‐steppe habitats near urban areas are popular sites for motorized recreation and nonmotorized recreation and can provide important habitat for protected species, including golden eagles. Our objective was to determine whether recreation use (i.e., number of recreationists) or recreation features (e.g., trails or campsites) predicted golden eagle territory occupancy, egg‐laying, or the probability a breeding attempt resulted in ≥1 offspring (nest survival). We monitored egg‐laying, hatching and fledging success, eagle behavior, and recreation activity within 23 eagle territories near Boise, Idaho, USA. Territories with more off‐road vehicle (ORV) use were less likely to be occupied than territories with less ORV use (β = ?1.6, 85% CI: ?2.8 to ?0.8). At occupied territories, early season pedestrian use (β = ?1.6, 85% CI: ?3.8 to ?0.2) and other nonmotorized use (β = ?3.6, 85% CI: ?10.7 to ?0.3) reduced the probability of egg‐laying. At territories where eagles laid eggs, short, interval‐specific peaks in ORV use were associated with decreased nest survival (β = ?0.5, 85% CI: ?0.8 to ?0.2). Pedestrians, who often arrived near eagle nests via motorized vehicles, were associated with reduced nest attendance (β = ?11.9, 85% CI: ?19.2 to ?4.5), an important predictor of nest survival. Multiple forms of recreation may have cumulative effects on local populations by reducing occupancy at otherwise suitable territories, decreasing breeding attempts, and causing nesting failure. Seasonal no‐stopping zones for motorized vehicles may be an alternative to trail closures for managing disturbance. This study demonstrates the importance of considering human disturbance across different parts of the annual cycle, particularly where multiple forms of recreation have varying spatiotemporal use patterns that create human–wildlife interactions.     

Effect of human activities on Egyptian vulture breeding success

Habitat loss, electrocution on power poles and persecution by humans are the main threats to birds of prey. Nevertheless, the effects of human disturbance on endangered species are becoming notorious due to the increasing recreational use of the natural environment. We evaluated the effects of human disturbances on Egyptian vulture Neophron percnopterus breeding success and developed conservation measures based on minimum distance of effect and buffer areas in a high human density area of northern Spain. A total of 100 breeding attempts of 15 breeding pairs were monitored over 8 years. Human disturbances affected 42 of the breeding attempts. Those disturbances related to and originating in forestry work had the most severe effect on breeding success, being associated with the loss of 100% of 13 breeding attempts, while human disturbances related to free-time activities caused 44% failures in 25 breeding attempts by four pairs, two of them within Natural Parks. The breeding success was significantly less in territories affected by disturbances than in those free of disturbances. Some pairs affected by disturbances changed their nest site, increasing breeding success. Adults were prevented from entering the nest to feed chicks when anyone was detected at an average distance of 307 m, while an average distance of 837.5 m allowed them access. The maximum alert distance was estimated at 605 m and the buffer area was 57 ha. We discuss the application of our results for management schemes and conservation of this species.     

Breeding success of the Eurasian eagle owl (<Emphasis Type="Italic">Bubo bubo</Emphasis>) and rodent population dynamics

The dependence of the breeding success of the eagle owl on the population dynamics of rodents, which are a staple of its diet in Mordovia, was traced. At peak numbers of rodents, large clutches and a high survival rate of fledglings were observed in the breeding pairs; after a year of depression of rodent populations, in the following year, the pairs do not nest at all or their breeding success is reduced to a minimum due to the death of the clutches. In 52% of the cases, the nesting pairs laid three eggs; in 31%, two eggs; in 9%, four eggs; in 4%, one egg or five eggs. The average clutch size was 2.78 ± 0.17. The average number of chicks grown from a successful nest was 2.41 ± 0.27. The nests in Mordovia were located at a distance of 1.1–3.7 km from residential areas.     

Lifespan Analyses of Forest Raptor Nests: Patterns of Creation,Persistence and Reuse

Structural elements for breeding such as nests are key resources for the conservation of bird populations. This is especially true when structural elements require a specific and restricted habitat, or if the construction of nests is costly in time and energy. The availability of nesting-platforms is influenced by nest creation and persistence. In a Mediterranean forest in southeastern Spain, nesting-platforms are the only structural element for three forest-dwelling raptor species: booted eagle Aquila pennata, common buzzard Buteo buteo and northern goshawk Accipiter gentilis. From 1998 to 2013, we tracked the fate of 157 nesting-platforms built and reused by these species with the aim of determining the rates of creation and destruction of nesting-platforms, estimating nest persistence by applying two survival analyses, describing the pattern of nest reuse and testing the effects of nest use on breeding success. Nest creation and destruction rates were low (0.14 and 0.05, respectively). Using Kaplan Meier survival estimates and Cox proportional-hazards regression models we found that median nest longevity was 12 years and that this was not significantly affected by nest characteristics, nest-tree dimensions, nest-builder species, or frequency of use of the platform. We also estimated a transition matrix, considering the different stages of nest occupation (vacant or occupied by one of the focal species), to obtain the fundamental matrix and the average life expectancies of nests, which varied from 17.9 to 19.7 years. Eighty six percent of nests were used in at least one breeding attempt, 67.5% were reused and 17.8% were successively occupied by at least two of the study species. The frequency of nest use had no significant effects on the breeding success of any species. We conclude that nesting-platforms constitute an important resource for forest raptors and that their longevity is sufficiently high to allow their reuse in multiple breeding attempts.     

Population size and breeding performance of the Lanner Falcon Falco biarmicus in Sicily: conservation implications

Capsule: We report a significant reduction in population size and breeding success for the Lanner Falcon Falco biarmicus in Sicily, its biggest stronghold in Europe, since the latest coordinated survey.

Aims: To provide updated information on current population size of Lanner Falcon in Sicily and to compare breeding parameters with those obtained in previous studies.

Methods: We performed an intensive coordinated field survey and literature review of breeding success parameters across the species range.

Results: Overall, we monitored 126 territories throughout Sicily where the species had been reported in the last 15 years. Lanner Falcons were present only in 60 of them. Mean nest productivity (± standard deviation) was 1.09?±?1.18 fledged young/checked pairs, flight rate was 2.22?±?0.52 fledged young/successful pairs and breeding success was 49.0%.

Conclusions: Indirect measures aimed at preventing abandonment of occupied territories should be applied, for instance by developing a network of priority areas and slowing down degradation of the pseudo-steppe habitats by agri-environmental schemes. Additionally, direct measures aimed at preventing nest robbery, including the organization of nest guarding activities, and reduction of anthropogenic disturbance and illegal shooting, must be encouraged in order to avoid territory abandonment.     

Feeding territoriality and the reproductive consequences in brown skuas<Emphasis Type="Italic"> Catharacta antarctica lonnbergi</Emphasis>

In the maritime Antarctic, brown skuas (Catharacta antarctica lonnbergi) show two foraging strategies: some pairs occupy feeding territories in penguin colonies, while others can only feed in unoccupied areas of a penguin colony without defending a feeding territory. One-third of the studied breeding skua population in the South Shetlands occupied territories of varying size (48 to >3,000 penguin nests) and monopolised 93% of all penguin nests in sub-colonies. Skuas without feeding territories foraged in only 7% of penguin sub-colonies and in part of the main colony. Females owning feeding territories were larger in body size than females without feeding territories; no differences in size were found in males. Territory holders permanently controlled their resources but defence power diminished towards the end of the reproductive season. Territory ownership guaranteed sufficient food supply and led to a 5.5 days earlier egg-laying and chick-hatching. Short distances between nest and foraging site allowed territorial pairs a higher nest-attendance rate such that their chicks survived better (71%) than chicks from skua pairs without feeding territories (45%). Due to lower hatching success in territorial pairs, no difference in breeding success of pairs with and without feeding territories was found in 3 years. We conclude that skuas owning feeding territories in penguin colonies benefit from the predictable and stable food resource by an earlier termination of the annual breeding cycle and higher offspring survivorship.Research licence: Umweltbundesamt Bonn 13.4-94003-1/5-7.     

Prey abundance and reproductive success of the golden eagle Aquila chrysaetos in Sweden

Reproductive output of the golden eagle Aquila chrysaetos (L.), was studied in two areas within the species distribution in northern Sweden during 1975–1980. Reproductive success was compared with the abundance (based on hunting bag statistics) of small game prey species and with the microtine cycles. The proportion of golden eagle pairs with successful breeding and the number of young produced per occupied territory varied greatly between years (21–85% and 0.27–1.24, respectively). In the northern study area I found a significant correlation between the proportion of pairs with successful breeding versus total hunting bag of small game species. There were also significant correlations between vole density and breeding success one year later. This was not the case in the southern study area, mainly due to a good reproductive year for golden eagles in 1977 when small game species were scarce. The good reproductive output in 1977 may be explained by favourable weather conditions that spring. Whether breeding occurs or not is probably determined by prey abundance early in spring just before the golden eagle female lays her eggs.     

A long-term large-scale study of the breeding biology of the Spanish imperial eagle (Aquila adalberti)

We present data from a 17-year study of the population biology of a growing population of Spanish imperial eagles Aquila adalberti across most of its breeding range. The objective of this study was to analyse the effects of age, supplemental feeding and rabbit haemorrhagic disease (RHD) on several breeding parameters of this population of eagles. Average clutch size was 2.2 eggs per clutch, and the average incubation time was 41.7 days per clutch. Fledging occurred an average of 76.8 days after hatching, the length of the fledgling period was not correlated to clutch size. The annual average percentage of pairs laying eggs was 88%. A significant reduction in the percentage of pairs laying eggs in the period 1992–1994 (from 91 to 81%) coincided with most of the eagles’ territories being affected by the rabbit epizootic disease, RHD, which reduced their food supply significantly. Average productivity was 1.23 chicks per monitored territory, average breeding success was 1.40 chicks in a clutch per territory and the average fledging rate was 1.69 chicks per territory with hatching success. The main known causes of breeding failure during incubation were nest collapse and human disturbance. During chick-rearing, total or partial chick losses were mainly caused by siblicide, disease, malnutrition or nest collapse. In 26.2% of the 1372 monitored breeding attempts, at least one of the breeding birds was a subadult (the male in 56.1% of the cases, the female in 15.5% and both sexes in 28.4% of cases). In cases of mixed-aged pairs (n = 205), 70.7% were the result of a substitution, and 29.3% were the result of the forming of a new pair. Egg laying took place significantly earlier and breeding success was higher in territories occupied by adults than in those occupied by subadults. Breeding parameters were higher in high-quality (rabbit-rich) territories than in low-quality (rabbit-poor) territories, but only for those territories occupied by adults. The values obtained in the territories occupied by adults were only significantly higher than in those of the subadults in high-quality territories. Age and territory quality thus simultaneously affected reproductive output.     

Availability of optimal‐sized prey affects global distribution patterns of the golden eagle Aquila chrysaetos

Climate and landscape change are expected to significantly affect trophic interactions, which will especially harm top predators such as the golden eagle Aquila chrysaetos. Availability of optimal prey is recognized to influence reproductive success of raptors on a regional scale. For the golden eagle, medium‐sized prey species between 0.5 and 5 kg are widely considered to be optimal prey during the breeding season, whereas smaller and larger species are deemed as energetically sub‐optimal. However, knowledge about the effects of optimal prey availability is still scarce on larger scales. To decrease this apparent knowledge gap, we combined biogeographical information on range margins with information about the foraging behaviour and reproductive success of golden eagles from 67 studies spanning the Northern Hemisphere. We hypothesized that availability of optimal prey will affect foraging behaviour and breeding success and, thus, distribution patterns of the golden eagle not only on a local but also on a continental scale. We correlated the diet breadth quantifying foraging generalism, breeding success and proportions of small (< 0.5 kg), medium (0.5–5 kg) and large‐sized (> 5 kg) prey species within the diet with the minimum distance of the examined eagles to the actual species distribution boundary. Closer to the range edge, we observed decreased proportions of medium‐sized prey species and decreasing breeding success of golden eagles. Diet breadth as well as proportions of small and large‐sized prey species increased, however, towards the range edge. Thus, availability of optimal‐sized prey species seems to be a crucial driver of foraging behaviour, breeding success and distribution of golden eagles on a continental scale. However, underlying effects of landscape characteristics and human influence on optimal prey availability has to be investigated in further large‐scale studies to fully understand the major threats facing the golden eagle and possibly other large terrestrial birds of prey.     

Role of breeding and natal movements in lifetime dispersal of a forest‐dwelling rodent

The lifetime movements of an individual determine the gene flow and invasion potential of the species. However, sex dependence of dispersal and selective pressures driving dispersal have gained much more attention than dispersal at different life and age stages. Natal dispersal is more common than dispersal between breeding attempts, but breeding dispersal may be promoted by resource availability and competition. Here, we utilize mark–recapture data on the nest‐box population of Siberian flying squirrels to analyze lifetime dispersal patterns. Natal dispersal means the distance between the natal nest and the nest used the following year, whereas breeding movements refer to the nest site changes between breeding attempts. The movement distances observed here were comparable to distances reported earlier from radio‐telemetry studies. Breeding movements did not contribute to lifetime dispersal distance and were not related to variation in food abundance or habitat patch size. Breeding movements of males were negatively, albeit not strongly, related to male population size. In females, breeding movement activity was low and was not related to previous breeding success or to competition between females for territories. Natal philopatry was linked to apparent death of a mother; that is, we did not find evidence for mothers bequeathing territories for offspring, like observed in some other rodent species. Our results give an example of a species in which breeding movements are not driven by environmental variability or nest site quality. Different evolutionary forces often operate in natal and breeding movements, and our study supports the view that juveniles are responsible for redistributing individuals within and between populations. This emphasizes the importance of knowledge on natal dispersal, if we want to understand consequences of movement ecology of the species at the population level.     

The effect of food supplementation on the reproductive success of Black-billed Magpies Pica pica

The effect of supplementary food on the reproductive success of Black-billed Magpies Pica pica was studied in an urban habitat in the city of Edmonton, Alberta, Canada. We provided supplementary food (pelleted dog food) regularly from August 1986 to June 1988 in a feeder situated outside all known magpie territories. Magpies using the supplementary food showed (i) an advancement of 7 days in the initiation of laying, (ii) no increase in clutch-size, (iii) higher survival of nest contents (eggs and young) during a spring snow storm, (iv) an increase in rate of nestling weight gain, and (v) an increase in fledging success. Supplementary food did not prompt the fed pairs to select nest sites nearer the feeder in 1988 than in 1987. The feeder did not affect the breeding density because the nearest neighbour distances of fed magpies did not differ from unfed magpies in either year, nor did they differ between the two years for either fed or unfed magpies. This study provided evidence that magpies frequently renest even if their initial nests fail after the eggs hatch—a practice that has been reported to be absent or rare in other studies. Although brood reduction was equally frequent in the nests of food-supplemented and control pairs, the actual number of nestlings disappearing from the former (1.56 per nest) was significantly smaller than that from the latter (2.60 per nest), suggesting that the availability of food plays a major role in magpie reproductive success.     

Nest site selection and patterns of nest re-use in the Hooded Crow Corvus cornix

Capsule: Hooded Crows Corvus cornix selected nesting trees based on species, height, grouping and distance from an occupied house. Nest re-use was common and pairs that re-used old nests produced more fledglings than those that built a new nest.

Aims: To determine the features of trees that influenced whether they were used by Hooded Crows as nest sites, to establish what factors influenced nest re-use between years and to explore potential costs or benefits of nest re-use.

Methods: In a large area of Orkney, Scotland, the features of trees that contained a Hooded Crow nest were compared to those of trees where nests were absent. Patterns of nest re-use between years were examined in relation to the availability of alternative sites, previous nesting success and the number of equivalent options to the tree used previously within 200?m of this site.

Results: Hooded Crows favoured spruce and pine trees as nest sites, above the most locally abundant tree species, elder and willow. Preference for trees increased with tree height, local tree density and distance from occupied houses. Over half of the crows studied re-used an old nest when one was available and crows that re-used an old nest fledged more offspring than those that built a new nest. The likelihood of a new nest being built increased as the number of potential locations to build increased. Territories where a nest survived the winter were more likely to be reoccupied the following year than those where nests fell, while territories with fewer trees around the old site were most likely to be abandoned, suggesting that those were territories of lower quality.

Conclusions: Hooded Crows displayed strong preferences for nest sites that might favour nesting success by offering concealment, shelter and protection from ground-based predators. Nest re-use was common, especially when alternative sites were scarce, and appeared to facilitate greater reproductive output.     

Within-season divorce in Blue Tits (Cyanistes caeruleus)

Divorce between breeding seasons, i.e. mate change while the old breeding partner is still alive, has been well-studied in several bird species. It can be viewed either as the result of reproductive decisions of individuals to maximize their own fitness or as the outcome of intra-sexual competition for mating opportunities. In contrast, divorce within a breeding season—also referred to as rapid mate switching—has received much less attention. Within-season divorce may occur after sudden changes in environmental conditions, such as those causing the disappearance or devaluation of nesting sites or territories, or when better breeding partners become available. Within-season divorce can be initiated by a member of the pair, or it can be the result of a take-over by an unpaired individual that competes for access to the resource. During a field experiment investigating the effects of limiting nesting sites on reproductive behaviour in Blue Tits, we recorded several cases of within-season divorce. The rate of divorce was not related to the experimental nest-site limitation, and pairs that changed their partner suffered reduced reproductive success compared to faithful pairs. Although there were no differences in the timing of breeding, clutch size or hatching success, pairs with a new partner also suffered a reduced fledging success, which was partly explained by complete brood failures. This study highlights that the pair bond prior to egg laying can be unstable when conditions force individuals to compete for a new partner or nest site and indicates the importance of the correct timing of divorce within the breeding cycle.     

The reproductive success of Blackbirds Turdus merula in relation to habitat structure and choice of nest site

The reproductive success of a population of Blackbirds Turdus merula occupying farmland and woodland was studied over 3 years to investigate the effects of habitat on breeding success. Territory distribution was patchy in both farmland and woodland; some areas were unoccupied, while other areas were occupied at variable densities. Habitat structure appeared to influence occupation: the index of habitat complexity ("cover score") was higher in occupied areas than in unoccupied areas and high-density territories had higher cover scores than low-density territories. However, habitat structure had no significant effect on reproductive success because the cover scores of territories where pairs were successful did not differ significantly from those of territories where there were no successful breeding attempts. There was no evidence of differential mortality rates in adults according to habitat. The height, bulk and exposure of c. 430 nests were measured to determine the effect of nest and nest-site characteristics on reproductive success. Nest exposure was the only feature that differed between successful and failed nests, successful nests being less exposed than failed nests. The major cause of breeding failure was nest predation, but the effect of nest exposure operated only during the laying and incubation period and not during the nestling period. The significance of habitat structure for variation in population densities between habitats is discussed.