Average Breeding Success Research Articles

The lifestyle and biology of the Hooded Vulture at Sokoto, northern Nigeria, 1970 to 1973.

The Hooded Vulture was studied in and around Sokoto town, in north-west Nigeria, in the years end-1970 to mid-1973. We estimated that 1500-2000 Hooded Vultures lived at Sokoto, based on counts at the abattoir. We mainly followed the nesting of the species, and to that end located more than 320 nesting trees. Faidherbia (Acacia) albida was the tree most used; the average height of nests in eight species of tree was 8.1 m (sd ± 3.2), being highest in the Borassus Palm. Overall egg-laying dates for first-laid eggs were 16 October to 20 February (averaging 15 December); replacement eggs were laid on average on 17 February. The incubation period of 13 eggs averaged 51 days (range 50 to 54 days). Growth in weight of chicks rose to about 1500 g at 70 days and levelled off. The wing grew straight from day 30 to day 90 at 5.2 mm per day. It peaked at day 120 at 96% of the adult’s wing length. The fledging period of 10 chicks averaged 108 days (range 98 to 123 days). During observations there was always an adult (sometimes two) present at the nest, and the chick is fed mouth-to-mouth by regurgitation. Average breeding success (chicks reared from eggs laid) was low at 41%, indicating some impact of our interference. Adults were caught on their nests eight times, and measured. There was little interaction between people and the vultures, and the local people were generally very tolerant of them.

Results of the re-introduction of the Griffon Vulture (Gypsfulvus) in Vrachanski Balkan Nature Park, Bulgaria - completion of the establishing phase 2010-2020.

The current study analyses and presents the results of the ten-year establishment phase of the Griffon Vulture (Gypsfulvus) local re-introduction in Vrachanski Balkan Nature Park, north-western Bulgaria. Between 2010 and 2020, 61 rehabilitated and captive-bred Griffon Vultures from Spain, France and several European zoos were released from an acclimatisation aviary. The first successful breeding in the wild was reported in 2015. Thus, the species has been restored as a nesting species in the area after more than 60 years of absence. In 2020, the local population accounted for some 55-70 individuals, consisting of about 20-23 breeding pairs in three-five separate colonies and two frequently-used roosting sites. Forty-two chicks fledged from 2010 to 2020, at an average breeding success of 0.46 chicks/territorial pair and productivity of 0.62 fledglings/breeding pair. The mortality rate is calculated at 0.34; an additional 0.07 of the released individuals have never been seen or found. The local nucleus of the Griffon Vulture now covers a territory of 1,478.58 km², calculated as a 95% home range, while the 50% core area is 9.07 ± 5.73 km2 (range 2.12-22.89 km2). With these results, we consider the establishment phase of the re-introduction of the species in Vrachanski Balkan Nature Park as completed.

Status of the Eastern Imperial Eagle in Hungary Between 2019 and 2023

The Hungarian population of the Eastern Imperial Eagle (Aquila heliaca) is closely monitored by MME BirdLife Hungary and the ranger service of the Hungarian national park directorates since 1980. The population is slowly, but continuously increasing during the last four decades and the average population growth rate is 11% and the number of estimated nesting population has increased from 230–240 pairs (2018) to 400–450 (2023). The distribution area is also still expanding to the south and the species colonised new areas especially in Bács-Kiskun, Csongrád and Győr-Moson-Sopron counties, but even spread to Serbia and Romania. The breeding density also increased in the core areas of the breeding range especially in Heves, Jász-Nagykun-Szolnok and Békés counties, which hold the most important stronghold of the species in the Pannonian basin. Besides, the average breeding success also increased to 1.54 fledgling/breeding pair and 1.91 fledgling/successful pair by 2022. The prevalence of raptor poisoning incidents decreased and were kept at a relatively low level between 2014–2022 thanks to specific and complex conservation measures with the help of the EU’s LIFE Nature programme (HELICON and PannonEagle LIFE projects). Due to the significant increase of the population and the limited capacity of the working group, the national monitoring protocol has been changed in 2023. The key parameters of the national population (i.e. no. of nesting pairs, no. of chicks) is still monitored by the ranger service of the Hungarian national park directorates. On the other hand, the complex monitoring program of MME BirdLife Hungary, which is gathering detailed data on breeding performance (including ringing and drone surveillance, diet and genetic analyses) is focusing only on selected sample areas, which recently covers 50% of the population (ca. 200 pairs).

Earlier and more frequent occupation of breeding sites during the non-breeding season increases breeding success in a colonial seabird.

Competition for high‐quality breeding sites in colonial species is often intense, such that individuals may invest considerable time in site occupancy even outside the breeding season. The site defense hypothesis predicts that high‐quality sites will be occupied earlier and more frequently, consequently those sites will benefit from earlier and more successful breeding. However, few studies relate non‐breeding season occupancy to subsequent breeding performance limiting our understanding of the potential life‐history benefits of this behavior. Here, we test how site occupancy in the non‐breeding season related to site quality, breeding timing, and breeding success in a population of common guillemots Uria aalge, an abundant and well‐studied colonially breeding seabird. Using time‐lapse photography, we recorded occupancy at breeding sites from October to March over three consecutive non‐breeding seasons. We then monitored the successive breeding timing (lay date) and breeding success at each site. On average, sites were first occupied on the 27th October ± 11.7 days (mean ± SD), subsequently occupied on 46 ± 18% of survey days and for 55 ± 15% of the time when at least one site was occupied. Higher‐quality sites, sites with higher average historic breeding success, were occupied earlier, more frequently and for longer daily durations thereafter. Laying was earlier at sites that were occupied more frequently and sites occupied earlier were more successful, supporting the site defense hypothesis. A path analysis showed that the return date had a greater or equal effect on breeding success as lay date. Pair level occupancy had no effect on breeding timing or success. The clear effect of non‐breeding occupancy of breeding sites on breeding timing and success highlights the benefits of this behavior on demography in this population and the importance of access to breeding sites outside the breeding season in systems where competition for high‐quality sites is intense.

Site-dependent regulation of breeding success: Evidence for the buffer effect in the common guillemot, a colonially breeding seabird.

Density‐dependent regulation can offer resilience to wild populations experiencing fluctuations in environmental conditions because, at lower population sizes, the average quality of habitats or resources is predicted to increase. Site‐dependent regulation is a mechanism whereby individuals breed at the highest quality, most successful, sites, leaving poorer quality, less successful sites vacant. As population size increases, higher quality sites become limiting but when populations decline, lower quality sites are vacated first, offering resilience. This process is known as the ‘buffer effect’. However, few studies have tested whether such regulation operates in populations experiencing changes in size and trend.We used data from a population of common guillemots Uria aalge, a colonially breeding seabird, to investigate the relationship between site occupancy probability, site quality and population size and trend. These data were collected at five sub‐colonies spanning a 38‐year period (1981–2018) comprising phases of population increase, decrease and recovery.We first tested whether site quality and population size in sub‐colonies explained which sites were occupied for breeding, and if this was robust to changes in sub‐colony trend. We then investigated whether disproportionate use of higher quality sites drove average site quality and breeding success across sub‐colony sizes and trends. Finally, we tested whether individuals consistently occupied higher quality sites during periods of decline and recovery.Higher quality sites were disproportionality used when sub‐colony size was smaller, resulting in higher average site quality and breeding success at lower population sizes. This relationship was unaffected by changes in sub‐colony trend. However, contrary to the predictions of the buffer effect, new sites were established at a similar rate to historically occupied sites during sub‐colony decline and recovery despite being of lower quality.Our results provide support for the buffer effect conferring resilience to populations, such that average breeding success was consistently higher at lower population size during all phases of population change. However, this process was tempered by the continued establishment of new, lower quality, sites which could act to slow population recovery after periods when colony size was low.

Artificial nesting platforms support population recovery of the Dalmatian pelican Pelecanus crispus along the Danube River in Bulgaria

The Dalmatian pelican Pelecanus crispus is a Near Threatened species of waterbird with populations in the wetlands of the Lower Danube River. Breeding populations declined due to habitat loss and wetland drainage and conservation efforts have focused on bringing breeding Dalmatian pelicans back to their former wetland sites in Bulgaria. Since 2008, conservation efforts have focused on building artificial nesting platforms at marshes along the Lower Danube River. These efforts resulted in considerable growth of the population in the country. Between 2011 and 2021, four wooden platforms were installed at the Belene Island wetland complex (Pechina and Martvo Marshes) and the Kalimok wetland complex. All four platforms were used successfully by pelicans, resulting in the formation of two new breeding colonies and a total of 91 pairs in 2021. The majority, 88 pairs, were recorded at the Belene Island marshes, with the remainder at the Kalimok colony. The average annual breeding success was 1.17 young per pair at Peschina Marsh (occupied from 2016-2021), 0.90 at Martvo Marsh (2020-2021), and 1.33 at Kalimok (2021). The average across all three colonies was 1.14 young per pair. By 2021, the breeding population of Dalmatian pelicans in Bulgaria had grown to 131-150 breeding pairs at three locations.

Comparison of Nesting Features and Breeding Success of Turtle Dove Streptopelia turtur between Orchards and Riparian Habitats

The European turtle dove Streptopelia turtur breeds in both farmlands and woodlands, and it is important to explore the difference in breeding ecology of this threatened game in these two ecosystem types. This study, carried out during four years (2015–2018), compares nesting features of this species and its breeding success between apple orchards and riparian vegetation in Midelt Province, Morocco. The main result revealed that the nest placement, including nesting-tree height and nest height, is similar between orchards and riparian trees. However, the nest dimensions (big and small diameters) were larger in orchards. Correlations were variable among nest placement parameters and dimensions. On the contrary, in four breeding seasons, where 566 nests were monitored (467 in orchards and 99 in riparian sites), the average breeding success was different (57% of chicks in apple farms and 53% in riparian vegetation). Moreover, in apple orchards, clutches’ failure is due to both predation (18.89% of eggs and 10.54% of chicks) and temperature lowering (5.03% of unhatched eggs and 5.49% of dead chicks), while in riparian vegetation, the loss is due to nest desertion (21.33% of clutches) and mostly predation (33.16% of clutches).

Climate warming induced a stretch of the breeding season and an increase of second clutches in a passerine breeding at its altitudinal limits.

The increase in the average air temperature due to global warming has produced an early onset of the reproduction in many migratory birds of the Paleartic region. According to the “mismatch hypothesis” this response can lead to a decrease in the breeding output when the conditions that trigger the departure from the wintering areas do not match the availability of food resources in the breeding ground. We used 653 brooding events registered during the period 1991–2013 to investigate the link between climatic variables and individual breeding performance of a partially migratory passerine, the Rock Sparrow Petronia petronia, breeding at the altitude limit of its distribution. The laying date (LD) of the earliest first clutch was associated with local spring (minimum) temperatures but did not show a significant trend during the period considered. The LD of the latest first clutch had a positive and statistically significant trend, unrelated to local covariates and resulting in a longer breeding season (∼1.5 days/year). A longer breeding season allowed birds to produce more second clutches, which proportion increased from 0.14 to 0.25. The average breeding success was also positively correlated with the average temperature in July and with the duration of the breeding season. Contrary to expectations, the most important climate-dependent effect was a stretch of the breeding season due to a significant increase of the LD of the latest first-clutches rather than an earlier breeding onset. We show how climate changes act on bird populations through multiple paths and stress the need to assess the link between climatic variables and several aspects of the breeding cycle.

Migratory Dates, Breeding Phenology, and Reproductive Success of European Turtle Doves between Lowlands and Highest Breeding Habitats in North Africa

The migratory time, breeding chronology, and reproductive success of the European turtle doves (Streptopelia turtur) were studied in Midelt as a high-altitude breeding habitat and Beni Mellal as a low-altitude breeding site from 2015 to 2018 in Morocco. Migration dates, breeding phenology, and breeding success were recorded from March to October for each season. As a result, during four years, arrival dates were earlier at the low breeding site, while departure dates were earlier at the high breeding site. Similarly, breeding phenology from nest building to fledging was early at low-altitude site. On the other hand, with four breeding seasons and 893 nests (467 at Midelt and 426 at Beni Mellal), average breeding success was 57% of chicks at Midelt compared to 60.15% at Beni Mellal. Moreover, at Midelt, 18.89% of eggs and 10.54% of chicks were predated, while at Beni Mellal 21.80% of eggs and 4.65% of chicks were deserted due to human disturbance. As a response, at Midelt breeding period was shorter and shifted to hot periods to ensure better reproductive success. Finally, our results highlight that the turtle dove breeding season is later and shorter at breeding highlands, which might allow this bird to avoid the vigorous climate conditions at mountains and their effect on reproductive success.

Effectiveness of a five-year project of intensive, regional-scale, coordinated management for little terns Sternula albifrons across the major UK colonies

Little terns in the UK have undergone a population decline of 37 % over the last three decades despite protection efforts at their breeding sites during this time. In 2014, a five-year Little Tern Recovery Project was launched in the UK through the European Union LIFE + funding stream, deploying a more coordinated, comprehensive and strategic programme of conservation measures at 29 breeding sites across England and Wales. We compared little tern productivity (fledging probability per egg) during the project with that experienced in the five years prior to the project and included data from both project sites and reference sites. We found a positive relationship between productivity and colony size as well as differences in productivity between ‘northern’ and ‘southern’ regions of the UK, with colonies in the northern region doing better. There was a positive effect on productivity associated with the project’s enhanced measures, but this interacted negatively with colony size, such that increased productivity was only seen for colonies with ≤91 pairs (which comprised 90 % of our dataset). Although average modelled breeding success across all project sites during the project was low (0.37 chicks/pair), we found it was on average more than double (164 % increase) than it would have otherwise been in the absence of the enhanced management measures. For colonies with ≤91 pairs, the predicted increase was 208 %. However, for colonies >91 pairs, there was a predicted 20 % reduction in breeding success in the presence of enhanced management. This counter-intuitive result may be due to threats at larger sites already being managed as far as practicable prior to the project, and poor breeding success during the project being mainly due to stochastic, less manageable threats such as severe weather. In addition, fewer data were available to inform the model for larger colonies. Overall, we estimate that an additional 1,785 chicks fledged as a result of the project. Those that survive their first few years are expected to recruit back into the UK and Irish population, benefitting from the legacy of protection, habitat restoration and creation measures established as part of the project. However, a simple population model indicates that even if the elevated breeding success levels achieved by the project were maintained, the projected population decline between 2020–2050 would only be slowed by 30 %, so more needs to be done to achieve population stabilisation. However, the project’s beneficial impact has bought valuable time that can be used to further develop and deploy innovative solutions.

Breeding biology of the chaffinch Fringilla coelebs africana in the El Kala National Park (North East Algeria)

This work focused on monitoring the reproductive phenology of the chaffinch Fringilla coelebs africana for two successive seasons (2016 and 2017) in the mountains of the El Kala National Park (North East Algeria). We searched for nests in the tree stratum or by direct observation of breeding pairs carrying building materials from mid-March to the end of June and found a total of 34 nests that were measured without affecting the breeding process. The results show that nests were built at an average height of 6.3m from the ground. The laying period was 46 days (Early April to mid-May) and the average laying date was May 2. The mean of clutch size is 3.2 eggs per brood. All the previous traits are similar to those recorded in Europe. The mean hatching success rate was 43.1% and the average breeding success rate was 36.3%. These two values are relatively low compared to those found in European populations.

Results of eighteen years (2000–2017) monitoring study of an extra-alpine Peregrine Falcon Falco peregrinus population in North-Western Italy

Abstract The Author reports the results of a survey about a breeding extra-alpine Peregrine Falcon population in Cuneo Province, NW Italy. The first reproduction was recorded in 2000, and from that year, the monitoring work localized 16 occupied territories in an area of 1,900 km2 covering the hills and nearby lowland areas. The average distance between breeding territories was of 7.6 km with a density 0.7 pairs/100 km2. During the eighteen years, 89 successful breeding attempts were recorded, 81 of them led to fledging of 206 young. Average breeding success was 2.5 juv/reproducing pairs. The nests are located on bridges (motorway viaducts and rail way bridges) and clay/sandstone cliffs. Preliminary prey analyses showed that pigeons and doves represent the 75% of their diet.

No short-term effect of closing a rubbish dump on reproductive parameters of an Egyptian Vulture population in Turkey

SummaryChanges in food availability that lead to lower reproductive output or lower survival probability are important drivers of the widespread declines in vulture populations. Permanent feeding stations for scavengers, such as vulture restaurants or rubbish dumps, may have both positive and negative effects on reproductive parameters. Here we examine the effects of the closure of a large communal rubbish dump on breeding success and fledging rate of a dense population of the ’Endangered’ Egyptian Vulture Neophron percnopterus in central Turkey to assess whether the closure may have affected the population. We monitored territories from 2011 to 2016, and tested whether the closure of the rubbish dump in early 2015 coincided with changes in reproductive parameters while accounting for confounding variables such as weather and the availability of other predictable foraging opportunities. We found an average productivity of 0.78 fledglings per territorial pair before the dump closed and 0.82 after the closure, an average breeding success of 0.64 before and 0.71 after the closure, and an average fledging rate of 1.17 fledglings per successful pair before and 1.26 after the closure of the rubbish dump. Once confounding variables were accounted for, the closure of the rubbish dump did not have a significant effect on reproductive parameters (P = 0.426 for nest survival and P = 0.786 for fledging rate). We speculate that the Egyptian Vulture population in central Turkey may have sufficient alternative food sources and high levels of intra-specific competition due to its density, so that the closure of the rubbish dump may not have resulted in detectable positive or negative effects. We recommend the maintenance of small traditional animal husbandry farms and disposal practices that mimic the spatio-temporally unpredictable supply of food sources that appears to be most beneficial for avian scavengers.

Fathers matter: male body mass affects life-history traits in a size-dimorphic seabird.

One of the predicted consequences of climate change is a shift in body mass distributions within animal populations. Yet body mass, an important component of the physiological state of an organism, can affect key life-history traits and consequently population dynamics. Over the past decades, the wandering albatross—a pelagic seabird providing bi-parental care with marked sexual size dimorphism—has exhibited an increase in average body mass and breeding success in parallel with experiencing increasing wind speeds. To assess the impact of these changes, we examined how body mass affects five key life-history traits at the individual level: adult survival, breeding probability, breeding success, chick mass and juvenile survival. We found that male mass impacted all traits examined except breeding probability, whereas female mass affected none. Adult male survival increased with increasing mass. Increasing adult male mass increased breeding success and mass of sons but not of daughters. Juvenile male survival increased with their chick mass. These results suggest that a higher investment in sons by fathers can increase their inclusive fitness, which is not the case for daughters. Our study highlights sex-specific differences in the effect of body mass on the life history of a monogamous species with bi-parental care.

Effects of prescribed fire on fuels, vegetation, and Golden-cheeked Warbler (Setophaga chrysoparia) demographics in Texas juniper-oak woodlands

The Golden-cheeked Warbler (Setophaga chrysoparia) is an endangered songbird that breeds in mature juniper-oak woodlands restricted to Central Texas. This habitat is increasingly susceptible to crown fire due to climate change, land use change, and fire suppression. Prescribed fire is a potential tool to reduce the risk of crown fire and may be a management tool to enhance juniper-oak woodlands for breeding warblers. However, no experimental study has been undertaken to investigate how well prescribed fire can meet these goals. We conducted a before-after control-impact study on three plot-pairs within Balcones Canyonlands National Wildlife Refuge, Texas, from 2012 to 2014 to evaluate the response of fuel loads, vegetation structure, and warblers to prescribed fire. We measured fuel loads and vegetation structure in summer 2012 (pre-treatment) and 2014 (post-treatment). We burned one randomly-chosen plot within each plot-pair during February 2013 and measured fire severity in May 2013. We monitored populations of warblers to determine plot abundance and breeding success each season. Impact of the prescribed fires was highly variable across treatment plots with ∼49% of points showing no effects of fire on junipers and ∼9% showing high mortality in the juniper canopy. All 12 fuel and vegetation measures responded to fire in the direction expected; however, only juniper seedling density, juniper sapling density, hardwood sapling density, canopy cover, litter cover, and litter depth were significant (i.e., year×treatment effect P<0.05). Warbler density decreased 23% and 40% in the two post-treatment years in response to fire but other demographics did not have significant year×treatment effects. Across all plots and years, 67–93% of males were aged after-second year (“ASY”), pairing success was high (94–100%), average breeding success was 50–63%, and mean daily nest survival was 0.957 (SE=0.010). Return rates averaged 45% and 35% for control and treatment plots. Discrete choice analysis based on locations of males in treated plots revealed the highest probability of use was in closed-canopy woodlands that experienced low to moderate fire severity effects from the fire (canopy intact but some intermediate level of subcanopy mortality) and the lowest probability of use was where fire severity was high. A single application of prescribed fire achieved many but not all fuel and vegetation objectives. The resulting reductions in warbler densities appeared due to avoidance of areas with high burn severity, whereas areas of low to moderate burn severity had high warbler use.

Low survival rather than breeding success explains little penguin population decline on Granite Island

Over the past decade, populations of little penguins (Eudyptula minor) have been seriously declining across South Australia for reasons still not fully understood. In the present study, I investigated breeding performance and return rates of little penguins on Granite Island for 17 years in relation to patterns of population decline. I focussed on the impacts of human disturbance, breeding site, abandonment and predation on breeding success. The average breeding success was 1.05±0.12 fledglings per pair; and breeding success increased since 1990, despite population decline. Breeding site was the main factor affecting breeding success. I found no effect of predation or abandonment on breeding success, but there was a negative effect of predation on the number of dead chicks found. In addition, I found a negative effect of human disturbance. Despite the observed increase in breeding success, return rates were extremely low for both adults (16.3%) and fledglings (2.3%). Population modelling confirmed the observed population decline on Granite Island, with subadult survival being the most critical variable affecting population growth. The present study thus highlighted the need for further studies into factors affecting survival of adults and subadults.

Nest fidelity is driven by multi-scale information in a long-lived seabird.

Although the reproductive success of most organisms depends on factors acting at several spatial scales, little is known about how organisms are able to synthesize multi-scale information to optimize reproduction. Using longitudinal data from a long-lived seabird, Monteiro's storm-petrel, we show that average breeding success is strongly related to oceanic conditions at the population level, and we postulate that (i) individuals use proximal information (their own reproduction outcome in year t) to assess the qualities of their mate and nest and to decide to retain them or not in year t + 1; (ii) the intensity of these responses depends on the quality of the oceanic environment in year t, which affects the predictability of reproduction outcome in year t + 1. Our results confirm that mate and nest fidelities are higher following successful reproduction and that the relationship between the success of a given pair and subsequent nest fidelity is stronger in years with unfavourable oceanic conditions, suggesting that individuals rely on distant information to modulate their use of proximal information and adjust their breeding strategy.

Environmental Instability as a Motor for Dispersal: A Case Study from a Growing Population of Glossy Ibis

Dispersal is a life-history trait directly affecting population dynamics and species range shifts and thus playing a prominent role in the response to climate change. Nonetheless, the relationship between extreme climatic events and dispersal has received little attention in birds. Here we focused on climatic, demographic and individual factors affecting the dispersal propensity of a major glossy ibis population. We performed a capture-resighting analysis on individuals born and observed at Doñana (South-West Spain) over fourteen years. We applied a multiple analytical approach to show that single-site capture-resighting estimates were a reliable index of dispersal propensity from the area. We focused on the emigration of Doñana-born individuals sporadically (transients) and regularly (residents) frequenting their natal area. Droughts during two out of 14 study years caused higher apparent dispersal rates, explaining most of the annual variation in these rates. The age structure of Doñana-born individuals resighted simultaneously locally and in Morocco in one week over the 2010 autumn confirmed that the 2005 drought boosted permanent emigration. As numbers increased steadily during non-drought years since the formation of the colony in 1996 to several thousand pairs, philopatry increased gradually, while transients probability appeared to be related to average breeding success. Age, sex, density, quality of foraging habitat and breeding success in the previous season were not found to directly affect apparent dispersal. Nonetheless, autumn sex ratio gradually switched from male (≈0.68) to female-skewed (≈0.44) by the end of the study period, suggesting that males and females respond differently to high densities reached in recent years. This study demonstrates the importance of extreme climatic events as a powerful motor for spread of species in expansion. Also, it suggests different factors drive emigration of individuals according to their amount of experience in the area (e.g. transients vs residents).

The interaction between reproductive cost and individual quality is mediated by oceanic conditions in a long-lived bird

Environmental variability, costs of reproduction, and heterogeneity in individual quality are three important sources of the temporal and interindividual variations in vital rates of wild populations. Based on an 18-year monitoring of an endangered, recently described, long-lived seabird, Monteiro's Storm-Petrel (Oceanodroma monteiroi), we designed multistate survival models to separate the effects of the reproductive cost (breeders vs. nonbreeders) and individual quality (successful vs. unsuccessful breeders) in relation to temporally variable demographic and oceanographic properties. The analysis revealed a gradient of individual quality from nonbreeders, to unsuccessful breeders, to successful breeders. The survival rates of unsuccessful breeders (0.90 +/- 0.023, mean +/- SE) tended to decrease in years of high average breeding success and were more sensitive to oceanographic variation than those of both (high-quality) successful breeders (0.97 +/- 0.015) and (low-quality) nonbreeders (0.83 +/- 0.028). Overall, our results indicate that reproductive costs act on individuals of intermediate quality and are mediated by environmental harshness.

Impact of climate change and prey abundance on nesting success of a top predator, the goshawk

Contemporary research has documented a large number of shifts in spring phenology and changes in distribution range although the average spring temperatures have increased by only 0.3-0.6 °C over the past 100 years. Generally, earlier breeding birds have larger clutch sizes, and the advancing spring could thus potentially increase breeding success. Shifts in spring phenology can, however, be crucial for bird reproduction, and mistiming the breeding event may even have negative consequences for population development. Our aim was to explore how weather and prey abundance relates to the breeding performance of a north European top predator, the northern goshawk Accipiter gentilis. Our nationwide dataset from Finland, spanning the period 1989-2004, shows that ambient weather has a greater impact on the timing and success of breeding than the density of grouse Tetraonidae, the main prey of goshawks. Higher early spring temperatures were associated with advancing hatching date of goshawks. Correspondingly, grouse density and temperature during laying and brooding were positively associated with brood size, while precipitation showed a negative connection. Applying our models to a future scenario of climate warming, combined with a 50 % reduction in grouse density, suggests that average breeding dates will advance only 2.5 days and average breeding success would remain the same. Notably, breeding success was not spatially equal throughout Finland, as northern and eastern populations suffered most from declining grouse densities. The observed pattern is thus the opposite to what is expected from a population situated at the northern edge of its distribution range, and thus may help to understand why populations may not increase at the northern edge of their thermal distribution due to climate change.