Effect of 2020-21 and 2021-22 Highly Pathogenic Avian Influenza H5 Epidemics on Wild Birds, the Netherlands.

The number of highly pathogenic avian influenza (HPAI) H5-related infections and deaths of wild birds in Europe was high during October 1, 2020-September 30, 2022. To quantify deaths among wild species groups with known susceptibility for HPAI H5 during those epidemics, we collected and recorded mortality data of wild birds in the Netherlands. HPAI virus infection was reported in 51 bird species. The species with the highest numbers of reported dead and infected birds varied per epidemic year; in 2020-21, they were within the Anatidae family, in particular barnacle geese (Branta leucopsis) and in 2021-22, they were within the sea bird group, particularly Sandwich terns (Thalasseus sandvicensis) and northern gannet (Morus bassanus). Because of the difficulty of anticipating and modeling the future trends of HPAI among wild birds, we recommend monitoring live and dead wild birds as a tool for surveillance of the changing dynamics of HPAI.

More management, less damage? With increasing population size, economic costs of managing geese to minimize yield losses may outweigh benefits.

Conflicts between farmers and geese are intensifying; yet, it remains unclear how interactions between goose population size and management regimes affect yield loss and economic costs. We investigate the cost-effectiveness of accommodation and scaring areas in relation to barnacle goose (Branta leucopsis) population size. We use an existing individual-based model of barnacle geese foraging in nature, accommodation, and scaring areas in Friesland, the Netherlands, to study the most cost-effective management under varying population sizes (i.e., between 20 and 200% of the current size). Our study shows that population size non-linearly affects yield loss costs and total costs per goose. The most cost-effective management scenario for intermediate to large populations is to avoid scaring of geese. For small populations, intensive scaring resulted in minimized yield loss costs and total costs, but also substantially lower goose body mass. Our results strongly suggest that scaring becomes a less effective management measure as goose populations increase.

Mercury-Modulated Immune Responses in Arctic Barnacle Goslings (Branta leucopsis) upon a Viral-Like Immune Challenge.

Historical mining activities in Svalbard (79°N/12°E) have caused local mercury (Hg) contamination. To address the potential immunomodulatory effects of environmental Hg on Arctic organisms, we collected newborn barnacle goslings (Branta leucopsis) and herded them in either a control or mining site, differing in Hg levels. An additional group at the mining site was exposed to extra inorganic Hg(II) via supplementary feed. Hepatic total Hg concentrations differed significantly between the control (0.011 ± 0.002 mg/kg dw), mine (0.043 ± 0.011 mg/kg dw), and supplementary feed (0.713 ± 0.137 mg/kg dw) gosling groups (average ± standard deviation). Upon immune challenge with double-stranded RNA (dsRNA) injection, endpoints for immune responses and oxidative stress were measured after 24 h. Our results indicated that Hg exposure modulated the immune responses in Arctic barnacle goslings upon a viral-like immune challenge. Increased exposure to both environmental as well as supplemental Hg reduced the level of natural antibodies, suggesting impaired humoral immunity. Hg exposure upregulated the expression of proinflammatory genes in the spleen, including inducible nitric oxide synthase (iNOS) and interleukin 18 (IL18), suggesting Hg-induced inflammatory effects. Exposure to Hg also oxidized glutathione (GSH) to glutathione disulfide (GSSG); however, goslings were capable of maintaining the redox balance by de novo synthesis of GSH. These adverse effects on the immune responses indicated that even exposure to low, environmentally relevant levels of Hg might affect immune competence at the individual level and might even increase the susceptibility of the population to infections.

Environmental change reduces body condition, but not population growth, in a high-arctic herbivore.

Environmental change influences fitness-related traits and demographic rates, which in herbivores are often linked to resource-driven variation in body condition. Coupled body condition-demographic responses may therefore be important for herbivore population dynamics in fluctuating environments, such as the Arctic. We applied a transient Life-Table Response Experiment ('transient-LTRE') to demographic data from Svalbard barnacle geese (Branta leucopsis), to quantify their population-dynamic responses to changes in body mass. We partitioned contributions from direct and delayed demographic and body condition-mediated processes to variation in population growth. Declines in body condition (1980-2017), which positively affected reproduction and fledgling survival, had negligible consequences for population growth. Instead, population growth rates were largely reproduction-driven, in part through positive responses to rapidly advancing spring phenology. The virtual lack of body condition-mediated effects indicates that herbivore population dynamics may be more resilient to changing body condition than previously expected, with implications for their persistence under environmental change.

Contrasting consequences of climate change for migratory geese: Predation, density dependence and carryover effects offset benefits of high-arctic warming.

Climate change is most rapid in the Arctic, posing both benefits and challenges for migratory herbivores. However, population-dynamic responses to climate change are generally difficult to predict, due to concurrent changes in other trophic levels. Migratory species are also exposed to contrasting climate trends and density regimes over the annual cycle. Thus, determining how climate change impacts their population dynamics requires an understanding of how weather directly or indirectly (through trophic interactions and carryover effects) affects reproduction and survival across migratory stages, while accounting for density dependence. Here, we analyse the overall implications of climate change for a local non-hunted population of high-arctic Svalbard barnacle geese, Branta leucopsis, using 28 years of individual-based data. By identifying the main drivers of reproductive stages (egg production, hatching and fledging) and age-specific survival rates, we quantify their impact on population growth. Recent climate change in Svalbard enhanced egg production and hatching success through positive effects of advanced spring onset (snow melt) and warmer summers (i.e. earlier vegetation green-up) respectively. Contrastingly, there was a strong temporal decline in fledging probability due to increased local abundance of the Arctic fox, the main predator. While weather during the non-breeding season influenced geese through a positive effect of temperature (UK wintering grounds) on adult survival and a positive carryover effect of rainfall (spring stopover site in Norway) on egg production, these covariates showed no temporal trends. However, density-dependent effects occurred throughout the annual cycle, and the steadily increasing total flyway population size caused negative trends in overwinter survival and carryover effects on egg production. The combination of density-dependent processes and direct and indirect climate change effects across life history stages appeared to stabilize local population size. Our study emphasizes the need for holistic approaches when studying population-dynamic responses to global change in migratory species.

Temperature and precipitation at migratory grounds influence demographic trends of an Arctic-breeding bird.

Anthropogenic climate disruption, including temperature and precipitation regime shifts, has been linked to animal population declines since the mid-20th century. However, some species, such as Arctic-breeding geese, have thrived during this period. An increased understanding of how climate disruption might link to demographic rates in thriving species is an important perspective in quantifying the impact of anthropogenic climate disruption on the global state of nature. The Greenland barnacle goose (Branta leucopsis) population has increased tenfold in abundance since the mid-20th century. A concurrent weather regime shift towards warmer, wetter conditions occurred throughout its range in Greenland (breeding), Ireland and Scotland (wintering) and Iceland (spring and autumn staging). The aim of this study was to determine the relationship between weather and demographic rates of Greenland barnacle geese to discern the role of climate shifts in the population trend. We quantified the relationship between temperature and precipitation and Greenland barnacle goose survival and productivity over a 50 year period from 1968 to 2018. We detected significant positive relationships between warmer, wetter conditions on the Icelandic spring staging grounds and survival. We also detected contrasting relationships between warmer, wetter conditions during autumn staging and survival and productivity, with warm, dry conditions being the most favourable for productivity. Survival increased in the latter part of the study period, supporting the possibility that spring weather regime shifts contributed to the increasing population trend. This may be related to improved forage resources, as warming air temperatures have been shown to improve survival rates in several other Arctic and northern terrestrial herbivorous species through indirect bottom-up effects on forage availability.

Ontogenetic niche shifts as a driver of seasonal migration.

Ontogenetic niche shifts have helped to understand population dynamics. Here we show that ontogenetic niche shifts also offer an explanation, complementary to traditional concepts, as to why certain species show seasonal migration. We describe how demographic processes (survival, reproduction and migration) and associated ecological requirements of species may change with ontogenetic stage (juvenile, adult) and across the migratory range (breeding, non-breeding). We apply this concept to widely different species (dark-bellied brent geese (Branta b. bernicla), humpback whales (Megaptera novaeangliae) and migratory Pacific salmon (Oncorhynchus gorbuscha) to check the generality of this hypothesis. Consistent with the idea that ontogenetic niche shifts are an important driver of seasonal migration, we find that growth and survival of juvenile life stages profit most from ecological conditions that are specific to breeding areas. We suggest that matrix population modelling techniques are promising to detect the importance of the ontogenetic niche shifts in maintaining migratory strategies. As a proof of concept, we applied a first analysis to resident, partial migratory and fully migratory populations of barnacle geese (Branta leucopsis). We argue that recognition of the costs and benefits of migration, and how these vary with life stages, is important to understand and conserve migration under global environmental change.

Climate warming may affect the optimal timing of reproduction for migratory geese differently in the low and high Arctic.

Rapid climate warming is driving organisms to advance timing of reproduction with earlier springs, but the rate of advancement shows large variation, even among populations of the same species. In this study, we investigated how the rate of advancement in timing of reproduction with a warming climate varies for barnacle goose (Branta leucopsis) populations breeding at different latitudes in the Arctic. We hypothesized that populations breeding further North are generally more time constrained and, therefore, produce clutches earlier relative to the onset of spring than southern populations. Therefore, with increasing temperatures and a progressive relief of time constraint, we expected latitudinal differences to decrease. For the years 2000-2016, we determined the onset of spring from snow cover data derived from satellite images, and compiled data on egg laying date and reproductive performance in one low-Arctic and two high-Arctic sites. As expected, high-Arctic geese laid their eggs earlier relative to snowmelt than low-Arctic geese. Contrary to expectations, advancement in laying dates was similar in high- and low-Arctic colonies, at a rate of 27% of the advance in date of snowmelt. Although advancement of egg laying did not fully compensate for the advancement of snowmelt, geese laying eggs at intermediate dates in the low Arctic were the most successful breeders. In the high Arctic, however, early nesting geese were the most successful breeders, suggesting that high-Arctic geese have not advanced their laying dates sufficiently to earlier springs. This indicates that high-Arctic geese especially are vulnerable to negative effects of climate warming.

Stress behaviour and physiology of developing Arctic barnacle goslings ( Branta leucopsis) is affected by legacy trace contaminants.

Natural populations are persistently exposed to environmental pollution, which may adversely impact animal physiology and behaviour and even compromise survival. Responding appropriately to any stressor ultimately might tip the scales for survival, as mistimed behaviour and inadequate physiological responses may be detrimental. Yet effects of legacy contamination on immediate physiological and behavioural stress coping abilities during acute stress are virtually unknown. Here, we assessed these effects in barnacle goslings ( Branta leucopsis) at a historical coal mine site in the Arctic. For three weeks we led human-imprinted goslings, collected from nests in unpolluted areas, to feed in an abandoned coal mining area, where they were exposed to trace metals. As control we led their siblings to feed on clean grounds. After submitting both groups to three well-established stress tests (group isolation, individual isolation, on-back restraint), control goslings behaved calmer and excreted lower levels of corticosterone metabolites. Thus, legacy contamination may decisively change stress physiology and behaviour in long-lived vertebrates exposed at a young age.

Respiratory mechanics and morphology of Tibetan and Andean high-altitude geese with divergent life histories.

High-altitude bar-headed geese (Anser indicus) and Andean geese (Chloephaga melanoptera) have been shown to preferentially increase tidal volume over breathing frequency when increasing ventilation during exposure to hypoxia. Increasing tidal volume is a more effective breathing strategy but is also thought to be more mechanically and metabolically expensive. We asked whether there might be differences in the mechanics or morphology of the respiratory systems of high-altitude transient bar-headed geese and high-altitude resident Andean geese that could minimize the cost of breathing more deeply. We compared these two species with a low-altitude migratory species, the barnacle goose (Branta leucopsis). We ventilated anesthetized birds to measure mechanical properties of the respiratory system and used CT scans to quantify respiratory morphology. We found that the respiratory system of Andean geese was disproportionately larger than that of the other two species, allowing use of a deeper breathing strategy for the same energetic cost. The relative size of the respiratory system, especially the caudal air sacs, of bar-headed geese was also larger than that of barnacle geese. However, when normalized to respiratory system size, the mechanical cost of breathing did not differ significantly among these three species, indicating that deeper breathing is enabled by morphological but not mechanical differences between species. The metabolic cost of breathing was estimated to be <1% of basal metabolic rate at rest in normoxia. Because of differences in the magnitude of the ventilatory response, the cost of breathing was estimated to increase 7- to 10-fold in bar-headed and barnacle geese in severe hypoxia, but less than 1-fold in Andean geese exposed to the same low atmospheric PO2.

Altitude matters: differences in cardiovascular and respiratory responses to hypoxia in bar-headed geese reared at high and low altitudes.

Bar-headed geese (Anser indicus) fly at high altitudes during their migration across the Himalayas and Tibetan plateau. However, we know relatively little about whether rearing at high altitude (i.e. phenotypic plasticity) facilitates this impressive feat because most of what is known about their physiology comes from studies performed at sea level. To provide this information, a comprehensive analysis of metabolic, cardiovascular and ventilatory responses to progressive decreases in the equivalent fractional composition of inspired oxygen (FiO2 : 0.21, 0.12, 0.09, 0.07 and 0.05) was made on bar-headed geese reared at either high altitude (3200 m) or low altitude (0 m) and on barnacle geese (Branta leucopsis), a low-altitude migrating species, reared at low altitude (0 m). Bar-headed geese reared at high altitude exhibited lower metabolic rates and a modestly increased hypoxic ventilatory response compared with low-altitude-reared bar-headed geese. Although the in vivo oxygen equilibrium curves and blood-oxygen carrying capacity did not differ between the two bar-headed goose study groups, the blood-oxygen carrying capacity was higher than that of barnacle geese. Resting cardiac output also did not differ between groups and increased at least twofold during progressive hypoxia, initially as a result of increases in stroke volume. However, cardiac output increased at a higher FiO2  threshold in bar-headed geese raised at high altitude. Thus, bar-headed geese reared at high altitude exhibited a reduced oxygen demand at rest and a modest but significant increase in oxygen uptake and delivery during progressive hypoxia compared with bar-headed geese reared at low altitude.

Forecasting spring from afar? Timing of migration and predictability of phenology along different migration routes of an avian herbivore.

Herbivorous birds are hypothesized to migrate in spring along a seasonal gradient of plant profitability towards their breeding grounds (green wave hypothesis). For Arctic breeding species in particular, following highly profitable food is important, so that they can replenish resources along the way and arrive in optimal body condition to start breeding early. We compared the timing of migratory movements of Arctic breeding geese on different flyways to examine whether flyways differed in the predictability of spring conditions at stopovers and whether this was reflected in the degree to which birds were following the green wave. Barnacle geese (Branta leucopsis) were tracked with solar GPS/ARGOS PTTs from their wintering grounds to breeding sites in Greenland (N = 7), Svalbard (N = 21) and the Barents Sea (N = 12). The numerous stopover sites of all birds were combined into a set of 16 general stopover regions. The predictability of climatic conditions along the flyways was calculated as the correlation and slope between onsets of spring at consecutive stopovers. These values differed between sites, mainly because of the presence or absence of ecological barriers. Goose arrival at stopovers was more closely tied to the local onset of spring when predictability was higher and when geese attempted breeding that year. All birds arrived at early stopovers after the onset of spring and arrived at the breeding grounds before the onset of spring, thus overtaking the green wave. This is in accordance with patterns expected for capital breeders: first, they must come into condition; at intermediate stopovers, arrival with the food quality peak is important to stay in condition, and at the breeding grounds, early arrival is favoured so that hatching of young can coincide with the peak of food quality. Our results suggest that a chain of correlations between climatic conditions at subsequent stopovers enables geese to closely track the green wave. However, the birds' precision of migratory timing seems uninfluenced by ecological barriers, indicating partly fixed migration schedules. These might become non-optimal due to climate warming and preclude accurate timing of long-distance migrants in the future.

Tropism of Highly Pathogenic Avian Influenza H5 Viruses from the 2020/2021 Epizootic in Wild Ducks and Geese.

Highly pathogenic avian influenza (HPAI) outbreaks have become increasingly frequent in wild bird populations and have caused mass mortality in many wild bird species. The 2020/2021 epizootic was the largest and most deadly ever reported in Europe, and many new bird species tested positive for HPAI virus for the first time. This study investigated the tropism of HPAI virus in wild birds. We tested the pattern of virus attachment of 2020 H5N8 virus to intestinal and respiratory tissues of key bird species; and characterized pathology of naturally infected Eurasian wigeons (Mareca penelope) and barnacle geese (Branta leucopsis). This study determined that 2020 H5N8 virus had a high level of attachment to the intestinal epithelium (enterotropism) of dabbling ducks and geese and retained attachment to airway epithelium (respirotropism). Natural HPAI 2020 H5 virus infection in Eurasian wigeons and barnacle geese also showed a high level of neurotropism, as both species presented with brain lesions that co-localized with virus antigen expression. We concluded that the combination of respirotropism, neurotropism, and possibly enterotropism, contributed to the successful adaptation of 2020/2021 HPAI H5 viruses to wild waterbird populations.

Evaluation of Nutritional Quality and Sensory Parameters of Meat from Mallard and Four Species of Wild Goose.

Future challenges concerning protein supply for food and feed include the management of all currently available resources. In Sweden, wildfowl are hunted for several reasons, one of which is to protect growing crops. In this study, meat from wild geese and mallard was evaluated with respect to its quality and sensory parameters. The most pronounced sensory differences were between meat from the barnacle goose and the Canada goose and between meat from mallards that were farmed and born wild. This study also provides measurements of values for the nutritional and heavy metal contents of the meat from these wildfowl species in order to elucidate their possible use as modern foods.

Does intensive goose grazing affect breeding waders?

Increasing goose population sizes gives rise to conflicts with human socioeconomic interests and in some circumstances conservation interests. Grazing by high abundances of geese in grasslands is postulated to lead to a very short and homogeneous sward height negatively affecting cover for breeding meadow birds and impacting survival of nests and chicks. We studied the effects of spring grazing barnacle geese Branta leucopsis and brent geese Branta bernicla on occupancy of extensively farmed freshwater grasslands by nesting and brood-rearing waders on the island Mandø in the Danish Wadden Sea. We hypothesized that goose grazing would lead to a shorter grass sward, negatively affecting the field occupancy by territorial/nesting and chick-rearing waders, particularly species preferring taller vegetation. Goose grazing led to a short grass sward (<5 cm height) over most of the island. To achieve a variation in sward height, we kept geese off certain fields using laser light. We analyzed effects of field size, sward height, mosaic structure of the vegetation, proximity to shrub as cover for potential predators, and elevation above ground water level as a measure of wetness on field occupancy by nesting and chick-rearing waders. The analysis indicated that the most important factor explaining field occupancy by nesting redshank Tringa totanus, black-tailed godwit Limosa limosa, oystercatcher Haematopus ostralegus and lapwing Vanellus vanellus as well as by chick-rearing black-tailed godwit and lapwing was short vegetation height. Distance to shrub cover and elevation were less important. Hence, despite very intensive goose grazing, we could not detect any negative effect on the field occupancy by nesting nor chick-rearing waders, including redshank and black-tailed godwit, which are known to favor longer vegetation to conceal their nests and hide their chicks. Possible negative effects may be buffered by mosaic structures in fields and proximity to taller vegetation along fences and ditches.

Arctic Geese Tune Migration to a Warming Climate but Still Suffer from a Phenological Mismatch.

Climate warming challenges animals to advance their timing of reproduction [1], but many animals appear to be unable to advance at the same rate as their food species [2, 3]. As a result, mismatches can arise between the moment of largest food requirements for their offspring and peak food availability [4-6], with important fitness consequences [7]. For long-distance migrants, adjustment of phenology to climate warming may be hampered by their inability to predict the optimal timing of arrival at the breeding grounds from their wintering grounds [8]. Arrival can be advanced if birds accelerate migration by reducing time on stopover sites [9, 10], but a recent study suggests that most long-distance migrants are on too tight a schedule to do so [11]. This may be different for capital-breeding migrants, which use stopovers not only to fuel migration but also to acquire body stores needed for reproduction [12-14]. By combining multiple years of tracking and reproduction data, we show that a long-distance migratory bird (the barnacle goose, Branta leucopsis) accelerates its 3,000 km spring migration to advance arrival on its rapidly warming Arctic breeding grounds. As egg laying has advanced much less than arrival, they still encounter a phenological mismatch that reduces offspring survival. A shift toward using more local resources for reproduction suggests that geese first need to refuel body stores at the breeding grounds after accelerated migration. Although flexibility in body store use allows migrants to accelerate migration, this cannot solve the time constraint they are facing under climate warming.

Reconciling competing values placed upon goose populations: The evolution of and experiences from the Islay Sustainable Goose Management Strategy.

The Scottish island of Islay hosts 45 000 barnacle geese Branta leucopsis (56% of the Greenland barnacle goose population, plus those passing through on migration), 5000 Greenland white-fronted geese Anser albifrons flavirostris (up to 30% of the world population) and 2500 greylag geese Anser anser, most of which feed on 9000 ha of grassland. The financial impacts of estimated agricultural damage have risen greatly over the past 20 years due to increasing goose numbers and higher farming costs. Mechanisms implemented to resolve conflict over time are reviewed for their effectiveness. Emphasis is placed on coordinating the implementation of strategic national conflict resolution at a local scale where the relative pressure from internationally important concentrations of geese on agriculture is acute. Despite the "local" nature of this problem, the benefit from the experience of decades of attempted conflict resolution and the effectiveness of existing programmes can contribute much to the regional and flyway dimensions of this international issue.

Pre-breeding survival of barnacle geese Branta leucopsis in relation to fledgling characteristics.

Pre-breeding survival, i.e. survival from fledging up to the third winter, of barnacle geese (Branta leucopsis) was analysed by using more than 45000 resightings of 1302 individually marked birds. Since observations from the wintering areas only were used, the survival estimates obtained were not confounded by natal dispersal. Post-fledging survival, i.e. survival from fledging to the start of the first winter, differed significantly among the 10 cohorts analysed. These differences were related to the mean body weight and mean tarsus length of cohorts just before fledging. In further analyses, when data from all cohorts were combined, body weight and tarsus length of individuals just before fledging were found to be positively related to Post-fledging survival. Post-fledging survival rates were, on average, lower than survival rates after the first winter. This age effect was largest in cohorts with low mean body weight at capture and was absent in cohorts with high mean body weight at capture. The age effect on survival could only partly be explained by individuals with low body weight having lower survival and successively disappearing from the cohort. It was therefore concluded that other factors must have contributed to the age effect as well. It is hypothesized that lightweight birds are more vulnerable to, for example, diseases, parasites and predation during the first months of their life. Once they have survived this critical period, the effects of characters related to low body weight at capture seem to disappear. Body size of juveniles just before fledging has previously been found to be greatly affected by weather factors and the availability of high quality food during the growth period. Hence, a considerable part of the observed variation in Pre-breeding survival appears to be outside the individual's control.

BODY SIZE DECLINES DESPITE POSITIVE DIRECTIONAL SELECTION ON HERITABLE SIZE TRAITS IN A BARNACLE GOOSE POPULATION.

Analyses of more than 2000 marked barnacle geese (Branta leucopsis) in the largest Baltic colony, Sweden, showed that structurally large females generally produced larger clutches and larger eggs, hatched their broods earlier in the season, and produced more and heavier young than smaller females. In males, the corresponding relationships between reproductive parameters and structural body size were weaker or nonsignificant. Because structural body size traits have previously been found to be significantly heritable and positively genetically correlated, an increase in mean structural body size of individuals as a response to selection might have been expected. By contrast, we found that the mean adult head length and mean adult tarsus length decreased significantly in the largest colony by approximately 0.7 and 0.5 standard deviations, respectively, in both males and females during the 13-year study period. Environmental factors, such as the amount of rain in different years, were found to affect the availability of high-quality food for growing geese. As a consequence of this temporal variability in the availability of high-quality food, the mean adult structural body size of different cohorts differed by up to 1.3 standard deviations. Comparisons of mean body size of cohorts born in different colonies suggest that the most likely explanation for the body-size decline in the main study colony is that a density-dependent process, which mainly was in effect during the very early phase of colony growth, negatively affected juvenile growth and final size. We conclude that large environmental effects on growth and final structural body size easily can mask microevolutionary responses to selection. Analyses of environmental causes underlying temporal and spatial body size variation should always be considered in the reconstruction and prediction of evolutionary changes in natural populations.

GENETIC AND SOCIAL INHERITANCE OF BODY AND EGG SIZE IN THE BARNACLE GOOSE (BRANTA LEUCOPSIS).

We present heritability estimates for final size of body traits and egg size as well as phenotypic and genetic correlations between body and egg traits in a recently established population of the barnacle goose (Branta leucopsis) in the Baltic area. Body traits as well as egg size were heritable and, hence, could respond evolutionarily to phenotypic selection. Genetic correlations between body size traits were significantly positive and of similar magnitude or higher than the corresponding phenotypic correlations. Heritability estimates for tarsus length obtained from full-sib analyses were higher than those obtained from midoffspring-midparent regressions, and this indicates common environment effects on siblings. Heritabilities for tarsus length obtained from midoffspring-mother regressions were significantly higher than estimates from midoffspring-father regressions. The results suggest that this discrepancy is not caused by maternal effects through egg size, nor by extra-pair fertilizations, but by a socially inherited foraging site fidelity in females.