First-year Birds Research Articles

Late-Hatched Great TitsParus major: How Do They Contribute to the Next Year Breeding Population?

Late-hatched Great Tits Parus major have often a lower local recruitment rate than those hatched early. However, estimates of recruitment usually neglect differential disappearance due to dispersal. We report on the proportions of late-hatched breeders in two Great Tit breeding populations (near St. Petersburg, NW Russia, and on the Courish Spit, Eastern Baltic). Using a novel method, late-hatched birds (hatching in July) were distinguished in the next breeding season by the incomplete moult of their tail feathers. Based on this method, we were able to estimate the percentage of late-hatched individuals in all first-year birds, including immigrants. The proportion of late hatched breeders was considerable in some years, and appeared related to the proportion of late fledglings in the previous year. In the Courish Spit population we found no significant difference in the global recruitment rate (accounting for both local birds and immigrants) of early- and late-hatched Great Tits. In the St. Petersburg population, the annual percentage of late-fledged breeders was significantly lower than the percentage of late-hatched fledglings in the previous year. However, in the St. Petersburg population we cannot definitely conclude that the global recruitment rate of late fledglings was lower than that of early fledglings because of peculiarities of the study site. We also can suggest that it may be a result of competition between early and late fledglings, winter residents and newcomers in spring (especially in males), as well as a result of different survival of early- and late-born individuals during the first year.

Are population dynamics of shorebirds affected by El Niño/Southern Oscillation (ENSO) while on their non-breeding grounds in Ecuador?

Declines in avian populations are a global concern, particularly for species that migrate between Arctic-temperate and tropical locations. Long-term population studies offer opportunities to detect and document ecological effects attributable to long-term climatic cycles such as the El Niño/Southern Oscillation (ENSO). In this study, we report possible population-level effects of such climatic cycles on shorebird species that use two non-breeding season sites in Ecuador (Santa Elena peninsula area, near La Libertad). During our 9-year study period (1991/1992–1999/2000), there was a particularly strong ENSO warm phase event during 1997/1998. Population trend data for three species of shorebird, Western Sandpipers ( Calidris mauri), Semipalmated Sandpipers ( C. pusilla), and Least Sandpipers ( C. minutilla), indicated abundances generally declined during the 1990s, but there was an increase in the proportion of first-year birds and their abundance in the years following the 1997/1998 ENSO warm phase. There was some support for variation in apparent survivorship associated with the onset of the ENSO warm phase event in our population models, based on capture–mark–recapture data. Following the 1997/1998 ENSO event onset, individuals for all three species were significantly lighter during the non-breeding season ( F 1,3789 = 6.6, p = 0.01). Least-squares mean mass (controlling for size, sex and day of capture) for first-year birds dropped significantly more than for adults following ENSO (first-year mass loss = 0.69 ± 0.12 g; adult mass loss = 0.34 ± 0.11 g, F 1,3789 = 5.31, p = 0.021), and least-squares mean mass dropped most during the period when sandpipers prepare for northward migration by gaining mass and moulting into breeding plumage. Least Sandpipers may have declined the most in mean mass following ENSO (0.76 ± 0.19 g), whereas Semipalmated Sandpipers were 0.52 ± 0.12 g lighter, and Western Sandpipers 0.40 ± 0.13 g lighter, but overall variation among species before (1992/1993–1996/1997) and after (1997/1998–1999/2000) ENSO was not significant ( F 2,3787 = 1.52, p = 0.22). Anomalously warm and wet conditions associated with strong ENSO warm phases during the non-breeding season may either act directly on shorebirds (increasing metabolic demands) and/or indirectly through reductions in prey availability. Reduced mass and mass gain may explain lower survivorship, particularly in adults, which are more likely to migrate northward in the spring than are first-year birds, at least in Western Sandpipers. Our results suggest a potential mechanism selecting for life history strategies suitable for withstanding long-term fluctuating climatic cycles such as ENSO.

Age‐specific variation in reproduction is largely explained by the timing of territory establishment in the New Zealand stitchbird Notiomystis cincta

1. Using data from 327 nests over a consecutive 8-year period we examined age-specific variation in reproduction in a population of stitchbirds (or hihi) Notiomystis cincta and related how differences in reproductive performance were linked to the timing of territory establishment and breeding. 2. Across the population all reproductive parameters showed a quadratic relationship with an increase mainly between the first and second breeding season and a decline after the fourth year. A longitudinal analysis showed evidence of senescence by the sixth year in the numbers of chicks fledged and recruited. 3. Reproductive increases between years 1 and 2 were the result of poor-quality females dying after their first breeding season (differential selection hypothesis) in combination with surviving females showing improvements in reproduction in their second year (individual improvement/constraint hypothesis). 4. There was no effect of mate experience or territory quality on improvements in breeding between years. 5. The key variable influencing reproductive output was the timing of breeding. Birds that started laying earlier were more likely to lay multiple clutches in any given season. This was the main difference between first-year and older birds; generally first-year birds initiated egg laying later and consequently laid fewer clutches. 6. Approximately half of all first-year birds did not establish their territory until after the breeding season had begun. This delay in territory establishment resulted in these birds delaying breeding, which resulted in them having a lower reproductive output relative to all other birds. First-year birds that managed to establish their territory before breeding commenced, had similar rates of reproduction as older birds. 7. There was a positive relationship between the timing of territory establishment during a female's first year and her hatching date in the previous breeding season. We hypothesize that this was because late-hatched females were less able to effectively compete for territories against earlier-hatched members of their cohort, and this delayed their establishment and breeding in their first year. Thus, this social constraint is likely to be a major factor driving age-specific reproductive variation in this population.

Annual Survival and Turnover Rates of an Afrotropical Robin in a Fragmented Forest

Whether or not subdivided populations persist in fragmented landscapes primarily depends on how well individuals can survive within discrete habitat patches. Using data from six capture–recapture sessions, survival probabilities of the white-starred robin were estimated in seven indigenous forest patches in the highly fragmented Taita Hills forests, SE Kenya. We found no significant differences in survival probability either among fragment-size categories (large 135 ha, medium 95 ha, and small 2–8 ha) or between adult and first-year birds. However, males had a higher probability of survival from one year to the next than females. Turnover rates of adult birds were higher for females than males, but also higher in the medium and small patches than in the large one within each sex. That survival probability was similar among fragments, but turnover rates differed denoted that different processes caused extirpation from the patches. We suggest that mortality associated with dispersal was probably a more important cause of extirpation than within-patch mortality in the largest habitat, which had the lowest turnover rates. Conversely, high within-patch mortality, for instance due to predation during incubation, could have been more important in the smaller, more disturbed habitats. These results lend support to the proposition that avian conservation efforts should be focussed both at the landscape level to improve connectivity between fragments and reduce mortality during dispersal, and at the patch level to exclude other mortality sources such as nest predation.

Net-entrapment of great cormorants Phalacrocorax carbo sinensis in relation to individual age and population size

Population control of great cormorants Phalacrocorax carbo sinensis has been suggested as a tool to mitigate fish losses by cormorants foraging in pound nets, gill nets and fykes. The objective of our study was to quantify the difference between first-year birds and older birds in the risk of drowning in fishing gear, and to explore the influence of population size on the proportion of individuals drowning. An index of the proportion of first-year birds and older birds that drowned was obtained for cormorants ringed in the Danish Vorsø colony using resightings of colour-ringed individuals and ring recoveries of individuals found dead. First-year birds were approximately 10 times more likely to drown than older birds. We used ring recoveries of cormorants ringed in Denmark and found dead in Denmark, North Germany and South Sweden (i.e. in the main post-breeding area) to reveal changes over a 25-year period in the proportion drowned among those recovered. Among first-year birds the proportion drowned declined from 66% in 1978–1984 to 24% in 2000–2002, and among older birds the proportion declined from 46% in 1978–1984 to 26% in 1999–2002. During 1978–2000, breeding numbers in Denmark increased from 1,400 to 42,500 nests, and the proportion of cormorants drowned among those recovered was significantly negatively correlated with population size. This suggests that the proportion of the population foraging in nets declined as the population increased. The damage caused by cormorants foraging in pound nets is, therefore, unlikely to decrease in proportion to reductions in population size reached through population control.

Adaptive benefits of differential post-fledging development patterns in the Lesser Flamingo (Phoenicopterus minor)

Measurements of five morphological components (mass, skull length, culmen, flattened wing and tarsus) and blood samples were taken from 154 fledged wild Lesser Flamingos Phoenicopterus minor captured during 2001 and 2002 at Lake Bogoria, Kenya (0°11'–20'N, 036°06'E). The sample included adults (>3 years old), immature birds (2–3 years old) and first-year juvenile birds of both sexes. The sex of each bird was determined by PCR amplification of the CHD-Z and CHD-W genes, using DNA extracted from blood samples. Within each gender, there were significant differences in mass and tarsus length amongst the three age groups, indicating that the skeletal size and mass of Lesser Flamingos continue to increase between fledging and attainment of adult plumage at three to four years of age. The different morphological components increased in size at different rates, although the same components appeared to increase at similar rates in both males and females. Skull and culmen lengths had reached adult size in juvenile birds, while juvenile wing length, tarsus length and mass were approximately 95%, 85% and 75% of adult size, respectively. The adaptive significance of these findings is discussed.

Sex determination in the Lesser Flamingo (Phoenicopterus minor) using morphological measurements

Morphological measurements and blood samples were taken from 154 Lesser Flamingos Phoenicopterus minor, including adults (>3 years old), immature sub-adults (2–3 years old) and first-year juvenile birds of both sexes, captured at Lake Bogoria, Kenya (0°11'–20' N, 036°06' E) during 2001 and 2002. PCR amplification of the CHD-Z and CHD-W genes using DNA extracted from the blood samples was used to determine the sex of each bird. There were significant differences in mass and tarsus length among the three age groups, indicating that Lesser Flamingos continue to grow in skeletal size and mass between fledging and the attainment of adult plumage at 3–4 years of age. On average, males were significantly larger than females in all age groups, although there was substantial overlap between the sexes in all morphological measurements. The element with the least amount of overlap was head-and-bill length. Discriminant functions utilising head-and-bill length that correctly predict the sex of juvenile and immature birds with approximately 93% accuracy are presented. By adding total tarsus length, the sex of wild adult Lesser Flamingos is correctly predicted with approximately 98% accuracy. The same discriminant function developed for wild adult birds predicted the sex of 19 captive adult Lesser Flamingos of known sex with 100% accuracy.

Estimating population size in Black-tailed Godwits Limosa limosa islandica by colour-marking

Capsule Adult population size of the Icelandic race of Black-tailed Godwit was estimated at 37 500 individuals. Aims To estimate the population size of the Icelandic race of Black-tailed Godwit and infer numbers wintering in areas where both the Icelandic and nominate races overlap. Methods Adult population size was estimated using resightings of individually colour-ringed birds in spring staging flocks in Iceland. Winter distribution was assessed by comparing January counts to the population estimate, assuming 0.5 chicks per breeding pair to estimate the number of first-year birds and the resulting winter population size. Results The adult population size of the Icelandic race of Black-tailed Godwit was estimated at about 37 500 individuals (range 35 565–41 999) between 1999 and 2002. This gives an estimated winter population of about 47 000 individuals. From counts in wintering areas and the calculated population estimate, we estimate that about 15 000 Icelandic Black-tailed Godwits winter in areas where the races overlap (mostly in Portugal, Spain and Morocco) but the numbers in each country remain unknown. Conclusion For species which can be fitted with conspicuous colour rings and for which a high proportion of individuals can be observed at particular times of year, mark–recapture methods can be very useful techniques to estimate the number of individuals of different races of birds wintering in the same areas.

Seasonal Biometric Differences Between Sex and Age Groups of the Graceful WarblerPrinia gracilisat Eilat (Israel)

An abundant resident in Israel, the Graceful Warbler breeds in the northern and central parts of the country and has recently invaded desert areas following their human settlement. Possible seasonal changes in age and sex structure were investigated, as were differences in body measurements in individual sex and age classes as well as changes in the numbers of the Graceful Warbler population in Eilat. No difference was recorded between spring and autumn in the proportion of males to females, nor were significant differences found in the numbers of males and females ringed during the spring and autumn seasons. There was a significant difference in the proportion of juvenile to adult birds trapped in spring and in autumn. Moreover, males had longer wings than females in both seasons. There were no differences in body mass or body condition between sexes in the two seasons. Furthermore, adults had longer wings than first-year birds. Juveniles had longer wings in autumn than in spring, but no differences were recorded in the adults in this respect. In addition, juveniles were in better condition in spring than in autumn; however, there was no difference in body condition of the adults between seasons. The fact that a significant trend was found in the numbers of Graceful Warblers trapped in spring but no such trend in autumn, that a large proportion of recaptured birds were noted in both seasons, and that a high number of individual birds were caught repeatedly during the study period, suggests the existence of a stable or increasing breeding population in Eilat all the year round. In addition, the Bird Sanctuary is like an oasis in the desert environs of Eilat. Hence, the lack of differences among the years in the proportions of males and females between the seasons suggests that it is mainly breeding pairs that occupy the area.

Factors influencing liver PCB concentrations in sparrowhawks ( Accipiter nisus), kestrels ( Falco tinnunculus) and herons ( Ardea cinerea) in Britain

Large scale temporal and spatial changes in the exposure of terrestrial vertebrates to PCBs have been monitored in the UK by measuring liver residues in sparrowhawks ( Accipiter nisus), kestrels ( Falco tinnunculus) and grey herons ( Ardea cinerea) from throughout the country. Residues in the three species are typically characterised by large intra- and inter-specific variation. Data for 306 sparrowhawks, 186 kestrels and 47 herons collected between 1992 and 1997 as part of a national Predatory Bird Monitoring Scheme were examined to determine how much of this variation was explained by body condition, age and sex, rather than other factors. In all three species, body condition was the single most important factor and accounted for up to 49% of the variation in PCB liver residues; starved birds had the highest liver concentrations. Age and sex were also significant but of lesser importance. Adult sparrowhawks and kestrels had liver PCB residues that were 2 to 10-fold higher than in first-year birds. Sex did not affect residue magnitude in a consistent manner. PCB concentrations in the liver were higher in males than females in both first-year and adult kestrels and in first-year sparrowhawks, but adult female sparrowhawks had similar PCB residues to adult males. Liver residues also varied seasonally. PCB concentrations in first-year sparrowhawks increased during the first year following fledging and a similar pattern was detected in adult female sparrowhawks following egg laying. When these physiological factors were taken into account, it was evident that while kestrels with high fat scores had significantly lower PCB concentrations than either sparrowhawks or herons, liver residues were similar in all three species when birds were in a starved condition. Overall during 1992–1997, the combined influence of body condition, age and sex explained more of the variation in liver PCB concentrations than species differences or other factors, such as geographical variation in exposure.

Breeding biology and taxonomy of the Red-breasted Swallow, Hirundo semirufa, in Zimbabwe

Nocturnal trapping and ringing of swallows around Harare, Zimbabwe, showed that at least five species have adapted their breeding behaviour to manmade road culverts, none more so than the Red-breasted Swallow, Hirundo semirufa, which may be found nesting in the smallest (46cm diameter) of pipe culverts. This species is double-brooded and returns to the same nest site year after year, though first-year birds appear to disperse well away from the natal area. There was some evidence of low divorce rates. Both adults roost in the nest at night, accompanied at times by a first brood of fledglings together with a second brood of naked young. Predation of nest contents is minimal, and adult longevity was about 5-6 years. Comparison of mensural data from Harare and Bloemfontein shows that the putative split of the nominate race of H. semirufa into (northern) short-tailed and (southern) long-tailed populations is not tenable on morphological grounds alone.

Demographic analysis from summaries of an age-structured population.

Demographic analyses of age-structured populations typically rely on life history data for individuals, or when individual animals are not identified, on information about the numbers of individuals in each age class through time. While it is usually difficult to determine the age class of a randomly encountered individual, it is often the case that the individual can be readily and reliably assigned to one of a set of age classes. For example, it is often possible to distinguish first-year from older birds. In such cases, the population age structure can be regarded as a latent variable governed by a process prior, and the data as summaries of this latent structure. In this article, we consider the problem of uncovering the latent structure and estimating process parameters from summaries of age class information. We present a demographic analysis for the critically endangered migratory population of whooping cranes (Grus americana), based only on counts of first-year birds and of older birds. We estimate age and year-specific survival rates. We address the controversial issue of whether management action on the breeding grounds has influenced recruitment, relating recruitment rates to the number of seventh-year and older birds, and examining the pattern of variation through time in this rate.

First-year and adult survival of the endangered Lesser Kestrel Falco naumanni in southern France

Capsule Local first-year survival is higher in La Crau than in Spain with a high proportion of first-year birds returning to their natal area. Aims To understand life history parameters that may be responsible for a population growth since the 1990s. Methods Between 1994 and 1998, 342 birds were ringed as fledglings. From capture–recapture data, annual local survival and the proportion of first-year birds returning to their natal area were computed. Results The annual survival probabilities of adult and first-year birds were constant during the study period (0.67 and 0.57, respectively). A high proportion (0.72) of birds ringed as fledglings returned to their natal area after one year. Of these, an estimated 53% bred in that first year. Local first-year survival is higher in La Crau than other areas in Spain. Conclusion The high local survival rate, the product of survival and philopatry, might be responsible for the positive trend of the population. A peculiar aspect of the La Crau population is that ground nests have been increasingly common since the 1990s despite a higher risk of breeding failure.

SEASONAL, AGE, AND SEX DIFFERENCES IN WEIGHT, FAT RESERVES, AND PLASMA CORTICOSTERONE IN WESTERN SANDPIPERS

Abstract Western Sandpipers (Calidris mauri) were followed throughout their annual cycle along the Pacific Coast of North America. Changes in body condition and corticosterone were tracked at two overwintering sites (Ensenada, Mexico, and San Diego, California), four migration stopover sites (Bodega Bay, California, Grays Harbor and False Bay, Washington, and Hartney Bay, Alaska), and a breeding site (Nome, Alaska). Adult males and females had elevated weight and fat scores during spring migration, breeding, and autumn migration relative to lean levels during winter. Although elevated mass and fat reserves may hinder escape from predators and are not necessary at wintering sites with benign conditions, the cost-benefit trade-offs with weight and agility shift during migration and breeding. Extra fat and muscle are necessary for fueling the long flight to and from the breeding grounds and serve as a hedge against unpredictable food and weather conditions on the breeding grounds. First-year birds weighed le...

Biometrics, moult and migration of Grey Plovers, Pluvialis squatarola, at Mida Creek, Kenya

Ringing data for 269 Grey Plovers, Pluvialis squatarola, caught between 1977 and 1988 were analysed. Adults started primary moult soon after arrival in August and September. Mean starting date of adult primary moult was estimated to be 13 September, primary moult duration as 130 days. First-year birds had not started primary moult by early May. Body mass of adults in primary moult was stable from September to February, averaging 205 g, but increased at the end of the moult in March and April. Pre-migratory body mass was estimated at 268 g and would allow a non-stop flight as far as the southeastern Mediterranean or the northern Persian Gulf. Mean wing length increased in March and April, possibly due to the arrival of longer-winged migrants from southern Africa or an earlier departure of males. A decrease in numbers was observed from the second half of April and by mid-May all adults had left the area. In June/July, the number of first-year birds was c. 25% of the total summer population.

Biometrics, possible breeding origins and migration routes of South African Grey Plovers, Pluvialis squatarola

Data on 355 Grey Plovers, Pluvialis squatarola, ringed in South Africa in 1971–1997 were analysed. Biometrics could not identify well-defined origins in the Siberian breeding grounds, but suggested the presence of birds from east of the Gydan Peninsula. Comparisons with populations spending the non-breeding period elsewhere did not allow a clear definition of the flyways used, although ringing recoveries indicated a migration route crossing the Mediterranean/Black Sea region during both southward and northward movements. First-year birds were on average smaller than adults. Bill length increased during the first year, reaching adult length by June. First-year wing length decreased by 4% before the outermost primary was shed, adult wing length did not change with season.

Numbers, migration phenology and survival of Purple Sandpipers Calidris maritima at Gourdon, eastern Scotland

Purple Sandpipers wintering on the Kincardine coast had a protracted autumn arrival (one-quarter and three-quarters of the birds arrived on 30 July and 21 October respectively, 83 days) but a faster spring departure (one-quarter and three-quarters departed on 9 April and 27 May respectively, 48 days). The long arrival period was partly due to differences in the migration phenology of the two main wintering populations: short-billed birds from Norway arrived before the long-billed birds, probably from Canada. There was a smaller difference in departure times of the two populations: short-billed birds left before the long-billed birds. Minimum annual survival was estimated from resightings of 92 marked birds. There was no evidence that survival differed between adults and first-years or between birds of different bill-size classes, which were of different sex and geographical origin. Minimum annual survival was estimated to be 79.5%(se = 2.8%). The similarity between the mortality rate (20.5%) and the percentage of first-year birds in populations of Purple Sandpipers probably reflects balanced population dynamics.

Long-Term Study of Apparent Survival in Pacific Golden-Plovers at a Wintering Ground on Oahu, Hawaiian Islands

AbstractWe monitored the apparent survival of territorial and nonterritorial Pacific Golden-Plovers (Pluvialis fulva) for 20 consecutive nonbreeding seasons at a wintering ground within Bellows Air Force Station (BAFS) on the eastern shore of Oahu, Hawaii. Territorial birds were especially site-faithful from season to season, and each surviving individual reoccupied the same territory held in previous seasons. On average, territorial birds were resighted for about twice as many postbanding seasons (4.2) as nonterritorial birds (1.8). Open-population modeling indicated that apparent survival varied by age and territorial status. Our most parsimonious model estimated apparent annual survival rates in territorial plovers as 0.90 for young birds (age determined from retained juvenal primaries) from their first through their second wintering season, and 0.80 for adults over numerous seasons. For nonterritorial plovers, the corresponding values were 0.82 and 0.67, respectively. Despite lower apparent survival in nonterritorial plovers, it remains uncertain whether nonterritoriality actually results in shorter life spans. Some surviving nonterritorial birds may have gone undetected (detection probability of 0.70) because of permanent emigration from the study area. Given strong site-fidelity of territorial birds and the relative certainty of detecting them (probability = 1.0), we regarded the disappearance of a plover from its territory as an indicator of mortality. From last-recorded sightings, we concluded that territorial birds died with about equal frequency during the nonbreeding and breeding seasons. Because the latter is of much shorter duration, time-relative hazards were greatest while birds were away from the wintering grounds. Winter mortality was caused by accidents (collisions with overhead wires and other obstructions), and probable predation by owls. We estimated mean additional life expectancy among territorial plovers at 5.1 years for first-year birds, and 4.5 years for unknown-age adults. The oldest known-age individual was a male that lived 13 years 10 months; in adults of uncertain ages, one male survived to a minimum age of 18 years 10 months, and two females to at least 17 years 10 months. Pacific Golden-Plovers wintering at BAFS, especially territorial birds, demonstrated relatively high rates of apparent survival combined with adaptability for coexistence with humans in an urban environment.

Age-related differences in abundance and habitat occupation of Steppe Buzzards, Buteo buteo vulpinus, in the Western Cape Province, South Africa

Linear densities of Steppe Buzzards, Buteo buteo, were studied along roads at the southernmost edge of the nonbreeding range in the Western Cape Province, South Africa. Adults arrived before first-year birds; peak populations of the former occurred from December to February, and of the latter from January onwards. Except during periods with rapidly changing numbers during arrival and departure, the proportion of first-year birds in three different areas was negatively related to the density of adults. This may be the result of competitive exclusion by adults, or of age-related differences in habitat preference. Adults dominated first-year birds at hunting perches. First-year birds were most frequently found in more closed habitat, where they attempted to kleptoparasitize adults in 18% of all observed instances of prey mantling.

Condition-dependent immune defence in the Magpie: how important is ectoparasitism?

The immune system has high costs of maintenance and production and expression of immune function should be dependent on the condition of the individual. The fitness cost that parasites impose on hosts may then be reflected in the investment that hosts make in immunity. Little is known about sources and consequences of intra- and interspecific variation in the presence and size of organs of the immune system in relation to ectoparasitism. Variation in the presence and size of the bursa of Fabricius and spleen size was measured to assess potential relationships between immune defence, nutritional condition and ectoparasites in the Magpie Pica pica. We found three species of lice (Mallophaga), two species of louse flies (Hippoboscidae), and the larvae of a tick (Ixodidae) infecting magpies. Age explained a significant proportion of variation in prevalence and number of parasite species, first-year birds being more parasitized than adults. Spleen size increased with nutritional condition, and the level of ectoparasitism negatively affected nutritional condition. Only one species of lice (Philopterus picae) was involved; the negative effect of this species on nutritional condition was more pronounced in males than females. These results are consistent with the hypotheses of condition-dependent investment in the immune system and sexual differences in susceptibility to parasite effects. Ectoparasites may thus play a role in modulating the relationship between condition and immune defence. We also found evidence for an interaction between the bursa and the spleen. Magpies with bursa had larger spleens than those where it had atrophied. This interaction did not affect ectoparasitism or nutritional condition.