Family-based winter territoriality in western bluebirds, Sialia mexicana: the structure and dynamics of winter groups

Winter residency is characteristic of the majority of cooperatively breeding birds, but the composition and dynamics of winter groups have been examined in relatively few. In 1996-1998, we examined winter territoriality in the western bluebird, a year-round resident that shows a limited degree of helping behaviour in central coastal California, U.S.A. In spring, most western bluebirds breed as socially monogamous pairs, but a small proportion of pairs (3-16%) have additional breeding-age males helping at the nest, usually assisting parents or brothers. We found that year-round residents commonly wintered in family groups that defended territories similar to those used in spring. Winter groups had an even sex ratio and formed early in the autumn, when hatch-year birds dispersed. More females than males left their natal groups to be replaced by an influx of immigrant hatch-year birds. Winter groups typically consisted of breeders and one or two sons from the prior breeding season along with one or more immigrant females. A second period of dispersal occurred in spring when winter groups broke up and most birds other than the breeding pair left the winter territory. When they bred, yearling males and females often bred with unrelated individuals from their winter groups. Sons were more likely to remain on the study area as yearlings when they wintered with both parents than when they wintered with just one parent. We suggest that young males stay the winter due to benefits of remaining in family groups on mistletoe-based winter territories. Subsequent localized dispersal of sons then leads to opportunistic kin-based interactions later in life. Copyright 2001 The Association for the Study of Animal Behaviour.     

Social organization in the pheasant (Phasianus colchicus): harem formation, mate selection and the role of mate guarding

Harem formation and mate selection were studied in the pheasant in order to determine the advantages of territorial harem defence polygyny to the two sexes. We investigated the factors affecting harem size and the advantage to a female in remaining with one territorial male during breeding.
Female group size declined during late March and early April as females moved from large overlapping ranges into smaller, more widely dispersed breeding ranges. The proportion of female groups accompanied by males increased during this period.
Some males had a disproportionate share of females. Settled females were monogamous but, because a female's nest was generally outside the male's territory, her home range was larger than his territory.
Harem members were usually from the same winter group. Harem size was not related to territory quality in terms of food supply or nesting cover. Females were loyal to one male in more than one year even if his territory position changed. Older, territory-owning males had more females, both adult and immature, than males with newly-established territories. Harem size was not correlated with territory size.
We conclude that the mating system of the pheasant is based on mate guarding which protects females not only from the risk of predation or injury, but also from excessive energy expenditure incurred through being chased by other males. When escorted by a territorial male, females spent three times as much time feeding, one-fifth as much time running, and one-tenth as much time alert, as they did when not guarded.     

High divorce rates in Corsican blue tits: how to choose a better option in a harsh environment

We investigate which hypothesis, the “better mate hypothesis” or the “better territory hypothesis” best explains the unusually high divorce rate (59%) in a population of blue tits (Parus caeruleus) living in a sclerophyllous habitat characterised by severe environmental constraints (trophic, parasitic, climatic) on the island of Corsica, France. Using data from the breeding seasons 1985–1998 and from a brood size experiment (1990–1993) we examined the causes of divorce and their consequences on breeding performance, mate assortment and territory choice. Breeding performance had no significant effect on whether birds re‐united or divorced in the next breeding season. Re‐uniting pairs did better than divorced females and the latter improved their breeding performance compared to prior to divorce, but this was mainly due to age and territory effects. There were no differences in male performance depending on whether they re‐united or divorced. The age combination of pairs did not differ between re‐uniting and divorcing pairs, but mate assortment changed after divorce with males re‐mating more often with older partners than females. Manipulation of brood size showed a trend for birds with enlarged broods to divorce more. Pairs responded significantly to territory quality by divorcing more often in poor than in good breeding sites. Both faithful pairs and male divorcees had shorter breeding dispersal distances than female divorcees. Divorce rates were determined by the large differences in quality among breeding sites. Males, whatever their status, usually retained their previous territory whereas divorced females moved significantly longer distances and improved their breeding site. Moving to a better territory after divorce benefits only females which appear to be the choosing sex in the decision to divorce. This study strongly supports the “habitat mediated hypothesis” and we suggest that the large observed intraspecific variation in the magnitude of divorce rates in many species of birds is mostly determined by habitat characteristics.     

Breeding and movements of goshawks in boreal forests in Sweden

Goshawks were studied in their boreal forest breeding habitat in central Sweden from 1977 to 1982. Nests were regularly spaced, with a nearest neighbour distance of 6.3 km. corresponding to a density of about 3 pairs 100 km⁻². Of initiated breedings. 68% were successful. There was no difference in breeding performance between years. Average brood size was 2.7. Sex ratio among first captures of adults was even, but males were recaptured more often than females. Nearly all juveniles left the area in winter. Among radio-tagged adults 20% of the males, and 50% of the females moved out of the area. When moving, both adults and juveniles moved into agricultural areas, the juveniles generally covering longer distances. From autumn to spring, body condition of females improved, whereas that of the males deteriorated. It is argued that the males' stronger tendency to stay in winter is due to the advantage of possessing a territory in early spring. Females, on the other hand, can move to areas with as good food conditions as possible.     

Effects of breeding success, mate fidelity and senescence on breeding dispersal of male and female blue-footed boobies

1. Understanding the effects of individual and population factors on variation in breeding dispersal (the movement of individuals between successive breeding sites) is key to identifying the strategies behind breeders' movements. Dispersal is often influenced by multiple factors and these can be confounded with each other. We used 13 years of data on the locations, mates, breeding success and ages of individuals to tease apart the factors influencing breeding dispersal in a colonially breeding long-lived seabird, the blue-footed booby Sula nebouxii. 2. Breeding dispersal varied among and within years. Males dispersed further in years of higher population density, and late breeding males and females dispersed further than early breeders. This temporal variation related to changes in competition for territory was taken into account in all tests of individual factors influencing breeding dispersal. 3. Individuals that retained their mates from the previous year dispersed shorter distances than those that changed their mates. 4. The effect of previous breeding success depended on mate fidelity. Unsuccessful breeding induced greater dispersal in birds that changed their mates but not in birds that retained their mates, indicating that breeders who change mates may take their own previous breeding experience into account during habitat selection. Faithful individuals may have to stay close to their previous sites to encounter their mates. 5. Male divorcees dispersed over shorter distances than their former mates, possibly because males contribute more than females to establishing territories. 6. Dispersal of males and females declined with increasing age over the first 10-11 years of life, then increased in old age, possibly due to senescent decay in the ability to compete for mates and territories.     

Kin-related social organization in a winter population of the voleClethrionomys rufocanus

Kinship amongClethrionomys rufocanus was investigated during the winter of 1992/93 in a 3-ha enclosure using both molecular and catch-mark-release techniques. Forty-six adult voles (22 males and 24 females) having high heterozygosities, which were collected from several natural populations, were released into the enclosure on 29 September 1992. Most fall-born individuals of both sexes stayed in their natal site during the non-breeding period (December–March), although reproductively active females dispersed during the fall breeding season (October–November). These philopatric individuals aggregated and formed an maternal family in the winter. Several females which failed to reproduce were solitary during this season. Some individuals which were derived from several families also aggregated into a mixed lineage group. Survival rate of fall-born voles from earlier litters was higher than that from later ones. Maternal families broke up soon after the onset of spring reproduction. Most females established a territory near the wintering site and made a kincluster, in which close relatives neighbored each other. Maternal families in winter bring about female kin-clusters in spring, which may influence reproductive output in the breeding season.     

The Cooperative Breeding System of the Red-cockaded Woodpecker

The breeding system of the red-cockaded woodpecker is described based on data collected over six years from a population of 500 marked individuals in the Sandhills of North Carolina. Male-female pairs were the most common social unit (59%), but 30% of social units contained one or more adult helpers, and 11% consisted of solitary males. Helpers were almost exclusively male: 27% of males remained in their natal group as helpers for at least one year, whereas only four (1%) females did. Most breeding females remained as breeders in the same group from one year to the next (56%), but a surprising number (12%) moved to another group. Many movements were related to incest avoidance or mate death, but 39% involved deserting a mate, usually following successful reproduction. We suggest that females sometimes are forced from groups by immigrants or other group members. The median distance of movements by adult females was only 1.3 km. In contrast to females, no breeding males switched groups. Survival of both breeding (76%) and helper (80%) males was higher than that of breeding females (69%). Males exhibited two distinct life-history strategies. Some remained as helpers on their natal territory for one or more years, and became breeders by inheriting breeding status in the natal group (17% per year) or by replacing a deceased breeder in a nearby group (13% per year, median distance moved 1.0 km). Other males dispersed from their natal group permanently during their first year. Some of these males were floaters at age one year, others were solitary, and a few became helpers in a non-natal group, but many were breeders. In contrast to males that first functioned as helpers, those that dispersed after fledging moved long distances (median dispersal distance 4.5 km), longer even than dispersing female fledglings moved (median distance 3.2 km). The habitat saturation model of the evolution of cooperative breeding is based on selection between the two life-history strategies exhibited by male red-cockaded woodpeckers. The model therefore may be tested directly with this species. Another indication that this model is appropriate for this species is the existence of a resource (cavity trees) that might provide an ecological basis for habitat saturation.     

Plumage brightness predicts non-breeding season territory quality in a long-distance migratory songbird, the American redstart Setophaga ruticilla

Many species of birds exhibit brilliant ornamental plumage, yet most research on the function and evolution of plumage has been confined to the breeding season. In the American redstart Setophaga ruticilla , a long-distance Neotropical-Nearctic migratory bird, the acquisition of a winter territory in high-quality habitat advances spring departure and subsequent arrival on breeding areas, and increases reproductive success and annual survival. Here, we show that males holding winter territories in high-quality, black mangrove habitats in Jamaica have brighter yellow-orange tail feathers than males occupying territories in poor-quality second-growth scrub habitats. Moreover, males arriving on the breeding grounds from higher-quality winter habitats (inferred by stable-carbon isotopes) also had brighter tail feathers. Because behavioral dominance plays an important role in the acquisition of winter territories, plumage brightness may also be related to fighting ability and the acquisition and maintenance of territories in high-quality habitat. These results highlight the need for further research on the relationships between plumage coloration, behavior, and the ecology of over-wintering migratory birds.     

DISPERSION, POPULATION ECOLOGY AND MIGRATION OF EASTERN GREAT REED WARBLERS ACROCEPHALUS ORIENTALIS WINTERING IN MALAYSIA

A population of 400–600 Acrocephalus orientalis wintering in a Phragmites habitat at 3°N in West Malaysia was studied during four northern hemisphere winters, by means of systematic mist-netting. Data from other study-areas, other habitats and other winters are also used. Intensive mist-netting appears to have made birds move over longer distances than they did in the absence of disturbance, and to have led to the emigration of marked birds from the study-area. Trapping also affected feeding behaviour, resulting in weight-loss; repeated trapping may have increased mortality. Males and females could be separated by means of wing-length in fresh plumage. Females were largely confined to Phragmites; males were more numerous on the edge of reed-beds and in scrub vegetation. Males suffered greater feather-wear than females. As measured by the trapping rate, birds were uniformly distributed throughout the Phragmites habitat, at the same density in different winters. Undisturbed birds used a “home-range” of 1–4 ha, overlapping with 15–50 other individuals. Disturbed birds overlapped with 100–200 others. Individual birds returned to exactly the same “home-range” in successive winters. After correcting for the effects of disturbance and incomplete sampling, the proportion of adults ringed in one winter which returned in the next is estimated as 65% in each of two study-areas. This is a minimum estimate of the annual survival rate for adults. Mean total body-weights were at a minimum in midwinter (November-February). Fat-free weights were also lower in midwinter than in autumn and spring. Body-moult was observed in March and April. Moult of the flight-feathers takes place between July and September, on the breeding grounds or slightly to the south. Females departed on spring migration between 10 and 25 May; males some 11–14 days earlier. Adults arrived in autumn between 8 September and 7 October; males and females often came in in separate “waves”. Females were absent for only about 127 days, about the minimum required for migration, breeding and moult. Dates of migration match those of the more northern breeding populations. Spring departure is later than dates of passage recorded in south China; hence birds of this population appear to make long nights. On average, birds departing in spring carried about 9 g of fat, roughly 40% of total fat-free body-weight. This is about half the energy reserve required for the entire journey. Dates of passage in central China are consistent with a hypothesis that they make the journey (4,500-5,000 km) in two “hops”. A few birds which remained light until very late in the spring showed a significantly lower return rate in the next year. Most birds arriving in autumn appear to have carried 1–2 g of fat, but some were at or below the normal fat-free weight. Many birds appear to have lost weight soon after arrival. Returning ringed adults were amongst the very first birds trapped in September. Individual birds appear to have migrated on very similar dates in different years: many of the dates of trapping differed by 2 days or less in successive years. Trapping rates reached a peak in early October and then declined rapidly, reaching the midwinter level by 21 October. The decline coincided with the differential disappearance of juvenile birds. However, birds collected at this time had adequate fat reserves, and the disappearance appears to have preceded the period of food-shortage. It is suggested that the loss of juvenile birds resulted from behavioural interactions favouring the more dominant individuals, as has been described for several temperate zone residents. The first few weeks in the wintering area may thus be the critical period of mortality during the year. Because birds from different breeding areas are expected to be mixed in the winter-quarters, and vice versa, local mortality factors in winter may affect a number of breeding populations. High adult survival rates have been recorded in several other birds which breed in the temperate zones and winter in the tropics. In general their breeding success appears to be high, so the first-year mortality must be high. The closely related A. arundinaceus, which winters in Africa, differs from A. orientalis in size, wing-shape, timing of spring migration and timing of moult. These differences can be interpreted as adaptations to different environmental (primarily climatic) factors experienced during migration and on the breeding grounds. The segregation of males and females into different habitats probably reduces inter-sexual competition in winter, but this is not necessarily its primary function. Males collected in the evening in Phragmites had smaller fat reserves than females, suggesting that the females are better adapted to this habitat. The large size of the males is probably maintained in part by sexual selection in the breeding season. On the other hand, the size of females and their habitat is probably limited by the specialisation of their nest. These factors would suffice to explain the sexual dimorphism in size and habitat.     

Cold tolerance,and not earlier arrival on breeding grounds,explains why males winter further north in an Arctic‐breeding songbird

Sex biases in distributions of migratory birds during the non‐breeding season are widespread; however, the proximate mechanisms contributing to broad‐scale sex‐ratio variation are not well understood. We analyzed a long‐term winter‐banding dataset in combination with spring migration data from individuals tracked by using geolocators to test three hypotheses for observed variation in sex‐ratios in wintering flocks of snow buntings Plectrophenax nivalis. We quantified relevant weather conditions in winter (temperature, snowfall and snow depth) at each banding site each year and measured body size and condition (fat scores) of individual birds (n > 5500). We also directly measured spring migration distance for 17 individuals by using light‐level geolocators. If the distribution pattern of birds in winter is related to interactions between individual body size and thermoregulation, then larger bodied birds (males) should be found in colder sites (body size hypothesis). Males may also winter closer to breeding grounds to reduce migration distance for early arrival at breeding sites (arrival timing hypothesis). Finally, males may be socially dominant over females, and thus exclude females from high‐quality wintering sites (social dominance hypothesis). We found support for the body size hypothesis, in that colder and snowier weather predicted both larger body size and higher proportions of males banded. Direct tracking revealed that males did not winter significantly closer to their breeding site, despite being slightly further north on average than females from the same breeding population. We found some evidence for social dominance, in that females tended to carry more fat than males, potentially indicating lower habitat quality for females. Global climatic warming may reduce temperature constraints on females and smaller‐bodied males, resulting in broad‐scale changes in distributional patterns. Whether this has repercussions for individual fitness, and therefore population demography, is an important area of future research.     

Seasonal movements of British Greenfinches Carduelis chloris

Ringing recoveries are used to investigate the seasonal movements of British Greenfinches. Significantly more females than males make such movements. Outward movement begins by early November and continues until at least late January. The return movement can last from January to May and is particularly rapid in March and early April. The directions of the outward movements are widely dispersed but show trends towards the south and, except in western areas, towards the west. In Britain south of latitude 54°N the proportion of birds leaving their breeding grounds in winter increases from west to east, being negligible in the south-west and largest in East Anglia, but relatively small near the south coast. Distances moved show a similar dependence. The numbers making seasonal movements in a given year are correlated with the breeding population, particularly in south-east England.     

Dispersal,ranging and settling behaviour of Marsh Tits Poecile palustris in a fragmented landscape in lowland England

Capsule Natal dispersal was rapid and distances were short. Winter ranging and breeding dispersal were limited. Few birds undertook large movements.

Aims To investigate the natal and breeding dispersal of Marsh Tits, including the timing of dispersal movements.

Methods Nestlings, juveniles and adults were ringed and searched for over 4500 ha during summer, autumn–winter, and spring over six years. Dispersal distances were measured as metric distances and multiples of territory widths. Ranging distances were compared with dispersal distances.

Results Median distances of natal dispersal were 2.6 territory widths for males (704.5 m) and 3.1 territory widths for females (1065.0 m). Median distances of breeding dispersal were 0.2 territory widths for males and females (58.6 and 53.1 m respectively). Most natal dispersal was completed soon after independence, with further movement in spring. Breeding dispersal was also detected during these periods. Median ranging distances were short, and some winter floaters were identified.

Conclusion Marsh Tits had short dispersal distances, with most dispersal activity occurring in June. Results suggested that dispersal behaviour was sensitive to habitat fragmentation, resulting in poor settling success outside of the natal wood. Habitat fragmentation may, therefore, be a contributory factor in the decline of the Marsh Tit population in Britain.     

Migration patterns, breeding and moulting locations of king eiders wintering in north-eastern Norway

The Norwegian coast is a very important winter area for king eiders (Somateria spectabilis), but their origin has been unknown. We determined spring and autumn migration routes, timing of migration and potential breeding areas of king eiders wintering in north-eastern Norway using implanted satellite transmitters. Five males and five females were equipped with transmitters in February 2008, and location data were received from six birds. All birds departed within 10 days in mid April and flew to the Pechora Sea and Kara Sea in western Russia where they staged until mid June. Subsequently, four of five birds with active transmitters (two females and two males) moved east to potential breeding locations on the Gydan and Taymyr Peninsulas. In early July, the males moved west to moult at Kolguyev Island and in the Pechora Sea. The two females departed in mid July, one probably moulting between the gulfs of the Ob and Yenisei Rivers, after which it moved to the Pechora Sea. The other female probably moulted in the eastern Taymyr, thereafter moving to Novaya Zemlya. This female returned to the north-eastern coast of Norway 1 December, while the other female returned 2 January. For the males, the transmitters stopped in December/January when they were still in the Pechora/Kolguyev area. King eiders wintering in north-eastern Norway thus originate from the western half of Arctic Russia, and the Taymyr Peninsula is probably the dividing point between the eastern and western flyways.     

Arrival and habitat use by Lapwings Vanellus vanellus in the early breeding season

The arrival on the breeding grounds and habitat use in relation to agricultural land use by Lapwings Vanellus vanellus was compared in rough grazing and arable study areas in the Midland Valley of Scotland. First arrivals took place in early February. Males preceded females and territory occupation by the males began soon after first arrival. During the pre-breeding period birds remained in flocks and day time habitat use and activity was strongly influenced by the lunar cycle. Birds fed mainly in those habitats in which prey (leatherjackets and earthworms) were most plentiful, and in which their feeding success was high. The choice of nesting habitat was not influenced by food availability in the immediate vicinity of the nest site but by the crypticity of the clutch and incubating adults. On rough grazing birds preferred unimproved land while on arable land spring cereal was the preferred habitat. The choice of actual nesting field on the arable land was influenced by the risk of predation and the proximity of suitable feeding fields for the adults and chicks.     

Turnover and dispersal in a Peregrine Falco peregrinus population

In south Scotland, most Peregrines returned to the same territories to breed in successive years, though a few females changed territory from one year to the next.
Annual mortality among breeding birds was at most 9% among females (or 11% in both sexes combined). There may have been considerable annual variation, however, and excluding one exceptional year out of five reduced the estimate for females to 7%. These estimates are maxima, but are still considerably lower than those obtained from ring recoveries of dead birds reported by members of the public.
Among trapped birds, four males first bred at age two years, one at three and another at four or five; two females first bred at one year, 13 at two years old and one at three. Five other females which were seen to be in first-year plumage but were not trapped, also laid eggs, and 12 other such paired females held territory but did not lay. Only one paired male held territory in first-year plumage.
In their movements between natal and breeding territories, some females moved further than males, with median distances of 83 and 58 km respectively. In addition, of birds trapped breeding in the study area, a greater proportion of the males than of the females had been born locally, despite an equal sex ratio among fledglings; this was also consistent with a greater dispersal of females. In general, Peregrines made much longer movements in their first year of life than subsequently. Movements were in any direction.     

SEASONAL CHANGES IN BODY-WEIGHT OF OYSTERCATCHERS HAEMATOPUS OSTRALEGUS

The body-weights of Oystercatchers Haematopus ostralegus wintering in Morecambe Bay, north-west England, showed marked seasonal changes between late summer and late winter, with considerable differences apparent between adult and immature birds. An attempt is made to relate these changes to recorded seasonal variations in prey biomass and to the annual cycles of breeding, moult and migration of the Oystercatcher. The mean weight of females invariably exceeded the mean weight of males in samples collected on the same dates, regardless of age. Adults returned from northern breeding areas in very lean condition, with mean weights ranging from 526 g in males to 540 g in females. Mean weight then increased progressively, due mainly to fat deposition, to a peak in March (up to 662 g in males and 675 g in females) around the time of their main departures for breeding. Heaviest birds then exceeded 800 g. Birds migrating to Iceland in spring would need to be of above average weight in March to make the shortest crossing (850 km, 13 h), via Scotland, while Oystercatchers of 700 g and over could probably make a direct flight (1500 km, 25 h) from Morecambe Bay in favourable weather. Breeding weights of British Oystercatchers were similar to those of post-breeders returning to Morecambe Bay in late August. The mean weights of first-year Oystercatchers arriving in August were very low, 449 g in males and 478 g in females. Their weights, and those of second- and third-year immatures, then rose rapidly in autumn, with some fat deposition, and reached mean values ranging between 551 g (males) and 597 g (females) by November-December. Mean weight then fell by 10–17% from December to March returning close to or below the September levels, whereas adults gained a further 6% during these winter months. Summer and autumn weight gains, and the major moult of adults and older immatures, occurred when the biomass of their two staple mollusc preys, Mytilus edulis (mussel) and Cardium edule (cockle), was maximal. Winter loss in mean weight of immatures corresponded with declining prey biomass, suggesting either that they were less efficient than adults in coping with deteriorating winter food supplies, or that they had no need to accumulate further (premigratory) fat reserves. The autumnal increases in mean weight of immatures are interpreted as an adaptation for withstanding adverse feeding conditions in winter. The Oystercatcher appears to be the only wader species in Britain in which adults increase, rather than lose, weight during the winter. This may be a consequence of an early breeding season, but it may be regarded also as a measure of the success Oystercatchers have achieved by specializing on a difficult but plentiful prey source.     

Breeding territory fidelity in a partial migrant, the American dipper Cinclus mexicanus

American dipper Cinclus mexicanus populations are frequently composed of resident individuals that occupy permanent territories year round and migratory individuals that overwinter with residents but migrate to breeding territories on higher elevation creeks each spring. Between 1999 and 2004 we examined how migratory strategy (resident/migratory) and sex differences influence breeding territory fidelity of American dippers occupying the Chilliwack River watershed, British Columbia, Canada. Counter to expectation we found that the migratory strategy of American dippers did not influence whether birds breeding in one year were found on their former breeding territory in the next. Migratory strategy also did not affect the probability that known surviving dippers occupied the same breeding territory in the following year. Males and females were equally likely to be found on their former territory in the following year (females 43%, males 41%) and known survivors had similar levels of breeding territory fidelity (females 74%, males 68%). However, breeding territory fidelity of males and females varied in response to different factors. Surviving female dippers were more likely to be found on their former breeding territory in the subsequent year following a successful breeding attempt than an unsuccessful breeding attempt. Prior reproductive performance did not influence whether surviving male dippers were found on their former breeding territory. Male dippers were more likely to be found on their former territory and, if they survived, have higher breeding territory fidelity when their mate also returned to that same territory. Mate retention also influenced whether females were found on their former territory in the following year but had no effect on the breeding territory fidelity of known survivors. We argue that sex‐specific dispersal decision rules in American dippers are driven by sex differences in the predictability of breeding performance between years and sex differences in how mate retention influences subsequent reproductive success.     

EVOLUTION OF PLUMAGE COLOR IN MALE PIED FLYCATCHERS (FICEDULA HYPOLEUCA): EVIDENCE FOR FEMALE MIMICRY

We present the first evidence for sexual deception by female mimicry in birds. Using live, caged birds we show that territorial male pied flycatchers behave aggressively toward bright-colored males but display sexually toward female-like male intruders. We also show that the males that are fooled are those that lack recent sexual experience. All male pied flycatchers are dull-colored in winter. It is possible that young males are more constrained during the spring molt than older males since the former are more dull-colored in spring. According to the molt-constraints hypothesis a subadult plumage would be maladaptive in the breeding season. Analysis of male settling pattern at breeding sites in spring suggests that brownish males are allowed to settle closer to already-established males than dark-colored males. This result suggests an adaptive value of having a subadult plumage color, in particular for young males arriving late from spring migration. However, we also show that mimicry incurs a cost, that of increased aggression from females, which may explain why female-like males have reduced mating success.     

Turnover, age and sex ratios of kestrels (Falco tinnunculus) in south Scotland

This paper examines the composition and turnover of a population of European kestrels Falco tinnunculus L. in the Southern Uplands of Scotland. Turnover was generally high, with most birds staying in the area for only one summer or winter. The rate at which birds arrived or left was highest in autumn and spring, and was independent of population size. During winter, breeding males were more likely to stay on their territories than females, and males that did so were more likely to retain their partners the following year. Breeding success was associated with a greater likelihood of return the following year, except in first-year females which were unlikely to return anyway. The sex ratio was biased toward males in winter, especially in poor vole years when there were also few first-year birds present. The study confirmed the high rate of turnover reported elsewhere in small raptors, and further indicated that the composition and turnover of the population were related to food supply and seasonal patterns of migration.     

The structure of a local population and dispersal pattern in the Styan's grasshopper warbler,Locustella pleskei

The breeding sites of Styan's grasshopper warblers are restricted to small islands and distributed discretely. The study on the marked population was conducted on two islets, Okitsu-jima and Ohtsukue-jima, located near Fukuoka, Japan, from 1981 to 1989. About 20 and 70 birds bred on the two islets, respectively, and the mean density of breeding pairs was 25.5 ha⁻¹. The dispersal pattern and population, structure were examined. Adult birds returned to the same breeding site. Males always reoccupied the same territory as in previous years but females not. Males were more philopatric and only female yearlings dispersed over long distances and moved between the breeding sites in Hakata Bay. The returning rate was 0.57 for adults, and 0.21 for juveniles. The age distribution of birds at Ohtsukue-jima was almost stable, while that at Okitsu-jima was unstable because it was a marginal habitat for the birds. Estimating from the dispersal pattern and age structure of this warbler the population near Fukuoka consists of only four islets and is almost stable.