Monitoring Svalbard rock ptarmigan: Distance sampling and occupancy modeling

The only resident terrestrial herbivorous bird species in high-Arctic Svalbard, Norway is the endemic Svalbard rock ptarmigan (Lagopus muta hyperborea) of which little is known of its population dynamics. We assessed temporal and spatial variability of the pre-breeding population of Svalbard rock ptarmigan males using: 1) distance sampling to estimate density (2000–2009) and 2) occupancy modeling to determine the proportion of survey points being occupied in relation to a habitat index for ptarmigan habitat suitability (2005–2009). Data were collected using a point-transect sampling design. We split the analysis according to type of survey point (non-random, random, and survey points combined). Our estimated spring densities were low (1.3–3.1 territorial male/km², non-random survey points, 2000–2009) with limited annual variability. The best models describing occupancy rates of territorial males at 2 different spatial scales (ptarmigan males observed ≤250 m and ≤450 m from the sampling point) were independent of spatial scales and the type of survey points. Occupancy dynamics were related to the habitat index whereas detection probability was year dependent. Extinction probability was negatively related to habitat quality (good habitats had lower extinction probability). We could not estimate the habitat effect on colonization precisely because initial occupancy rates were high at both spatial scales (estimated average initial occupancy at scale ≤250 m = 0.96; scale ≤450 m = 0.97). Colonization appeared to be positively related to the habitat index for the random survey points (including mainly marginal habitats), but the small sample size led to large uncertainty in the parameter estimate. Detection probabilities varied greatly between study years, thus demonstrating the importance of estimating detection probability annually. We recommend that future surveys are stratified with respect to habitat quality and to integrate the 2 methodologies in population monitoring of Svalbard rock ptarmigan. © 2011 The Wildlife Society.     

Spacing behaviour and habitat use of rock ptarmigan (Lagopus mutus) at low density in the Italian Alps

Studies of rock ptarmigan Lagopus mutus in northern latitudes have shown that, in the breeding season, the majority of cocks pair monogamously and defend small territories, whilst in most populations, a small proportion of cocks are polygynous or remain unmated. Little is known, however, on spacing behaviour and habitat use of alpine rock ptarmigan populations occurring at low densities at the southern edge of the species’ range. From 1995 to 1997, we trapped, radio-tracked and observed birds in the Central Italian Alps (elevation 2,000–3,000 m) in order to investigate spacing behaviour and habitat use in alpine landscapes where habitats offering rich food and cover are patchy. Both sexes were selective in their habitat use, and cocks did not establish territories on bare ground, artificial or nutrient rich grasslands or bogs. In the breeding season, cocks had larger home ranges than hens (cocks 99 ± 57 ha, hens 50 ± 25 ha) that overlapped with the ranges of neighbouring cocks and hens. Cocks were monogamous and defended relatively large territories (core areas of home ranges: cocks 37 ± 26 ha, hens 24 ± 12 ha), which corresponded with low spring densities (0.47–2.29 cocks km⁻² and 0.35–1.60 pairs km⁻²). Territory size of individual cocks was negatively correlated with the amount of high quality habitat in the core-area. Our results suggest increased home ranges and large breeding territories in low density alpine rock ptarmigan populations, compared to populations occurring at higher densities in the central and northern alps, and on subarctic and arctic grounds at northern latitudes, confirming the predictions of models on food-based territoriality.     

Spring density and summer habitat use of alpine rock ptarmigan Lagopus muta helvetica in the southeastern Alps

In the Alps tourism, land-use practices and climate change may cause a loss of suitable habitat of rock ptarmigan (Lagopus muta helvetica). Hence, there is a need for basic research on population densities and habitat requirements of this species as a basis for long-term monitoring studies. So far, in the Austrian Alps, no accurate data on densities and trends of rock ptarmigan populations are available. We carried out counts of calling cocks in the southeastern part of the Austrian Alps (Nockberge National Park, Carinthia, Austria) between 31 May and 1 July 2003 and evaluated the distribution of male rock ptarmigan territories. The spring density of 5.4 territorial cocks per square kilometre was similar to densities in the Swiss Alps but higher than spring population densities in the Italian Alps and the Pyrenees. To investigate summer habitat use, we established a grid mapping of habitat parameters and rock ptarmigan signs. In total, 38 squares with signs of rock ptarmigan presence were recorded (i.e. 32% of all recorded squares). The presence of rock ptarmigan was significantly correlated with cover of rock and rock ptarmigan preferred habitat patchiness. Our study provides a useful reference for future monitoring of this species in the alpine distribution range considering the potential decrease and fragmentation of alpine habitats resulting from climate change and human impacts.     

Locomotor development in the Svalbard rock ptarmigan (Lagopus muta hyperborea)

Juvenile animals often suffer from high levels of predation. Development of an effective and efficient locomotor system is therefore likely to be crucial towards ensuring their survival. However, our understanding of locomotor efficiency, at least in terms of energetic cost in young animals is poor. We performed this study as Svalbard rock ptarmigan, Lagopus muta hyperborea must rapidly develop the ability to locomote prior to the onset of their first winter, during which conditions are extreme. To aid survival, adult ptarmigan deposit large winter fat stores, whilst at the same time males exhibit a reduced metabolic cost of locomotion. Sub-adult males, however, are unable to fully acquire fat stores during their first winter and the maturity of their locomotor systems is unknown. Here, we investigate the energetics and kinematics of terrestrial locomotion in sub-adult male birds using flow-through respirometry and high-speed video recordings, respectively. We demonstrate that in terms of running speed and metabolic cost, sub-adult ptarmigan develop a mature functioning locomotor system prior to the onset of winter. This research indicates that achieving a mature locomotor system allows young males to emerge from the winter with the ability to compete for territories and mates during the breeding season.     

GENETIC VARIATION AND TERRITORIALITY IN WILLOW PTARMIGAN (LAGOPUS LAGOPUS LAGOPUS)

We examined eight polymorphic esterase loci in 526 juvenile and adult willow ptarmigan (Lagopus lagopus lagopus) collected during autumn and spring over five years. The genetic structure of territorial birds during spring differed from birds on the study area in autumn. This can not be explained by selective winter mortality since juvenile birds in the autumn had the same genetic structure as the adults who had lived through at least one winter. In the spring, birds with intermediate heterozygosity had the largest territories and were more frequently mated than expected from random mating among autumn birds. The results suggest selective access to territories by genotype and stabilizing selection, especially among males, since natural selection is assumed to favor large territories and reproduction. Our data also suggest that the spring population consisted of both territorial and non-territorial birds. The genetic relationship between birds of established pairs in spring was lower than that between randomly drawn birds on the study area in the autumn. This together with the finding that parents with an intermediate level of genetic relationship produced the largest broods, suggest that optimal rather than maximal outbreeding is the most successful breeding strategy in this species.     

EXPERIMENTS ON THE LIMITATION OF BIRD NUMBERS BY TERRITORIAL BEHAVIOUR

(1) This paper examines removal and other experimental studies on the role of territorial behaviour in the limitation of bird densities. Experimental design is discussed, as are the types of conclusions that can be drawn. (2) Experiments have been conducted on more than 40 species from a wide range of taxonomic groups. Most provided evidence for density limitation, probably mediated by territorial behaviour, and for the existence of a non-territorial sector able to take territories when territory owners were removed or when additional habitat was made available. Experiments in the breeding season indicated that some surplus birds, although younger on average then territorial birds, were sexually mature and able to breed when given the chance. In some species, replacement birds bred less well than other territorial birds, but in other species no difference was apparent between the two groups. (3) Other experiments indicated that, while density was limited by territorial behaviour in good habitats, this was not the case in poor habitats, and that some individuals would move from poor to good habitat when territory owners were removed from good habitat. (4) In some species, non-territorial birds of breeding age were present in the population despite the existence of vacant territorial sites, while in others replacements were observed on good territories but not on poor ones. The implications were that site quality influenced whether settlement, defence and breeding occurred, and that some individuals had more stringent site requirements than others. In some seabirds' sites in the centre of a colony were more attractive than sites on the edges. (5) Many land-bird species that have been studied show two peaks of territorial activity each year, in autumn and in spring, and a limitation on breeding density can occur at either or both seasons, depending on conditions. (6) The fact that densities of many bird species fluctuate greatly from year to year is not inconsistent with the limitation of density by territorial behaviour. Several mechanisms are apparent through which density might be limited by territorialism at different levels in different years, so that surplus non-territorial birds are present in years of low territorial density, as well as in years of high territorial density. (7) Experiments have shown that density is limited by the presence of territorial birds, in some species at different levels in different areas or years. The next step is to find whether density is regulated by the presence of territorial birds over a period of years in a density-dependent manner. For this, observational data are required to find whether the proportion of birds that is excluded by territorial behaviour each year varies with the total number available for settlement. Such studies can be made only on species in which non-territorial birds can be counted accurately, as well as territorial ones.     

Group size and composition are correlated with population density in the group-territorial blue korhaan (Eupodotis caerulescens)

The blue korhaan (Eupodotis caerulescens) is a poorly known bird endemic to the grasslands of southern Africa. It lives in permanently held territories defended by groups of individuals. During 2001 and 2002, 61 transects were carried out along 544 km of roads in Mpumalanga province (South Africa) to collect data on territory density and group size and composition in this species. Habitat, altitude and topography variables were measured to evaluate their influence on population density. Average territory density was 1.56 ± 0.19 (SE) territories per 5 km and maximum densities attained 5.6 territories per 5 km. Within its area of occurrence, higher densities occurred along transects with larger proportion of short grassland (either burned or grazed) and flatter topography. Mean group size was 3.03 ± 0.11 birds (n = 119), with 87.4% of the groups having one to four birds. Most common group composition was one male and one female‐like (34.5%), followed by two male and one female‐like (17.2%) and two male and two female‐like (13.8%). Single individuals were always males, and the male/female‐like ratio peaked at a group size of three and decreased thereafter. Mean group size increased with territory density (r_s = 0.42, P = 0.007, n = 41). There were no differences in group size across habitat types. The occurrence of habitat saturation and cooperative breeding in this species could explain the observed results.     

Stepping stones of life: natal dispersal in the group-living but noncooperative speckled warbler

In most cooperatively breeding birds the offspring of one sex, usually male, delays dispersal to remain on the natal territory and helps its parents to rear subsequent young. Thus delayed dispersal could be the first step in the evolution of cooperative breeding. We studied natal dispersal in a population of the group-living speckled warbler, Chthonicola sagittata, based on observations of a colour-banded population over 3 years. Unlike other group-living members of the Acanthizinae, all juvenile males in this population dispersed to settle on foreign territories as subordinates, which do not help rear the young. Speckled warblers showed all the life history traits that are thought to result in a saturated habitat and lead to delayed dispersal: they were sedentary, had high adult survival and had a male-biased sex ratio. However, they differed from other acanthizids in occurring at low density (0.18 birds/ha) on large breeding territories (6-12 ha), with a maximum of two males per territory. This may allow subordinates to live on foreign territories yet avoid aggression from dominants. A benefit of dispersal is that it provides an additional route to gaining a breeding vacancy. Dispersers can acquire vacancies on their new territory or on a neighbour's, but incest avoidance would be likely to constrain nondispersing males to neighbours' territories. A model of relative lifetime success showed that the survival benefits of natal philopatry are unlikely to outweigh this benefit of dispersal.     

Understanding sex differences in the cost of terrestrial locomotion

Little is known regarding the physiological consequences of the behavioural and morphological differences that result from sexual selection in birds. Male and female Svalbard rock ptarmigans (Lagopus muta hyperborea) exhibit distinctive behavioural differences during the breeding season. In particular, males continuously compete for and defend territories in order to breed successfully, placing large demands on their locomotor system. Here, we demonstrate that male birds have improved locomotor performance compared with females, showing both a lower cost of locomotion (CoL) and a higher top speed. We propose that the observed sex differences in locomotor capability may be due to sexual selection for improved male performance. While the mechanisms underlying these energetic differences are unclear, future studies should be wary when pooling male and female data.     

Population genetic structure of rock ptarmigan in the 'sky islands' of French Pyrenees: implications for conservation

Expected consequences of global warming include habitat reduction in many cool climate species. Rock ptarmigan is a Holarctic grouse that inhabits arctic and alpine tundra. In Europe, the Pyrenean ptarmigan inhabits the southern edge of the species' range and since the last glacial maximum its habitat has been severely fragmented and is restricted to high-alpine zones or 'sky islands'. A recent study of rock ptarmigan population genetic in Europe found that the Pyrenean ptarmigan had very low genetic diversity compared with that found in the Alps and Scandinavia. Habitat fragmentation and reduced genetic diversity raises concerns about the viability of ptarmigan populations in the Pyrenees. However, information on population structuring and gene flow across the Pyrenees, which is essential for designing a sound management plan, is absent. In this study, we use seven microsatellites and mitochondrial control region sequences to investigate genetic variation and differentiation among five localities across the Pyrenees. Our analyses reveal the presence of genetic differentiation among all five localities and a significant isolation-by-distance effect that is likely the result of short dispersal distances and high natal and breeding philopatry of Pyrenean ptarmigan coupled with severe habitat fragmentation. Furthermore, analysis of molecular variance, principal component analysis and Bayesian analysis of genetic structuring identified the greatest amount of differentiation between the eastern and main parts of the Pyrenean chain separated by the Sègre Valley. Our data also show that the Canigou massif may host an isolated population and requires special conservation attention. We propose a management plan which includes the translocation of birds. If a sky island structure affects genetic divergence in rock ptarmigan, it may also affect the genetic structure of other sky island species having low dispersal abilities.     

Spatial distribution and settlement strategies in willow ptarmigan

In heterogeneous landscapes individuals select among several habitat patches. The fitness rewards of these choices are assumed to play an important role in the distribution of individuals across landscapes. Individuals can either use environmental cues to directly assess the quality of breeding sites, or rely on social cues to guide the settlement decision. We estimated the density of adult birds and per capita reproductive success of willow ptarmigan over 5–15 years in 42 survey areas, nested within 5 spatially separated populations in south-central Norway. Our aims were to (1) examine spatial and temporal patterns of variation in densities of adult birds (i.e., the breeding densities) and reproductive success (juveniles/pair) measured in autumn and (2) evaluate which habitat distribution model best described the distribution of willow ptarmigan across heterogeneous mountain landscapes. Variation in density of adult birds was primarily attributable to variation between survey areas which could arise from spatial heterogeneity in adult survival or as a consequence of spacing behavior of juveniles during the settlement stage. In contrast, reproductive success was more variable between years and did not vary consistently between survey areas once year effects were accounted for. The lack of any relationship between the density of adult birds and reproductive success supported the predictions of an ideal free distribution (IFD), implying that within years, the mean reproductive success was approximately equal across survey areas. However, analysis based on Taylor's power law (i.e., the relationship between logarithms of spatial variance and mean density of adult birds) suggested that aggregation was stronger than expected under IFD. This implies that the relative change in density of adult birds was larger in areas with high mean densities than in areas with low densities. The exact mechanisms causing this statistical pattern are unclear, but based on the breeding biology of willow ptarmigan we suggest that yearlings are attracted to areas of high densities during the settlement period in spring. Our study was conducted during a period of low overall density and we suggest that this pattern might be particular to such situations. This implies that the presence of conspecifics might represent a cue signaling high adult survival and thus high habitat quality.     

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.     

Forest fragmentation is associated with primary brood sex ratio in the treecreeper (Certhia familiaris)

We studied the primary brood sex ratio of an old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), along a gradient of forest fragmentation. We found evidence that male nestlings were more costly to produce, since they suffered twofold higher nestling mortality and were larger in body size than females. Furthermore, the proportion of males in the brood was positively associated with the provisioning rate and the amount of food delivered to the nestlings. During the first broods, a high edge density and a high proportion of pine forests around the nests were related to a decreased production of males. The densities of spiders, the main food of the treecreeper, were 38% higher on spruce trunks than on pine trunks. This suggests that pine-dominated territories with female-biased broods may have contained less food during the first broods. The observation was further supported by the fact that the feeding frequencies were lower in territories with high proportions of pines. In the second broods, territories with a high forest patch density produced female-biased broods, whereas high-quality territories with a large amount of deciduous trees and mixed forests produced male-biased broods. Our results suggest that habitat quality as measured by habitat characteristics is associated with sex allocation in free-living birds.     

Distribution and reproductive success of the Pied Flycatcher Ficedula hypoleuca in relation to forest patch size and vegetation characteristics; the effect of scale

We studied habitat choice, distribution and reproductive success in the Pied Flycatcher Ficedula hypoleuca in relation to the spatial structure of a fragmented forest area. Two hierarchical levels of forest habitat structure were used: (1) forest patch size (macrohabitat) and (2) vegetative structure within forest patches (microhabitat). In spring, both males and females settled preferentially in large and medium-sized forest stands (>1 ha) where breeding density was also higher than in small stands (<1 ha). Stands <5 ha were occupied later, and the proportion of unpaired males was higher there than in large stands (>5 ha). The known age distributions of breeding birds and breeding success were independent of forest patch size. Nest predation rate was not associated with stand size or nest distance from the forest edge. The preference of breeding birds for large forest patches was presumably related to the higher amount of resources (nest sites, mates, food) a large patch can offer for reproduction compared with a small patch. At the microhabitat level, territories of old males were characterized by relatively more deciduous trees that contained more invertebrate food than coniferous tree-dominated territories of yearling males. The reproductive success of old males, as measured by the number of fledged young per male, was higher than that of yearling males. This suggests that the larger body size and blacker plumage of old males possibly contributed to male dominance and that old males excluded younger males from preferred habitats through territorial behaviour. Our results suggest that habitat choice of the Pied Flycatcher was affected by both micro- and macrohabitat. On the basis of settlement pattern, density and reproductive success, the distribution of Pied Flycatchers across forest stands of different sizes followed the ideal-free distribution model, whereas at the microhabitat level, age-related unequal distribution of males followed the ideal-despotic model of Fretwell and Lucas.     

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.     

The effects of season and sex upon the morphology and material properties of keratin in the Svalbard rock ptarmigan (Lagopus muta hyperborea)

The material properties and morphologies of the modified integumentary organs of birds (the keratinous bills, claws and feathers) have evolved to withstand the variety of mechanical stresses imposed by their interaction with the environment. These stresses are likely to vary temporally in seasonal environments and may also differ between the sexes as a result of behavioural dimorphism. Here we investigate the morphology and material properties of the claws of male and female Svalbard ptarmigan (Lagopus muta hyperborea) during the summer and winter using nanoindentation. Despite differences in locomotor demands between the sexes and pronounced seasonal differences in environmental conditions, like ground substrate, ambient temperature and day length, there was no significant difference in Young׳s modulus or hardness between the seasons for each sex. However, when comparing males and females, female claws were significantly harder than those of males and both sexes had significantly wider claws during winter. We propose that wider claws may follow winter claw moulting as the claws are regrown and form an important part of the ptarmigan׳s snowshoe-like foot that is an adaptation to locomotion on snow. Future work focusing on growth rates and more broad measures of material properties in both captive and wild birds is required to determine the extent of seasonal and sex differences in the material properties of their keratinous structures.     

Point–count method for estimating rock ptarmigan spring density in the Pyrenean chain

This paper describes a census technique that gives estimates of rock ptarmigan cocks’ number per unit area during the breeding season. This method originated from the need for an effective technique for estimating bird densities in mountainous and inaccessible zones like the Pyrenean chain. The bird census was carried out using a point–count method, which is recommended for uneven areas. To maximize sampling efficiency, we established sampling points at <500-m distances, in order to detect a maximum of calling birds within the sample areas. Birds localized at distances >250 m were excluded. We postulate that all birds are recorded in a 250-m radius around the observer. We carried out counts of calling rock ptarmigan cocks in the border between the Principality of Andorra and Ariège Department, France, from April to June during three consecutive years (2005–2007). The estimated spring density of 10.4 cocks per 100 ha was higher than densities reported in literature, in other parts of the Pyrénées and the Alps. Our study provides a useful reference for future monitoring of this species in its mountainous distribution range.     

Population fluctuations in relation to seasonal habitat preferences of the Swainson's spurfowl, Pternistis swainsonii

Counts on Swainson's spurfowl Pternistis swainsonii were made during 1998–1999 within an intensive, fine‐grained, agricultural landscape to estimate population parameters, seasonal dispersion and habitat preferences. Radio‐transmitters were fitted to four birds to note habitat use and home ranges within the Summer breeding season. During Winter, population densities peaked, and birds exploited agricultural crops extensively. At the onset of Spring, densities dropped as birds paired to establish non‐overlapping breeding territories over a number of habitats with apparently sufficient cover and ‘natural’ food. Expanding grazed grassland appears to be the greatest threat to Swainson's spurfowl due to a lack of cover and food. The matrix of habitats within the landscape plays important roles in the success of this opportunistic spurfowl. Agricultural crops in the Winter sustain the population until the following Summer when natural savanna and ungrazed grasslands provide complementary foraging, nesting and roosting sites.     

First record of the fledging period of Temminck's Courser,Cursorius temminckii

The Seychelles Fody, Foudia sechellarum, is a ploceid weaver occurring naturally on three islands in the Seychelles group in the Indian Ocean. The population on Cousine Island was studied between 30 June and 25 August 1997. The size of the population on the island was estimated at 458–614 individuals and densities varied in different habitat types. As Seychelles Fodies in non-breeding plumage are difficult to sex, we provide sexing criteria based on wing length. Breeding pairs form small, probably temporary, territories that are defended by both partners against other fodies, including the introduced Madagascar Fody, Foudia madagascariensis. The Seychelles Fody often breeds semi-colonially and we observed up to five nests close together. The birds are socially monogamous and both sexes share in nest building, nest defense and provisioning of the young. Only females incubate. Many nests were deserted before eggs were laid, including some that had been accepted by the female. Additional males and females were sometimes seen helping provision the young, but this cooperative breeding behaviour appeared to be uncommon. Non-breeding individuals congregate in large flocks, sometimes joined by breeding birds.     

Photoperiodic and ambient temperature control of the annual body mass cycle in Svalbard ptarmigan

The present study describes the photoperiodic control of annual body mass changes in captive. Svalbard ptarmigan, in particular the onset of autumnal increase and midwinter decrease in fat content under natural and simulated daylight-conditions in Tromsø (69° 46 N). Autumnal fattening commences when the birds become photorefractory and presumably depends on this condition. At present it is not known if any causal relationship is involved. Under outdoor ambient temperature, body mass begins to decline in November. However, when Svalbard ptarmigan are caged indoors at higher ambient temperatures and exposed to natural or simulated annual changes in daylength, body mass remains high until February. In these birds the depletion of fat stores appears to be triggered by the increasing daylength, since body mass remained high under permanent exposure to short days throughout spring but decreased promptly following photostimulation in May. When ptarmigan caged indoors were starved in midwinter body mass fell but increased briefly upon refeeding and thereafter declined as in the control birds throughout spring. This indicates that the winter body mass profile in Svalbard ptarmigan is not merely the passive outcome of shifts in the energy expenditure associated with thermoregulation, and that a sliding set point for body mass exists and is temporally fixed at the seasonal maximum in mid winter in birds caged under indoor ambient temperatures. The possibility is discussed that the decline in body mass seen outdoors may be associated with the increased hypothalamus-pituitary-gonadal activity which follows the breaking of photorefractoriness, and that this activity is sufficiently suppressed in Svalbard ptarmigan caged indoors under exposure to short days, to delay the reduction until they are photostimulated.Abbreviations BM body mass - GH growth hormone - HPG hypothalamic-pituitary-gonadal system - LL continous light - NL natural light - SD short day (4 hours light per day) - SL simulated annual changes in daylength - T _a ambient temperature