Hematological stress indices reveal no effect of radio‐transmitters on wintering Hermit Thrushes

ABSTRACT Radio‐telemetry is often used to track birds, and several investigators have examined the possible effects of radio‐transmitters on birds. One approach to this question is to determine if transmitters induce physiological stress. Using hematological indicators of stress (heterophil‐lymphocyte [H/L] ratios), studies of captive birds have revealed no evidence that radio‐transmitters cause stress. However, studies in captivity may not reflect conditions faced by birds in the wild, especially during energetically demanding times, such as the overwintering period. We examined the possible effect of backpack‐style transmitters on overwintering Hermit Thrushes (Catharus guttatus) in South Carolina by examining H/L ratios from blood smears made before and after transmitter attachment. We captured and recaptured eight thrushes in an early winter period and 11 in a late winter period, and in both cases found that H/L ratios did not change after 1 mo of carrying transmitters. H/L ratios also did not differ significantly (in either trial) between thrushes with and without transmitters. Furthermore, thrushes that carried transmitters during both of our trials did not have higher final H/L ratios than those that carried transmitters during just one trial (the late winter). These results indicate that carrying backpack‐style transmitters for 1 mo did not induce stress in overwintering Hermit Thrushes. These results are consistent with studies of captive birds, and provide support for the continued use of backpack radio‐transmitters in this species, as long as weight recommendations are met.     

Wintering destinations of Monterey Hermit Thrushes (Catharus guttatus slevini)

Across their broad North American distribution, Hermit Thrushes (Catharus guttatus) exhibit extensive yet subtle intraspecific variation in morphology and diverse migration patterns, causing considerable debate regarding their systematics and an incomplete understanding of their migratory geography. To better understand the fall migration and wintering destinations of Hermit Thrushes in coastal California, we deployed geolocators on individuals of the subspecies C. g. slevini breeding in the Santa Cruz Mountains of the Coast Ranges. In 2014, we captured 20 Hermit Thrushes in Big Basin Redwoods State Park using mist‐nets and attached geolocators. In 2015, we retrieved tags from 13 birds. Tagged Hermit Thrushes left the breeding area between 1 and 19 September 2014 and arrived in wintering areas in Baja California Sur and northwestern mainland Mexico between 24 September and 13 October 2014. The average distance between breeding and wintering areas was 1617 ± 217 (SD) km, and the average duration of fall migration was 22.5 ± 6.4 (SD) days. Our results suggest that Hermit Thrushes breeding in Big Basin winter in a highly concentrated region of western Mexico including Baja California Sur and southwestern Sonora or northwestern‐most Sinaloa; we found no evidence that Big Basin birds overwintered in the southwestern United States. Our results also confirm the existence of chain migration for Hermit Thrushes in California. Because C. g. slevini exhibits a limited distribution in both breeding and wintering areas and their morphology and song suggest adaptation to their habitat, we recommend exploration of fine‐scale genetic structure of coastal California’s Hermit Thrushes to determine the extent of evolutionary divergence in this subspecies.     

Fine‐scale habitat use during the non‐breeding season suggests that winter habitat does not limit breeding populations of a declining long‐distance Palearctic migrant

For migrant birds, which habitats are suitable during the non‐breeding season influences habitat availability, population resilience to habitat loss, and ultimately survival. Consequently, habitat preferences during winter and whether habitat segregation according to age and sex occurs directly influences migration ecology, survival and breeding success. We tested the fine‐scale habitat preferences of a declining Palearctic migrant, the whinchat Saxicola rubetra, on its wintering grounds in west Africa. We explored the influence of habitat at the territory‐scale and whether dominance‐based habitat occupancy occurs by describing the variation in habitat characteristics across wintering territories, the degree of habitat change within territories held throughout winter, and whether habitat characteristics influenced territory size and space‐use within territories or differed with age and sex. Habitat characteristics varied substantially across territories and birds maintained the same territories even though habitat changed significantly throughout winter. We found no evidence of dominance‐based habitat occupancy; instead, territories were smaller if they contained more perching shrubs or maize crops, and areas with more perching shrubs were used more often within territories, likely because perches are important for foraging and territory defence. Our findings suggest that whinchats have non‐specialised habitat requirements within their wintering habitat of open savannah and farmland, and respond to habitat variation by adjusting territory size and space‐use within their territories instead of competing with conspecifics. Whinchats show a tolerance for human‐modified habitats and results support previous findings that some crop types may provide high‐quality wintering habitat by increasing perch density and foraging opportunities. By having non‐specialised requirements within broad winter habitat types, migrants are likely to be flexible to changing wintering conditions in Africa, both within and across winters, so possibly engendering some resilience to the rapid anthropogenic habitat degradation occurring throughout their wintering range.     

Reproductive ecology of Brown-cheeked Laughing Thrushes (Garrulax henrici) in Tibet

ABSTRACT.   Most species in the genus Garrulax are found in subtropical or tropical areas of southeastern Asia, with few species adapted to temperate or high-altitude climates. We examined the behavior and reproductive ecology of Brown-cheeked Laughing Thrushes ( G. henrici ) from 1999 to 2005 at Xiongse Valley, Tibet. Cup-shaped nests located 0.4–2.6 m above ground were found in 13 different species of plants, with roses and barberries the most common nest substrates. Laughing thrushes were socially monogamous and no territorial behaviors were observed. Egg-laying was initiated in early May and ended by late August, with clutch sizes of either 2 or 3 eggs     . Incubation started with the first egg being laid and the incubation period lasted 13–17 d     . Young fledged at 14–16 d posthatching     . Of 38 nesting attempts with known fates, at least one nestling hatched in 32 nests (84%) and at least one young fledged from 21 nests (55%). The mean number of fledglings per successful nest was 2.2 (range 1–3; N = 21 nests). Compared to lowland congeners, Brown-cheeked Laughing Thrushes produced smaller clutches and larger eggs. Such differences suggest that laughing thrushes may invest more energy in fewer offspring at higher altitudes, as reported for other avian species and populations, and this may represent an adaptive response to high-altitude environments where climates are harsh and food availability may be limiting.     

Winter territoriality of Mute Swans Cygnus olor

There was considerable variation in winter territoriality among Mute Swan pairs around Oxford, though most pairs maintained their territories for at least half the winter. Variation was associated with temperature and with territory quality pairs left their territories most commonly during the coldest months of the winter, and pairs on good territories, with abundant vegetation and accessible pasture. stayed longer on territory than those on poor territories with sparse vegetation and little pasture. As in other species, exclusive territorial defence was maintained at intermediate levels of resource availability, and flocks were able to settle on a few particularly good territories. Circumstantial evidence suggested that winter territories may be maintained not only to provide an adequate food supply for the owners but also to ensure access to a brerding site by preventing other individuals from taking over the area.     

Shiny Cowbird parasitism of a low quality host: effect of host traits on a parasite's reproductive success

ABSTRACT.   The reproductive success of parasitic cowbirds ( Molothrus spp.) varies among host species and is influenced by the degree of synchronization in timing of egg laying, the duration of parasite and host incubation periods, and the ability of hosts to incubate and rear parasite young. We studied the reproductive success of Shiny Cowbirds ( Molothrus bonariensis ) that parasitized the nests of Creamy-bellied Thrushes ( Turdus amaurochalinus ) in the Monte desert region of Argentina. Shiny Cowbirds frequently parasitized Creamy-bellied Thrush nests (60%), and most cowbirds synchronized egg laying with that of thrushes (79%). Most parasitic eggs (80%) hatched within 1 d of the hatching of the first host egg, and more than 91% of the eggs survived until the end of the incubation. However, only 60% of the cowbird eggs hatched and 52% of young survived. The proportion of Shiny Cowbirds eggs laid in Creamy-bellied Thrush nests that resulted in fledged young was 0.03, including eggs and young lost due to predation or desertion. Despite this low reproductive success, Creamy-bellied Thrushes were heavily parasitized by Shiny Cowbirds in our study area. Shiny Cowbirds may continue to parasitize these thrushes because of diffuse selection or because Shiny Cowbird chicks are more likely to fledge from Creamy-bellied Thrush nests in years or areas with greater food availability when brood reduction does not occur.     

Family-based territoriality vs flocking in wintering common cranes Grus grus

We describe winter territoriality in common cranes, Grus grus , a long-lived migrant species with long-lasting pair bonds and parental care extending throughout the winter. Cranes are territorial in the breeding season, and usually gregarious during migration and wintering. Only 2% of the families present in our study area were territorial, all other families foraged in flocks with immatures and adult pairs. Territorial pairs defended the same winter territory year after year, but only when they had offspring. They were gregarious otherwise. The average breeding success measured throughout several years was higher in territorial pairs. Winter territories were small (0.7 km² on average), but included a higher diversity of habitats than the areas visited by gregarious birds (11.7 km² on average). Adults of territorial families showed longer vigilance times, and lower food intake rates than did adults in flocks, which were compensated with a longer time spent foraging per day. The accumulated daily food intake did not differ between adults in flocks and in families. We suggest that winter territoriality is a facultative strategy, conditioned by parental experience and habitat availability.     

THE SOCIAL BEHAVIOUR OF THE WHITE WAGTAIL MOTACILLA ALBA ALBA WINTERING IN ISRAEL

White Wagtails Motacilla alba wintering in Israel are partly territorial, mostly around human habitations, and partly live in flocks around temporary food sources. Individual birds may spend part of the season (or the day) in the territory and the other part with a flock. Experiments with artificial distribution of food, in a natural habitat, brought about a change from flocking to territorial behaviour. Preliminary observations suggest that in the natural situation the pattern of food distribution may be the proximate factor which regulates the birds' behaviour, by determining whether they have to fight for their food. Pairs are formed on many territories, and may last for long or short periods. Pair formation is initiated by females, who when seeking food appease the territorial males and are able to stay with them on their territories. Females also manifest territorial behaviour. Although pairing in winter territories is similar, in the behaviour involved, to sexual pairing, it is very unlikely that winter pairing continues into, or influences, pairing for breeding. It is suggested that the function of winter pair formation is that it allows two birds to exploit one territory, and that the main advantage is to the female which is the subordinate bird of the pair. This kind of pair-formation may be analogous to non-breeding group territories reported in some other birds.     

Prey availability affects territory size,but not territorial display behavior,in green anole lizards

The availability of food resources can affect the size and shape of territories, as well as the behaviors used to defend territories, in a variety of animal taxa. However, individuals within a population may respond differently to variation in food availability if the benefits of territoriality vary among those individuals. For example, benefits to territoriality may differ for animals of differing sizes, because larger individuals may require greater territory size to acquire required resources, or territorial behavior may differ between the sexes if males and females defend different resources in their territories. In this study, we tested whether arthropod abundance and biomass were associated with natural variation in territory size and defense in insectivorous green anole lizards, Anolis carolinensis. Our results showed that both male and female lizards had smaller territories in a habitat with greater prey biomass than lizards in habitats with less available prey, but the rates of aggressive behaviors used to defend territories did not differ among these habitats. Further, we did not find a relationship between body size and territory size, and the sexes did not differ in their relationships between food availability and territory size or behavioral defense. Together, these results suggest that differences in food availability influenced male and female territorial strategies similarly, and that territory size may be more strongly associated with variation in food resources than social display behavior. Thus, anole investment in the behavioral defense of a territory may not vary with territory quality.     

Flexible spatial organization of urban foxes, Vulpes vulpes, before and during an outbreak of sarcoptic mange

The social and spatial organization of urban fox groups prior to and during an outbreak of sarcoptic mange was compared with predictions derived from the resource dispersion hypothesis (RDH). We investigated the availability of three key resources. Neither daytime rest sites nor breeding sites appeared to be limited in availability. The availability of food deliberately supplied by local householders was examined by questionnaire surveys. The daily and weekly amount of food supplied was greatly in excess of the minimum requirements of a pair of foxes, but was consistent between territories. The availability of this food source increased markedly as a result of more people feeding the foxes. In agreement with the RDH, group size prior to the outbreak of mange increased from 2.25 animals (N=4) to 6.57 animals (N=7). Before the outbreak of mange, two territories were divided. Increased scavenge availability on smaller territories may have promoted these changes. Excluding these spatial changes, territories were very stable between years. After the outbreak of mange, group size declined as a direct result of mange-induced mortality. Surviving animals increased their ranges only after neighbouring groups had died out. Ranges did not increase in size in response to a decline in food availability. Nor were the increases in range size associated with the relinquishment of parts of the existing territory. These postmange changes are contrary to the RDH. Three factors may have promoted these changes: the elimination of interstitial space, the forced dispersal of young or future division of the territory. Copyright 2000 The Association for the Study of Animal Behaviour.     

Goose grazing selectivity along a depletion gradient

The micro-habilat food selectivity along a gradient of increasing grazing intensity of wintering pink-footed geese Anser brachyrhynchus and barnacle geese Branta leucopsis foraging on winter wheat Triticum aestivum and smooth meadow grass Poa pratensis was investigated. Protein content of ungrazed primary and secondary leaves of winter wheat did not differ, whereas the protein content of tertiary leaves was significantly lower. Secondary leaves were significantly longer than both primary and tertiary leaves, whereas tertiary leaves were significantly longer than primary leaves. Protein content of primary and secondary leaves of Poa pratensis did not differ, whereas the protein content of tertiary leaves was significantly lower. Primary leaves were significantly shorter than both secondary and tertiary leaves, which did not differ significantly.
For all leaf categories, there was an increase in the removal with increasing goose grazing intensity. The geese showed a sequential selection of winter wheat leaves according to the protein content, initially preferring primary and secondary leaves, but increasingly selecting tertiary leaves of poorer quality as high-quality leaves became depleted. Foraging on Poa pratensis. geese selected secondary leaves according to the protein content, whereas tertiary leaves were preferred to primary leaves, although these had a protein content equal to secondary leaves. This, however, may be explained by the primary leaves being too short to maintain intake rates.
It is concluded that feeding preferences cannot be established on the basis of protein contents of food plants alone and that green biomass estimates not necessarily reflect resource availability.     

Habitat effects on resource tracking ability: do wintering Blackcaps Sylvia atricapilla track fruit availability?

If resource availability shapes population distribution, changes in resource abundance should cause parallel changes in population numbers. However, tracking ability may be disrupted by different environmental and behavioural factors that act at different spatial and temporal scales. Here we analyse the ability of wintering Blackcap Sylvia atricapilla populations to track spatio-temporal variation in fruit availability in southern Spain in two habitats (forests and shrublands) with different population structure. Former studies had shown that forests are equally used by both adult migrant and local Blackcaps, whereas shrublands are nearly monopolized by juvenile migrants. These differences might affect resource tracking: it should be disrupted in forests, as local birds remain over winter in their breeding territories, but not in shrublands where similarly competitive juvenile migrants can freely track the spatial distribution of fruits. We analysed the fruit-tracking ability of Blackcap populations among sites and years in both habitat types using a habitat-matching model, which predicts spatio-temporal changes in population abundance proportional to changes in resource availability. We counted Blackcaps and fruiting shrubs (dominated by Lentiscs Pistacia lentiscus and Wild Olives Olea europaea sylvestris ) during four winters in forest and shrubland patches. The abundance of fruits was always higher in shrublands than in forests. In shrublands, Blackcaps seemed to move freely across fruit-rich habitat patches, tracking changes in fruiting-shrub abundance among sites and years. However, such tracking was not observed in forests. This supports the view that fruit-tracking ability may be constrained by local factors, such as the social structure of populations occurring in different habitat types, which introduces spatio-temporal variation in the way fruit availability shapes the abundance distribution of these birds in their Mediterranean wintering grounds.     

Determinants of territory size in the pomacentrid reef fish,Parma victoriae

Summary Factors governing the size of territories defended by the pomacentrid reef fish, Parma victoriae, were investigated, prompted by contradictory predictions in the literature concerning the effects of food supply and competitors. Observations were carried out over the non-breeding period (March–October) on a medium density population in which territories were partially contiguous. The territory size of adult fish varied between 3 and 26 m², and was inversely correlated with local densities of conspecifics. The same range in territory size was found for both males and females, which did not differ in the time they spent on territory defence and foraging activities. No correlation existed between territory size and the abundance of algal food, body size, age or time spent on territory defence. Also, there was little variation in territory size over time, despite seasonal changes in the abundance of food algae.Experimental reduction of food supplies on isolated territories of males and females had no effect on territory size. In a higher density habitat an experiment was carried out in which population density and food abundance were simultaneously manipulated. This showed that territory size was primarily determined by intraspecific interactions, as territories exhibited considerable increases in size upon removal of neighbours. No changes in the size of defended areas resulted from either artificial increases or decreases of food levels. There were also no changes in the time spent on defence of territories, foraging time or feeding rates associated with food manipulations or territory expansion, which suggested that food was not a limited resource. This conflicted with current theories proposed to explain territory defence and expansion. It is hypothesized that intraspecific interactions constrain territory size well below the optimum in terms of the abundance of preferred food algal species.     

Radio‐transmitters do not affect seasonal mass change or annual survival of wintering Bicknell's Thrushes

ABSTRACT Studies of the effects of transmitters on passerines have provided mixed results, but many have revealed no negative impacts. Most such studies have been conducted during the breeding season and, as a result, little is known about the possible effects of transmitters on wintering birds. We examined the effects of transmitters on Bicknell's Thrushes (Catharus bicknelli) wintering in the Dominican Republic. We used long‐term mark‐recapture data to compare birds with radio‐transmitters (N= 5 years, 64 individuals) to birds banded, but not radio‐tagged (N= 10 years, 164 individuals). For a subset of birds in each category, we measured the change in mass between early‐winter and late‐winter and, for all birds, we calculated return rates and modeled annual survival by sex and age classes. Return rates of radio‐tagged (16%) and banded‐only individuals (17%) did not differ, and survival models including the effect of transmitter attachment were poorly supported. We also found no sex or age‐specific effects of transmitter attachment. Radio‐tagged and banded‐only birds did not differ either in the proportion of birds gaining mass during the winter or amount of mass gained. Our results indicate that attaching light‐weight transmitters (<5% of body mass) to wintering Bicknell's Thrushes did not adversely affect either body condition or annual survival.     

Migratory and resident Blackcaps Sylvia atricapilla wintering in southern Spain show no resource partitioning

When different populations of the same bird species share non‐breeding habitats, competition for food may promote resource partitioning. We studied food choice by resident and migratory Blackcaps Sylvia atricapilla in sympatric wintering grounds in southern Spain. Resident Blackcaps have a larger bill, which may allow them to feed on a broader range of fruit sizes, and they may know the distribution of food better than do migrants. Based on fruit and bird counts, we transformed both fruit crop and bird abundance to a common energy currency. During two winters with low and high fruit production, available energy from fruit in mid‐January was estimated to be 80 and 1300 times, respectively, the daily requirements of Blackcaps. Furthermore, Blackcap numbers did not track between‐winter changes in fruit abundance during 10 consecutive years of monitoring, further suggesting that fruit food is not limiting. Analysis of food items from 760 samples of 717 individuals showed that migrants and residents fed primarily on fruits of Wild Olive Olea europaea sylvestris, the most energetic fruit resource. There was no evidence that the larger bills of resident Blackcaps provided any foraging benefit. Migratory Blackcaps fed on Wild Olives and invertebrates, two resources with high energetic and structural value, more frequently than did residents. This food choice could be more important for migratory Blackcaps because they have lower body mass to reduce wing load. Our results suggest that the wintering grounds of Blackcaps in Iberia provide abundant food that is used by sympatric migrants and residents without resource partitioning. Slight differences in food choice suggest that migrants might benefit from feeding on more nutritive food than residents to counteract the energetic constraints associated with a smaller body size.     

Causes and consequences of individual variation in territory size in the American red squirrel

For most territorial species that defend food or other resources, territory size is inversely related to resource density. However, in some food-based territorial species, larger territories are known to contain greater absolute resource availability. For these latter species, both the specific determinants of territory size, and the relationship between territory size and fitness, remain unclear. We predicted that in American red squirrels ( Tamiasciurus hudsonicus ), where larger territories tend to hold increased food resources, survival should be correlated positively with territory size and that both individual (body mass, sex, duration of territory ownership) and habitat-specific (population density) attributes would contribute to variation in territory size. We monitored survival and territory size of 58 squirrels during a two-year period using live-trapping and radio-telemetry. Survival and territory size were correlated positively, with each 0.1 ha decrease in territory size typically resulting in a 16.6% increase in mortality risk; mortality risk for squirrels with the smallest versus largest territories differed by up to 23-fold. Territory size was negatively related to local population density, but not to any attributes of individual squirrels, implying that, in this species, territory size may not be influenced strongly by individual quality. We conclude that while larger territories may confer a survival advantage to squirrels, territory size is unrelated to individual physical attributes, likely because costs associated with the acquisition of a larger or richer territory outweigh potential benefits of increased food access.     

Winter territoriality and flock cohesion in the black-capped chickadee Parus atricapillus

In northwest Massachusetts, black-capped chickadees shifted foraging groups repeatedly each day during the winter. As a result, there was no clear distinction between flock members and floaters, but instead a continuous gradation from birds that formed relatively stable associations to those that shifted groups more frequently. Group territories were absent and individual territories were at most poorly defended, if present at all. This picture differs from other recently studied populations, where flock membership is stable and flock territories are clear. Comparison with other populations indicates that winter social structure in the black-capped chickadee may be sensitive to both winter food levels and the proportion of year-round residents. This indicates that cost-benefit analyses of winter social organization should consider factors acting throughout the year as well as the residence status of the wintering birds. The data suggest that flock size, population density and home range size are sensitive to winter food on the study area, whereas territoriality and flock cohesion may respond more clearly to the local availability of breeding resources or the proportion of year-round residents.     

Food resource availability for American black ducks wintering in southern New Jersey

Midwinter waterfowl survey data indicates a long-term decline in the number of wintering American black ducks (Anas rubripes), potentially due to habitat limitations. In order for future estimates of carrying capacity to be determined, it is critical that regional food availability is estimated. We collected pairs of habitat core samples (n = 510) from 5 habitat types in southern New Jersey, USA, during October, January, and April 2006–2008 to estimate resource availability and variability. We collected upper gastrointestinal tracts from hunter-killed birds (n = 45) and late season collections (n = 19) to identify food items and limited our estimates of resource availability to only winter food items; thereby reducing the availability of seed foods found in our core samples by 38% and animal foods by 96%. We did not detect differences in years or sampling period, but did between habitat types. Mudflat habitat had the greatest availability of invertebrate and vertebrate food items and appeared to supply the bulk of energy to black ducks wintering in southern New Jersey. We suggest conservation efforts to be focused on restoring or enhancing mudflat habitat as an integral component of an ecologically functioning salt marsh to increase food availability. © 2011 The Wildlife Society.     

Can increased niche opportunities and release from enemies explain the success of introduced Yellowhammer populations in New Zealand?

Some introduced species succeed spectacularly, becoming far more abundant in their introduced than in their native range. 'Increased niche opportunities' and 'release from enemy regulation' are two hypotheses that have been advanced to explain the enhanced performance of introduced species in their new environments. Using an introduced bird species, the Yellowhammer Emberiza citrinella , which was first released in New Zealand in 1862, as a model, we tested some predictions based on these hypotheses. By quantifying habitat availability and quality, and measuring nest predation rates, we investigated whether increased niche opportunities or release from nest predation could explain the higher density of the Yellowhammer in New Zealand farmland, compared to farmland in their native Britain. Yellowhammer territory densities were over three times higher in New Zealand (0.40 territories per ha) than in comparable British farmland (0.12 territories per ha), and Yellowhammer densities remained significantly higher in New Zealand, after accounting for differences in habitat availability. The density and diversity of invertebrates, a key food resource for nestling Yellowhammers, was significantly lower in New Zealand than in Britain. Hence, these aspects of niche availability and quality cannot explain the higher density of Yellowhammers in New Zealand. Nest predation rates in New Zealand were similar to those in Britain, suggesting that release from nest predation also could not account for the higher density of Yellowhammers in New Zealand. Differences in winter survival, due to differences in winter food supply or the severity of the winter climate, along with release from other types of 'enemy' regulation are possible alternative explanations.     

Territory strategy of the migratory Brown Shrike Lanius cristatus

Brown Shrikes Lanius cristatus are both transients and winter residents in Taiwan. Large numbers of birds crowd into the available habitat during the transient period, and the cost of maintaining territories changes greatly, depending on the number of shrikes present. This study examined how wintering Brown Shrikes adapt to the changing level of competition in territory establishment. About 24% of autumn transients arrived before any wintering birds, while 73% of wintering birds arrived during autumn transient time. The frequency of aggression recorded was significantly related to the number of birds seen along the transect line that day. Among ringed birds that returned during transient time, 80% returned directly to their previous territories, but some waited for the departure of transient birds before establishing territories. Territory compression occurred at high population densities. Departure from and arrival at the study area were frequent during the winter, but no birds moved territories within the study area. On average, 25% of ringed wintering shrikes returned to the study site in the subsequent years, while no transients returned. This low site tenacity may be. in part, a result of a high persecution rate along the migration route.