Changes in adult annual survival rates in a western European population of the White Stork Ciconia ciconia

The population of the White Stork Ciconin ciconia in Alsace (northeastern France) has decreased considerably since 1961. Using recent captureerecapture models, we analysed resighting data on ringed birds resulting from a long-term ringing programme carried out in 1947–1985. Time-dependence and age-dependence in annual survival rates are examined with reference to environmental variables in the Sahel wintering area. Our results show a decrease in adult annual survival rate linked to the severity of the drought in the Sahelian zone.     

Predation of codling moth Cydia pomonella L. by the Silvereye Zosterops lateralis (Latham)

Predation of cocooned larvae of codling moth Cydia pomonella by silvereyes Zosterops lateralis was studied in an apple orchard in Nelson, New Zealand. Apple logs with known larval densities were made available to the birds for known periods of time, either in cages or exposed in the apple orchard. The numbers of silvereyes and the natural predation of codling moth were recorded in the same orchard. Predation was density dependent. On caged logs with an initial high density of 32 larvae, 1.1 larvae were consumed per bird‐hour; in contrast, one larva was consumed per 34.5 bird‐hours at three larvae per log. A curvilinear relationship was demonstrated between larval density and the bird‐hours required for predation; this relationship was consistent with the known density dependence of silvereye predation of codling moth. A regression of the total annual winter bird predation of larvae in the orchard on bird numbers was significant. However, the density dependence of predation resulted in declining rates of predation over the winter as larval density declined; the first birds to arrive in the orchard benefitted from particularly high predation rates. As a consequence, fluctuations in bird numbers during the winter had only a secondary influence on predation rates. The numbers of silvereyes in the orchard showed no relationship to the density of the codling moth population present. This study confirmed the importance of silvereyes in the predation of codling moth and a functional, not numerical, rseponse of these birds to codling moth density.     

Impact of wind farms on soaring bird populations at a migratory bottleneck

Collision with turbines at wind farms is expected to have a greater impact on birds at particular sites where high concentrations of individuals occur, such as migration bottleneck areas. The Strait of Gibraltar (southern Spain) has long been recognized as the most important bottleneck in western Europe for soaring bird migration. Moreover, this area is within one of the most important potential areas for wind energy generation in Spain. Here, we examine monthly migratory soaring bird abundance in relation to long-term avian mortality rates at 21 wind farms located near the Strait of Gibraltar using zero-inflated hurdle negative binomial and gamma models. Best fit models included an effect of season in the collision mortality rates and in the proportion of adult individuals within the total deaths. However, monthly bird abundance was not directly related to the number of fatalities over the year. The accumulated fatalities during autumn migration constitute a small percentage (1%) of the total migrating population size. Moreover, mortality peak during autumn migration is largely attributable to juvenile birds. In contrast, the number of fatalities coinciding with the breeding period constitutes a substantial proportion (6%) of the local population, and it involved substantial losses among adult birds. Our results show that wind farms probably have an individually low impact on the migratory population of soaring birds. On the contrary, annual losses among adult local birds are remarkably high considering the small size of the local populations, and they may have population level effects.     

The hunting of migratory birds in Europe: a ringing recovery analysis

The hunting of 20 species of migratory birds in Europe and countries bordering the Mediterranean Sea was investigated using ringing recovery data. The intensity of the hunting of birds in each country was measured by the calculation of an index which controls for ringing effort and reporting rates. The hunting of birds was shown to be consistently high in western Mediterranean countries, particularly in southern France, northern and southwestern Iberia, northern Italy and in northwest Africa. Geographical patterns in the relative magnitude of hunting indices for different breeding populations were species-specific. In Europe most birds are taken during autumn and winter but in North Africa hunting is almost equally prevalent in spring. All species investigated, other than those that remain legitimate quarry species, showed a general reduction in index values after 1980. Analysis of long-term trends in index values since 1950 indicated a statistically significant overall decrease in the hunting of the majority of species. These changes are thought to be at least partly attributable to a real decline in the taking of birds but they may also reflect changed attitudes to reporting the hunting of species which are now protected. This analysis provides the first quantitative Europe-wide assessment of geographical and temporal trends in the hunting of migratory birds.     

A Comprehensive Analysis of Small-Passerine Fatalities from Collision with Turbines at Wind Energy Facilities

Small passerines, sometimes referred to as perching birds or songbirds, are the most abundant bird group in the United States (US) and Canada, and the most common among bird fatalities caused by collision with turbines at wind energy facilities. We used data compiled from 116 studies conducted in the US and Canada to estimate the annual rate of small-bird fatalities. It was necessary for us to calculate estimates of small-bird fatality rates from reported all-bird rates for 30% of studies. The remaining 70% of studies provided data on small-bird fatalities. We then adjusted estimates to account for detection bias and loss of carcasses from scavenging. These studies represented about 15% of current operating capacity (megawatts [MW]) for all wind energy facilities in the US and Canada and provided information on 4,975 bird fatalities, of which we estimated 62.5% were small passerines comprising 156 species. For all wind energy facilities currently in operation, we estimated that about 134,000 to 230,000 small-passerine fatalities from collision with wind turbines occur annually, or 2.10 to 3.35 small birds/MW of installed capacity. When adjusted for species composition, this indicates that about 368,000 fatalities for all bird species are caused annually by collisions with wind turbines. Other human-related sources of bird deaths, (e.g., communication towers, buildings [including windows]), and domestic cats) have been estimated to kill millions to billions of birds each year. Compared to continent-wide population estimates, the cumulative mortality rate per year by species was highest for black-throated blue warbler and tree swallow; 0.043% of the entire population of each species was estimated to annually suffer mortality from collisions with turbines. For the eighteen species with the next highest values, this estimate ranged from 0.008% to 0.038%, much lower than rates attributed to collisions with communication towers (1.2% to 9.0% for top twenty species).     

Refining Estimates of Bird Collision and Electrocution Mortality at Power Lines in the United States

Collisions and electrocutions at power lines are thought to kill large numbers of birds in the United States annually. However, existing estimates of mortality are either speculative (for electrocution) or based on extrapolation of results from one study to all U.S. power lines (for collision). Because national-scale estimates of mortality and comparisons among threats are likely to be used for prioritizing policy and management strategies and for identifying major research needs, these estimates should be based on systematic and transparent assessment of rigorously collected data. We conducted a quantitative review that incorporated data from 14 studies meeting our inclusion criteria to estimate that between 12 and 64 million birds are killed each year at U.S. power lines, with between 8 and 57 million birds killed by collision and between 0.9 and 11.6 million birds killed by electrocution. Sensitivity analyses indicate that the majority of uncertainty in our estimates arises from variation in mortality rates across studies; this variation is due in part to the small sample of rigorously conducted studies that can be used to estimate mortality. Little information is available to quantify species-specific vulnerability to mortality at power lines; the available literature over-represents particular bird groups and habitats, and most studies only sample and present data for one or a few species. Furthermore, additional research is needed to clarify whether, to what degree, and in what regions populations of different bird species are affected by power line-related mortality. Nonetheless, our data-driven analysis suggests that the amount of bird mortality at U.S. power lines is substantial and that conservation management and policy is necessary to reduce this mortality.     

Population limitation in migrants

Unlike resident bird species, the population sizes of migratory species can be influenced by conditions in more than one part of the world. Changes in the numbers of migrant birds, either long‐term or year‐to‐year, may be caused by changes in conditions in the breeding or wintering areas or both. The strongest driver of numerical change is provided in whichever area the per capita effects of adverse factors on survival or fecundity are greatest. Examples are given of some species whose numbers have changed in association with conditions in breeding areas, and of others whose numbers have changed in association with conditions in wintering areas. In a few such species, the effects of potential limiting factors have been confirmed locally by experiment. In theory, population sizes might also be limited by severe competition at restricted stopover sites, where bird densities are often high and food supplies heavily depleted, but (with one striking exception) the evidence is as yet no more than suggestive. In some species, habitats occupied in wintering and migration areas, and their associated food supplies, can influence the body condition, migration dates and subsequent breeding success of migrants. Body reserves accumulated in spring by large waterfowl serve for migration and for subsequent breeding, and females with the largest reserves are most likely to produce young. Hence, the conditions experienced by individuals in winter in one region can affect their subsequent breeding success in another region. Such effects are apparent at the level of the individual and at the level of the population. Similarly, the numbers of young produced in one region could, through density‐dependent processes, affect subsequent overall mortality in another region. Events in breeding, migration and wintering areas are thus interlinked in their effects on bird numbers. Although in the last 30–40 years the numbers of some tropical wintering birds have declined in western Europe and others in eastern North America, the causes seem to differ. In Europe, declines have mainly involved species that winter in the arid savannas of tropical Africa, which have suffered from the effects of drought and increasing desertification. In several species, annual fluctuations in numbers and adult survival rates were correlated with annual fluctuations in rainfall, and by implication in winter food supplies. In North America, by contrast, numerical declines have affected many species that breed and winter in forest, especially those eastern species favouring the forest interior. Declines have been attributed ultimately to human‐induced changes in the breeding range, particularly forest fragmentation, which have led to increases in the densities of nest predators and parasitic cowbirds. These in turn are thought to have caused declines in the breeding success of some neotropical migrants, which is now too low to offset the usual adult mortality, but as yet convincing evidence is available for only a minority of species. The breeding rates and population changes of some migratory species have been influenced by natural changes in the availability of defoliating caterpillars. In other species, tropical deforestation is likely to have played the major role in population decline, and if recent rates of tropical deforestation continue, it is likely to affect an increasing range of migratory species in the future. Not all such species are likely to be affected adversely by deforestation, however, and some may benefit from the resulting habitat changes.     

Social and demographic effects of anthropogenic mortality: a test of the compensatory mortality hypothesis in the red wolf

Whether anthropogenic mortality is additive or compensatory to natural mortality in animal populations has long been a question of theoretical and practical importance. Theoretically, under density-dependent conditions populations compensate for anthropogenic mortality through decreases in natural mortality and/or increases in productivity, but recent studies of large carnivores suggest that anthropogenic mortality can be fully additive to natural mortality and thereby constrain annual survival and population growth rate. Nevertheless, mechanisms underlying either compensatory or additive effects continue to be poorly understood. Using long-term data on a reintroduced population of the red wolf, we tested for evidence of additive vs. compensatory effects of anthropogenic mortality on annual survival and population growth rates, and the preservation and reproductive success of breeding pairs. We found that anthropogenic mortality had a strong additive effect on annual survival and population growth rate at low population density, though there was evidence for compensation in population growth at high density. When involving the death of a breeder, anthropogenic mortality was also additive to natural rates of breeding pair dissolution, resulting in a net decrease in the annual preservation of existing breeding pairs. However, though the disbanding of a pack following death of a breeder resulted in fewer recruits per litter relative to stable packs, there was no relationship between natural rates of pair dissolution and population growth rate at either high or low density. Thus we propose that short-term additive effects of anthropogenic mortality on population growth in the red wolf population at low density were primarily a result of direct mortality of adults rather than indirect socially-mediated effects resulting in reduced recruitment. Finally, we also demonstrate that per capita recruitment and the proportion of adults that became reproductive declined steeply with increasing population density, suggesting that there is potential for density-dependent compensation of anthropogenically-mediated population regulation.     

Population dynamics of the bearded vulture Gypaetus barbatus in southern Africa

The essential features of mortality and survivorship of bearded vultures Gypaetus barbatus in southern Africa were deduced from age class plumage characteristics. The population consisted of about 204 adult pairs within a breeding range of about 35,000 sq km. Pairs bred every year and produced, on average, about 0·9 young per pair per year. Young birds made up about 37% of the population, subadults 3·5% and adults 60%. About 182 fledged young were recruited to the population each year. The proportion of young birds in the population in different areas was inversely related to the breeding density of adult birds (range 24–47%). Young bird mortality over the four years to subadult age was 87%, the survival rate of adults was 94% and the mean lifespan of birds surviving to adulthood was 21·4 years. This study demonstrates the need to understand the relationship between adult breeding density and young bird numbers in different parts of their range to accurately deduce population dynamics characteristics.     

Training Kenyan bird ringers

Jackson, J. 2000. Training Kenyan bird ringers. Ostrich 71 (1 & 2): 342.

Capturing and ringing birds is a fundamental ornithological field technique, and one that underlies much bird conservation research. In several countries, particularly in Britain and parts of western Europe, ringing is largely an amateur activity. Enthusiasts ring birds in their free time and take part in national surveys, enabling the effective monitoring of bird populations and movements. Until recently ringing in Kenya has been undertaken almost entirely by a handful of expatriate researchers and enthusiasts. Over the last 1CL15 years, it has also been largely confined to one site: Ngulia in Tsavo West National Park. Ringing effort has targeted Palaearctic migrants, mainly ignoring Afrotropical birds. The number of amateur and professional Kenyan ornithologists is fast increasing, but only a handful have any ringing experience. The remainder lack the resources to obtain the necessary equipment or the opportunity to undergo the long training required to become a qualified ringer. In June 1994, the, Nairobi Ringing Group was formed, based at the Department of Ornithology, National Museums of Kenya, Nairobi. The Group provides an opportunity for interested Kenyans to train as amateur ringers to a high standard, using departmental equipment and expertise. Ringing takes place twice weekly at the Nairobi Museum grounds and monthly at a constant effort site in a forest near Nairobi. Excursions have been made to other sites including Lake Magadi, for training in ringing waders, and Ngulia. The Group has a core of seven to ten members and a further 30 have taken part to varying degrees. Over 2 500 birds have been ringed so far and much learned about the identification, ageing and moult of the commoner species.     

Comparing population trend estimates of migratory birds from breeding censuses and capture data at a spring migration bottleneck

Europe has a well‐established network of breeding bird monitoring that is used to produce supranational indices of population trends for many species. However, a comparison of breeding bird censuses with other methods may be beneficial to confirm the validity of such indices. The aim of this study was to assess the value of standardized capture data of migratory birds at migration bottlenecks as an indicator of the effective breeding populations. One limitation to this method is that several populations are co‐occurring at these bottlenecks and their catchment areas need to be clearly identified to allow extrapolation of population indices. Here, we used standardized trends in capture numbers of 30 species on the island of Ponza, a migration bottleneck in the central Mediterranean, and compared them to population trends estimated in the putative catchment breeding areas between 2005 and 2016. The catchment areas were identified through the analysis of ring recoveries during the breeding season of birds passing through Ponza. Our results show an agreement between the population trends observed on Ponza and those in the breeding areas in 15 out of 30 species. The correlations were strongest in species with a more robust definition of the catchment areas, that is, species with more than 10 recoveries, and for which the recoveries were most likely of breeding birds. The main reason for disagreement between the two indices in the remaining species might be related to different intensity of sampling in different areas. This issue can be solved by further developing monitoring projects in underrepresented countries, as well as by intensifying monitoring through ringing, both in the breeding grounds and at migration bottlenecks. These results show that spring migration monitoring at bottlenecks has the potential to provide a valuable complement and an independent control of breeding bird surveys, allowing raising early warnings of population declines and contributing to their conservation.     

Managing an invasive predator pre-adapted to a pulsed resource: a model of stoat (Mustela erminea) irruptions in New Zealand beech forests

The stoat (Mustela erminea) is a specialist predator that evolved to exploit the unstable populations of northern voles and lemmings. It was introduced to New Zealand, where it is pre-adapted to respond with a population irruption to the resource pulses that follow a heavy seedfall of southern beech (Nothofagus spp.). Culling stoats during an irruption is necessary to reduce damaging predation on nesting endemic birds. Culling might not reduce the stoat population long term, however, if high natural mortality exceeds culling mortality in peak years. During other phases of the beech-mast cycle, culling might have a greater effect on a smaller stoat population, whether or not damage prevention is critical. We developed a 4-matrix model to predict the effects of culling on λ, the annual rate of change in the size of the stoat population, through the four annual phases of an average masting cycle, explicitly distinguishing between apparent and real culling. In the Post-seedfall phase of the cycle, large numbers of stoats are killed, but little of this extra mortality is additive; in other phases, culling removes larger proportions of smaller total numbers of stoats that would otherwise have lived. Culling throughout all phases is most effective at reducing stoat populations, but is also the most expensive option. Culling in Post-seedfall plus Seed or Crash years is somewhat less effective but better than culling in one phase only. Culling has different short-term effects on stoat age distribution depending on the phase of the cycle when culling begins.     

BIRD MIGRATION OVER THE MALTESE ISLANDS

The present paper is based on data obtained during several years' observations and three recent surveys. Little has previously been published on migration through the Maltese Islands. The geographical, vegetational and climatic factors of the islands are discussed in so far as they affect the migrants. Visible migration is seen with anticyclonic weather and westerly winds. Birds are found grounded after night migration in cyclonic weather with southwesterly or easterly winds, much larger numbers and variety being seen with the latter. When there is a deterioration in the weather during the night, a large influx of birds is seen on the following morning, and in addition large flocks of migrating Turtle Doves are seen. Several trans-Saharan migrants may pass in smaller numbers during autumn than spring, but the difference may be more apparent than real because in early autumn the birds may depart after only a very short stay, and a few conspicuous species are absent or scarce. By contrast several species which winter north of the Sahara pass only or in much larger numbers during autumn, and these more than make up for those which are absent or rare. There is no evidence from bird ringing that in spring Malta regularly gets birds from Tunisia, at any rate from that part covered by the ringing stations (Cap Bon, Enfidaville, Gabes). The migrants which pass through Malta probably originate from an area in North Africa around Tripoli and some way westwards of it. During autumn the bulk of recoveries is from eastern European countries with a smaller percentage from northern and central Europe. Several species or groups of species are dealt with individually. In the discussion stress is laid on the very close relationship between migration and weather, especially the wind component. The comparatively small numbers of birds seen at Malta probably form part of a larger movement travelling on a broad front. It is argued that the large “falls” of migrants in bad weather result from drift acting on a mass passing mainly to one side or the other of the islands. Since much larger densities are seen with easterly than with westerly winds, it follows, if the hypothesis of drift be correct, that the numbers of birds travelling to the east of Malta are larger than those to the west of it. Moreover, since day migrants are seen with westerly winds and the bulk of night migrants with easterly ones, it is inferred that day migrants normally pass to the west, and the bulk of night migrants to the east, of the Maltese Islands. A parallel is drawn between the autumn migration and the performance of racing pigeons which are flown from the north and NE at this season.     

Revised estimate of numbers of wintering Red-throated Divers Gavia stellata in Great Britain

Capsule An estimated 17 000 Red-throated Divers winter around Great Britain.

Aim To produce an up-to-date assessment of the numbers of Red-throated Divers wintering around Great Britain.

Methods A revised Great Britain (GB) wintering population estimate for Red-throated Divers was compiled using data primarily from systematic line transect surveys by aircraft over marine nearshore areas conducted during 2001–06, supplemented with county bird records and Wetland Bird Survey (WeBS) counts, both from 1995 to 2005. Population estimates for each survey area around GB were calculated as the mean of either annual means (aerial surveys) or annual maxima (WeBS, county bird records) of all data collected within January and February of each year. These were summed to give a revised GB wintering population estimate.

Results A total of 17116 (13198–21034, 95% confidence interval) Red-throated Divers were estimated to winter around GB. Distribution was uneven. By far the greatest numbers were found off southeast and east Britain (59.3% of the total was between Flamborough Head, Yorkshire, and Dungeness, Kent), with large concentrations off the English south coast (10.9%), north Wales and Liverpool Bay (9.8%), and eastern Scotland (6.1%).

Conclusion This is a minimum plausible population estimate and the true population size may be larger. The new 1% of population threshold for statutory conservation work is 170 birds.     

A RINGING STUDY OF MIGRATORY BARN SWALLOWS IN WEST MALAYSIA

The Barn Swallow is a non-breeding winter visitor to West Malaysia (Malaya), abundant in season, by day feeding aerially over a wide range of habitats and by night normally roosting gregariously in trees, reed-beds or on service wires in towns. Records of ringed birds have demonstrated that those reaching Malaya breed in the Palaearctic region from 108°E eastwards between 37° and 51°N. Recoveries south of the breeding range suggested that migrating birds may follow either a continental route or a more easterly track through the Philippines and Borneo. Counts at roost sites in a reed-bed and in towns demonstrated a seasonal increase in numbers from late July to a peak in November, followed by a decline of about 20% to a level maintained until mid-February when departure commenced. Most birds had left by early May, but a few lingered and possibly overlapped with the first returning migrants in June. There was no evidence that any individuals remained in Malaya through the nuptial period. Repeats during winter at three regularly sampled urban roosts indicated that many birds on passage were present until November and again in late March–early April; from December to February the winter population was relatively stable and comparatively sedentary. Although the distances between towns were small in relation to the demonstrated foraging range of Barn Swallows, only 17% of 1,955 repeats of ringed birds represented a shift in roost site. Most shifts were towards the centrally situated and most populous roost of the three; interchanges between the outer pair of towns were few. A complete moult occurred on the wintering grounds, during which young of the year acquired adult plumage. Replacement of the primaries extended virtually throughout the moulting period, at an average rate of 2.4 feathers per month in the proximal part of the tract and 1.3 feathers per month in the distal part. Adults on average moulted slightly earlier than juveniles, but there was a wide scatter in timing between individuals of both age groups. There was no evidence that the initiation of moult was related to the dates of post-nuptial migration. The date of departure on prenuptial migration, however, was normally delayed until primary moult was complete. Large weight gains in March and April occurred only in swallows which had completed the moult. At this period the mean weight of birds in fresh plumage was about 30% above the lowest winter mean, and was significantly higher than that of contemporary samples of birds in which moult was continuing. In final samples in late April and early May mean weights showed a decline, indicating that late birds departed with reduced deposits of metabolic reserves. The gonads of adults of both sexes among passage and arriving birds in July and August had largely completed post-nuptial regeneration, and subsequently remained quiescent. Preliminary stages of recrudescence were observed in females from February onwards, and in males from March. Recrudescence was most advance in specimens which had completed the moult, but did not approach breeding condition in any bird before departure. Returning birds tended to be conservative in their choice of winter roost. Among 1,276 records, 82% were recaptured in the town of original ringing. Again shifts towards the centrally situated roost were more numerous than between the peripheral pair. The frequency of returns varied significantly with the month of ringing, being higher for December-March, lower for July-November and April-May. Survival rates, calculated from returns after one and two breeding seasons, indicated an annual mortality of 60–72%, higher among juveniles than adults. Comparison of results of successive years suggested that unfavourable conditions in 1967 resulted in lower survival of juveniles in particular than in 1966. There was no evidence of mortality at the roost sites, and it is argued that heavy losses probably occur during the migratory journeys.     

Population and Harvest Dynamics of Midcontinent Sandhill Cranes

Sandhill cranes (Antigone canadensis) inhabiting the midcontinent of North America have been hunted since the 1960s under management goals of maintaining abundance, retaining geographic distribution, and maximizing sustainable harvest. Some biologists have raised concerns regarding harvest sustainability because sandhill cranes have lower reproductive rates than other game birds. We summarized demographic information in an age-structured matrix model to better understand population dynamics and harvest. Population indices and recovered harvest since the early 1980s suggest midcontinent sandhill cranes have experienced an average long-term annual growth of 0.9%; meanwhile, harvest has increased 1.8% annually. Adult survival and recruitment rates estimated from field data required modest adjustments (1–3%) so that model-derived growth rates matched growth estimated from a long-term survey (0.887 adult survival and 0.199 females/breeding female). Considering 0.9% long-term annual growth, sandhill cranes could be harvested at a rate of 6.6% if harvest was additive to natural mortality (assumed to be 0.05) or 11.3% if harvest mortality compensated for natural mortality. Life-history characteristics for long-lived organisms and demographic evidence suggested that hunter harvest was primarily additive. Differential harvest rates of segments of sandhill cranes in the midcontinent population derived from differential exposure to hunting suggested potentially unsustainable harvest for greater sandhill cranes (A. c. tabida) from 2 breeding segments. Overall, demographic evidence suggests that the harvest of sandhill cranes in the midcontinent population has been managed sustainably. Monitoring activities that reduce nuisance variation and estimate vital and harvest rates by subspecies would support continued management of sandhill cranes that are of interest to hunters and bird watchers. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.     

How many juvenile Teal Anas crecca reach the wintering grounds? Flyway-scale survival rate inferred from wing age-ratios

Autumn postnuptial migration is critical in the dabbling duck annual cycle, when first-year birds in particular suffer high losses to natural and hunting mortality. Mortality rates in this age-class are generally unknown in Europe where winter ringing predominates. We used data from large-scale wing collections from hunters in Finland, Denmark and France to test the prediction that juvenile proportions among killed Teal (Anas crecca) would decline with distance along the flyway. As expected, this proportion decreased from 89% in Northern Finland to 58% in Western France. Potential biases linked with age determination from the wings, differential migration of age-classes, relative susceptibility to different forms hunting and gradual improvement of juvenile survival as they learn to avoid hunters could not explain the observed decline of juveniles in the shot population. This pattern was therefore considered to be genuine, the result of the cumulative depletion of first-years along the flyway, likely through hunting. On this assumption, combined with known adult monthly survival rates during August–November (94.2%), monthly juvenile survival rate was estimated at 52.8%, i.e. 14.7% (range 13.9–15.4% based on extreme values of adult survival) amongst Scandinavian juveniles reaching wintering quarters in Western France. Despite lack of precision in such estimates based on relative proportions, there is little doubt about the magnitude of autumn juvenile mortality and its consequences for the population dynamics of Teal. Lack of correlations between annual proportions of juveniles in the hunting bag and an index of Teal breeding success in Finland may result from such high and variable inter-annual mortality.     

Urban bird declines and the fear of cats

The role of domestic cats Felis catus in the troubling, on-going decline of many urban bird populations in the UK is controversial. Debate, in the UK and elsewhere, has centred on the level of avian mortality directly imposed by cats, and on whether this is principally compensatory (the 'doomed surplus' hypothesis) or additive (the 'hapless survivor' hypothesis). However, it is well established that predators also have indirect, sub-lethal effects on their prey where life-history responses to predation risk affect birth and death rates. Here, using a simple model combining cat predation on birds with a sub-lethal (fear) effect of cat density on bird fecundity, we show that these sub-lethal effects may be substantial for urban songbirds. When cat densities are as high as has been recorded in the UK, and even when predation mortality is low (e.g. <1%), a small reduction in fecundity due to sub-lethal effects (e.g. <1 offspring year⁻¹ cat⁻¹) can result in marked decreases in bird abundances (up to 95%). Thus, low predation rates in urban areas do not necessarily equate with a correspondingly low impact of cats on birds. Sub-lethal effects may depress bird populations to such an extent that low predation rates simply reflect low prey numbers.     

Effects of vegetation, corridor width and regional land use on early successional birds on powerline corridors

Powerline rights-of-way (ROWs) often provide habitat for early successional bird species that have suffered long-term population declines in eastern North America. To determine how the abundance of shrubland birds varies with habitat within ROW corridors and with land use patterns surrounding corridors, we ran Poisson regression models on data from 93 plots on ROWs and compared regression coefficients. We also determined nest success rates on a 1-km stretch of ROW. Seven species of shrubland birds were common in powerline corridors. However, the nest success rates for prairie warbler (Dendroica discolor) and field sparrow (Spizella pusilla) were <21%, which is too low to compensate for estimated annual mortality. Some shrubland bird species were more abundant on narrower ROWs or at sites with lower vegetation or particular types of vegetation, indicating that vegetation management could be refined to favor species of high conservation priority. Also, several species were more abundant in ROWs traversing unfragmented forest than those near residential areas or farmland, indicating that corridors in heavily forested regions may provide better habitat for these species. In the area where we monitored nests, brood parasitism by brown-headed cowbirds (Molothrus ater) occurred more frequently close to a residential area. Although ROWs support dense populations of shrubland birds, those in more heavily developed landscapes may constitute sink habitat. ROWs in extensive forests may contribute more to sustaining populations of early successional birds, and thus may be the best targets for habitat management.     

Large and irregular population fluctuations in migratory Pacific (Calidris alpina pacifica) and Atlantic (C. a. hudsonica) dunlins are driven by density-dependence and climatic factors

Understanding the forces driving population dynamics is critical for species conservation and population management. For migratory birds, factors that regulate population abundance could come from effects experienced on breeding areas, wintering grounds, or during migration. We compiled survey data for Pacific and Atlantic subspecies of dunlins (Calidris alpina pacifica and C. a. hudsonica) from range-wide Christmas bird counts (1975–2010), and investigated the influences on this population index of density-dependence, falcon numbers, a set of seasonal environmental conditions during breeding, migration and non-breeding periods, and large-scale meteorological measures. For both sub-species, numbers fluctuated irregularly, varying threefold over the survey period, with no long-term upward or downward trend. Based on Royama's general model framework, the change in numbers between successive years for both sub-species was negatively affected by the total count in the previous year (i.e., negative density-dependence) and by the eastward component of storm movement during fall migration, with slower motion associated with higher population growth. The remaining environmental factors differed between the sub-species (snowmelt date on the Pacific, temperature on the Atlantic) or acted in opposite directions (soil moisture). The directional effects of each of these factors are consistent with the biology of dunlin, and together they explain 67.4 (72.9 %) of the variation in the rate of change of Pacific (Atlantic) dunlin annual counts. Falcon numbers were not predictive, despite a tenfold increase in abundance, suggesting compensatory mortality. This study highlights directions for future studies, and provides a model for the analysis of other migratory species.