Seasonal survival rates and causes of mortality of Little Owls in Denmark (original) (raw)
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Oecologia, 2011
The little owl (Athene noctua) has declined significantly in many parts of Europe, including the Netherlands. To understand the demographic mechanisms underlying their decline, we analysed all available Dutch little owl ringing data. The data set spanned 35 years, and included more than 24,000 ringed owls, allowing detailed estimation of survival rates through multi-state capturerecapture modelling taking dispersal into account. We investigated geographical and temporal variation in agespecific survival rates and linked annual survival estimates to population growth rate in corresponding years, as well as to environmental covariates. The best model for estimating survival assumed time effects on both juvenile and adult survival rates, with average annual survival estimated at 0.258 (SE = 0.047) and 0.753 (SE = 0.019), respectively. Juvenile survival rates decreased with time whereas adult survival rates fluctuated regularly among years, low survival occurring about every 4 years. Years when the population declined were associated with low juvenile survival. More than 60% of the variation in juvenile survival was explained by the increase in road traffic intensity or in average temperature in spring, but these correlations rather reflect a gradual decrease in juvenile survival coinciding with longterm global change than direct causal effects. Surprisingly, vole dynamics did not explain the cyclic dynamics of adult survival rate. Instead, dry and cold years led to low adult survival rates. Low juvenile survival rates, that limit recruitment of first-year breeders, and the regular occurrence of years with poor adult survival, were the most important determinants of the population decline of the little owl.
European Journal of Wildlife Research, 2019
Anthropogenic mortality has a considerable impact on populations of long-lived species, such as raptors, which increasingly inhabit human-dominated landscapes. Here, we analyzed long-term mortality data for two rapidly declining owls, Little Owl Athene noctua and Barn Owl Tyto alba, in the Czech Republic. We evaluated relative mortality rates with respect to owl age, month of carcass recovery, and two time periods (before and after year 2000). We examined 961 mortality records (199 Little Owls and 762 Barn Owls) derived from six distinct database sources totally spanning the period of years 1934-2017 and the entire Czech Republic. Natural causes, entrapment in vertical hollow objects and drowning in liquid reservoirs (entrapment), and collision with vehicles accounted for the highest proportion of mortality cases in Little Owl, while collision with vehicles and entrapment represented the most important mortality sources in Barn Owl. Relative mortality rates in Little Owl caused by entrapment, non-vehicle collision, electrocution at power lines and confinement in buildings increased after the year 2000. In turn, the relative mortality rate due to collision with vehicles increased after 2000 in Barn Owl. Persecution, collision with vehicles, and entrapment accounted for higher relative mortality rates in first-year than adult Little Owls. In Barn Owls, higher relative mortality rates due to collision with vehicles and entrapment were detected in adult compared to first-year birds. Finally, relative mortality rates differed between age classes according to the month of carcass recovery for both species. For Little Owl, the highest relative mortality rates in first-year individuals were detected during July and September, whereas adult Little Owls suffered the highest relative mortality rates during March, November and December. In Barn Owls, the relative mortality rates of first-year individuals peaked in November and December, whereas adult birds suffered the highest relative mortality rate during July, January and February. This study strongly suggests that reducing the risk of anthropogenic mortality may be crucial to halt the decline of Little Owl and Barn Owl populations.
POPULATION DECLINE OF THE LITTLE OWL ATHENE NOCTUA SCOP. IN THE CZECH REPUBLIC
2008
A change of land use is often cited as a causal factor in the decline of many species of farmland birds. Populations of the Little Owl (Athene noctua Scop., 1769) have notably decreased throughout Europe in the last 60 years, including the Czech Republic. The aims of this study were to estimate the recent population trend of the Little Owl and to analyze the importance of altitude and grassland habitat within Little Owl territories. The population trend of the Little Owl in the Czech Republic has still decreasing tendency. The population density dropped from 0.33 breeding pairs (bps) 10 km -2 to 0.12 bps 10 km -2 in the first (1993)(1994)(1995) and second (1998)(1999) monitoring program, respectively. The decline is apparent also from results from last Little Owl monitoring program which were carried out in 2005-2006 on 35 study plots (4607 km 2 ). The average population density was estimated at 0.1 bps 10 km -2 . A distinct feature of these recent populations is that they occur in the places with relatively high local density (core areas) in comparison to the surroundings, which are unoccupied. At present, the Little Owl rarely breeds in natural tree cavities, but rather the majority of nesting sites are situated in human artifacts, especially within agricultural objects. Areas in which the Little Owl occurs have a significantly larger proportion of grasslands and are situated at lower altitudes. We suggest that the changes in agricultural landscape associated with disappearance of traditional farming management of grassland habitats, forceful pasturage and regular mowing were the main factors in this long-term population decline. The recent decrease of Little Owls could be also the consequence of the existence of small isolated populations in which mortality is not balanced by immigration from surrounding areas.
Breeding season food limitation drives population decline of the Little Owl Athene noctua in Denmark
Ibis, 2010
Many farmland bird species have declined markedly in Europe in recent decades because of changes in agricultural practice. The specific causes vary and are poorly known for many species. The Little Owl, which feeds extensively on large invertebrates and is strongly associated with the agricultural landscape, has declined over most of northwestern Europe, including Denmark. We investigated the likely reasons for the population decline in Denmark by identifying patterns of local extinction (scale, 5 · 5 km 2 ) and estimating demographic parameters affecting local survival, focusing on changes over time and their relationship to habitat characteristics. The distribution of the Little Owl in Denmark contracted considerably between 1972-74 and 1993-96. The extent of contraction varied across the country, and the only habitat correlate was that local disappearance was associated with smaller amounts of agricultural land. Analyses of ring recovery data suggested a constant annual adult survival rate of 61% from 1920 to 2002, which is similar to estimates from countries with stable populations. First-year annual survival rates were much lower than values previously reported. From the 1970s into the 21st century, the mean number of fledglings declined from around 3 to < 2 young per territory, but the decline in clutch size was considerably less. Reproductive parameters were higher closer to habitat types known to be important foraging habitats for Little Owls, and were also positively correlated with the amount of seasonally changing land cover (mostly farmland) within a 1-km radius around nests as well as temperatures before and during the breeding season. Experimental food supplementation to breeding pairs increased the proportion of eggs that resulted in fledged young from 27 to 79%, supporting the hypothesis that the main proximate reason underlying the ongoing population decline is reduced productivity induced by energetic constraints after egg-laying. Conservation efforts should target enhancement of food availability during the breeding season. Other farmland species dependent on large invertebrates are likely to share the problems that Little Owls face in modern agricultural landscapes.
Journal of Avian Biology, 2016
Recoveries of marked animals hold long‐term, large‐scale information on survival and causes of mortality, but are prone to bias towards dead recoveries and casualties in the range of presence of potential finders. Thus, accounting for circumstance‐related recovery probabilities is crucial in statistical approaches. For the little owl, a species of conservation concern in central Europe, raw ring recoveries suggested a strong human‐related impact on survival. We analysed the proportions of the main causes of death using a large sample of radio‐tracked birds as a reference. We compared ring recoveries in southern Germany collected 1950–2012 (n = 465 dead recoveries of 2007 recoveries of 30 623 ringed birds) with data from a radio‐tracking study in the same region 2009–2012 (n = 177 dead recoveries of 377 tagged individuals). Two assumptions of multi‐state ring recovery modelling were unrealistic. First, not all dispatched rings remained available to potential finders. Instead, 34% of ...
2010
The modern anthropized landscape is a major source of hazards for large animals such as raptors. Collisions with cables, vehicles and trains, as well as electrocution cause casualties, which may negatively impact populations. Yet, demographic studies of that impact remain scarce, which is an impediment to evidence-based conservation action. We studied the dynamics of an eagle owl (Bubo bubo) population in the northwestern Alps (Switzerland). We estimated, firstly, its demographic parameters using a Bayesian integrated population model; secondly, the frequency of different types of casualty through radiotracking. Thirdly, we investigated the effects of reductions of human-related mortality on population trends. The breeding population was small but remained fairly stable during 20 years, suggesting that it was apparently in a good shape. However, survival probabilities of all age classes were very low (60.61), productivity fairly good (0.93), and immigration very high (1.6 females per pair and year), indicating that the population operated as a sink. Half of the mortality was caused by infrastructure, with electrocution accounting for 24% of all fatalities. The elimination of electrocution would result in a strong population increase (17% annually). Under that scenario, immigration rate could decline by 60% and the population would still remain stable. Given that the supply of recruits from elsewhere is likely to continue, we can expect a rapid local population recovery if dangerous electric pylons are mitigated systematically. Our study demonstrates that detailed demographic analyses are necessary to diagnose problems occurring in populations and to identify efficient conservation actions.
Use, interpretation, and implications of demographic analyses of northern spotted owl populations
1996
Raphael et al. 103 or quality at the nest-site or landscape level. Methods have been proposed that utilize resighting data from radiotelemetry studies to assess relationships between survival rates and habitat (Conroy 1993). Although these methods are relatively new and untested in field studies, they appear to offer some promise for assessing relationships between habitat conditions and survival. DETERMINATION OF RECOVERY AND DELIST~G Delisting of endangered or threatened species by the U.S. Fish and Wildlife Service has generally been based on numeric goals for population abundances rather than on changes in basic demographic rates. A recent example is the delisting of the gray whale (Eschrichtius glaucus), now relatively common along the Pacific Coast after being nearly eradicated by over-harvest in the late 1800' s. However, demographic data could be used as a more reliable indicator of population trend and status. For example, the recovery plan for the desert tortoise (Gopherus agassizi) included a requirement that the population must exhibit a statistically significant upward trend or remain stationary for at least 25 years before delisting would be considered (USDI 1994). The draft recovery plan for the Northern Spotted Owl proposed the use of demographic data to assess recovery (USDI 1992a). INTERPRETATION OF EXISTING NORTHERN SPOTTED OWL DATA Bumham et al. (this volume) synthesized results from 11 large scale Spotted Owl demography studies in a meta-analysis and estimated an average rate of population decline of 4.5% per year (with a 95% confidence interval of 0.6% to 8.4%) after adjusting estimates of juvenile survival for permanent emigration. Based on this analysis, Bumham et al. (this volume) concluded that the finite rate of population change (X) was probably < 1 .O for the population of territorial females, based on the years of study (see Franklin et al. this volume). The most parsimonious statistical model in a meta-analysis of adult survival indicated adult female survival rates were declining over time (Burnham et al. this volume). Based on this finding, the rate of population decline was apparently accelerating during the period of study. Although the meta-analysis conducted by Burnham et al. (this volume) indicated an overall decline in survival rates of adult females, estimated rates of adult survival varied among individual study areas. The best capture-recapture models identified in the individual study areas indicated declining linear trends in adult survival in some areas, constant rates of adult survival in other areas, and non-linear trends in others. However, the individual studies were unable to achieve the same statistical power to detect underlying trends as the meta-analysis. MODEL-BASED VERSUS SURVEY-BASED ESTIMATES OF POPULATION TREND The conclusion by Burnham et al. (this volume) that the territorial owl population was de
Age distribution and longevity in a breeding population of Swainson’s Hawks, Buteo swainsoni
Journal of Ornithology, 2020
Age structure of breeding populations may be important in the evaluation of population health and may indicate when populations are susceptible to decline. However, few long-lived species have well understood age distributions. We investigated the breeding age distribution in a long-lived species, the Swainson's Hawk, in a population that was monitored for 40 years and whose population has grown fourfold over the course of the study. We observed 279 known-aged Swainson's Hawks 1315 times during our study. The average age of breeding Swainson's Hawks in the Butte Valley across all surveyed years was 8.3 ± 4.3 years. The oldest known-aged individual in our population was 26 years old and the average age at death (individuals presumed dead) was 9.2 ± 5.5 years old (n = 160). There was no difference in longevity between males (mean ± SE; 9.7 ± 0.6 years old, n = 85) and females (9.0 ± 0.7 years old, n = 69). The proportion of subadults breeding (n = 56) was related to population growth, with a greater proportion of subadults breeding when the growth rate was higher. Keywords Known age • Long-term • Population growth • Subadult Zusammenfassung Altersverteilung und Langlebigkeit in einer Brutpopulation des Präriebussards Buteo swainsoni Die Altersstruktur von Brutpopulationen kann wichtig für die Beurteilung der Populationsgesundheit und ein Indikator dafür sein, dass Populationen Gefahr laufen abzunehmen. Allerdings gibt es nur wenige langlebige Arten mit gut untersuchter Altersverteilung. Wir untersuchten die Altersverteilung brütender Tiere bei einer langlebigen Art, dem Präriebussard, in einer über 40 Jahre hinweg beobachteten Population, welche sich im Untersuchungszeitraum vervierfacht hat. Während unserer Studie sammelten wir 1315 Beobachtungen von 279 Präriebussarden bekannten Alters. Das Durchschnittsalter brütender Präriebussarde im Butte Valley (Kalifornien, USA) über alle Untersuchungsjahre hinweg lag bei 8.3 ± 4.3 Jahren. Das älteste Individuum mit bekanntem Alter in unserer Population war zum Zeitpunkt seines Todes 26 Jahre alt; das Durchschnittsalter zum Todeszeitpunkt (als tot angenommener Individuen) lag bei 9.2 ± 5.5 Jahren (n = 160). Es gab keinen Unterschied in der Langlebigkeit zwischen Männchen (Mittel ± Standardfehler; 9.7 ± 0.6 Jahre, n = 85) und Weibchen (9.0 ± 0.7 Jahre, n Communicated by O. Krüger.