Raptor Migration Research Articles

Accounting for differential migration strategies between age groups to monitor raptor population dynamics in the eastern Black Sea flyway

Migration counts can offer a cost‐effective method for monitoring the state of migrant raptor populations. However, differential migration strategies between inexperienced juveniles and experienced non‐juveniles are rarely accounted for when inferring population trends from raptor migration counts. Since 2011, the Batumi Raptor Count (BRC) monitors the autumn migration of more than 1 million raptors along the eastern Black Sea coast in the Republic of Georgia. We also systematically sampled age information to assess differential migration timing between age groups and estimate age‐specific linear trends in abundance between 2011 and 2018 for eight focal species. In so doing we aimed (1) to reassess the global relevance of BRC counts for each species and the potential for monitoring abundance of juveniles and non‐juveniles, and (2) to identify demographic changes underlying recent trends in overall abundance. We found that the mean annual passage of non‐juveniles at Batumi represents at least 1% of the estimated global breeding population of five study species. As expected, counts of juveniles were more variable than counts of non‐juveniles. Yet despite our short monitoring period our models had sufficient statistical power to detect changes in abundance of 10%/year or less for at least one age group in all species except Pallid Harrier Circus macrourus. Our results indicate stable abundance and demography for half of the study species. We also found strong and significant increases in the abundance of Black Kites Milvis migrans and Short‐toed Eagles Circaetus gallicus that were primarily due to increasing numbers of non‐juveniles. By contrast, juvenile Montagu's Harriers Circus pygargus and Booted Eagles Hieraaetus pennatus significantly decreased in abundance. The first decade of BRC surveys offers an important benchmark for monitoring raptor populations using the eastern African–Palearctic flyway in the 21st century. We discuss possible causes of the observed trends and hope our work will stimulate demographic monitoring at migration count sites.

Hypatia-trackRadar: A software for animal tracking using marine surveillance radars

Hypatia-trackRadar is a Java standalone application designed to help biologists extract and process bird movement data from marine surveillance radars. This application integrates simultaneous collection of radar data and field observations by allowing the user to link information gathered from visual observers (such as bird species and flock size) to the radar echoes. A virtual transparent sheet positioned on the radar screen allows the user to visually follow and track the echoes on the radar screen. The application translates the position of the echoes on the screen in a metric coordinate system. Based on time and spatial position of the echoes the software automatically calculates multiple flight parameters, such as ground speed, track length and duration. We validated Hypatia-trackRadar using an unmanned aerial vehicle. Here we present the features of this application software and its first use in a real case study in a raptor migration bottle-neck.

Batumi Raptor Count: autumn raptor migration count data from the Batumi bottleneck, Republic of Georgia.

One of the most important geographical bottlenecks for migrating raptors in the east African-Palearctic migration system is situated between the easternmost tip of the Black Sea and the Lesser Caucasus, just north of Batumi, in the Republic of Georgia. Since 2008, citizen scientists of the Batumi Raptor Count (BRC) have monitored the autumn raptor passage daily from mid-August until mid-October, collecting also detailed information about the age and sex of focal species. The full BRC dataset was recently made available through the Global Biodiversity Information Facility (GBIF). Here we describe how count data were collected, managed, and processed for trend analysis over the past 10 years. This dataset offers a unique baseline for monitoring the state of migrant raptor populations in the east African-Palearctic flyway in the 21st century. We discuss potential pitfalls for users and hope that the open access publication of our data will stimulate flyway-scale and continent-wide collaboration for raptor migration monitoring in the Old World.

Flight feather moult in Western Marsh Harriers during autumn migration

BackgroundMost of long-distance migratory raptors suspend moult during migration but detailed information is patchy for most of the Palearctic species. The aim of this research is to verify if active moulting in migrating Western Marsh Harriers occurs and to quantify the extent of moulting along the season focusing on primary feathers.MethodsDuring a whole post-breeding migration at the Strait of Messina in Southern Italy, we gathered information about symmetrical flight feather moult from 221 adults by taking pictures of raptors passing at close range.ResultsWe found active moulting primaries during autumn migration in 48.4% of our samples. Slight differences on the extension and timing among sex classes were recorded during the season, with adult females showing a more advanced moult stage than adult males.ConclusionThe finding that the extension of the suspended moult was already defined in migratory individuals might be explained as an adaptation to minimize the energy required for moulting during migration.

Feather mercury concentrations in North American raptors sampled at migration monitoring stations.

We assessed total mercury (THg) concentrations in breast feathers of diurnal North American raptors collected at migration monitoring stations. For 9 species in the Pacific Flyway, we found species and age influenced feather THg concentrations whereas sex did not. Feather THg concentrations µg/g dry weight (dw) averaged (least squares mean ± standard error) higher for raptors that generally consume > 75% avian prey (sharp-shinned hawk Accipiter striatus: n = 113; 4.35 ± 0.45 µg/g dw, peregrine falcon Falco peregrinus: n = 12; 3.93 ± 1.11 µg/g dw, Cooper's hawk Accipiter cooperii: n = 20; 2.35 ± 0.50 µg/g dw, and merlin Falco columbarius: n = 59; 1.75 ± 0.28 µg/g dw) than for raptors that generally consume < 75% avian prey (northern harrier Circus hudsonius: n = 112; 0.75 ± 0.10 µg/g dw, red-tailed hawk Buteo jamaicensis: n = 109; 0.56 ± 0.06 µg/g dw, American kestrel Falco sparverius: n = 16; 0.57 ± 0.14 µg/g dw, prairie falcon Falco mexicanus: n = 10; 0.41 ± 0.13 µg/g dw) except for red-shouldered hawks Buteo lineatus: n = 10; 1.94 ± 0.61 µg/g dw. Feather THg concentrations spanning 13-years (2002-2014) in the Pacific Flyway differed among 3 species, where THg increased for juvenile northern harrier, decreased for adult red-tailed hawk, and showed no trend for adult sharp-shinned hawk. Mean feather THg concentrations in juvenile merlin were greater in the Mississippi Flyway (n = 56; 2.14 ± 0.18 µg/g dw) than those in the Pacific Flyway (n = 49; 1.15 ± 0.11 µg/g dw) and Intermountain Flyway (n = 23; 1.14 ± 0.16 µg/g dw), and Atlantic Flyway (n = 38; 1.75 ± 0.19 µg/g dw) averaged greater than the Pacific Flyway. Our results indicate that raptor migration monitoring stations provide a cost-effective sampling opportunity for biomonitoring environmental contaminants within and between distinct migration corridors and across time.

Messy eaters: Swabbing prey DNA from the exterior of inconspicuous predators when foraging cannot be observed.

Complex coevolutionary relationships among competitors, predators, and prey have shaped taxa diversity, life history strategies, and even the avian migratory patterns we see today. Consequently, accurate documentation of prey selection is often critical for understanding these ecological and evolutionary processes. Conventional diet study methods lack the ability to document the diet of inconspicuous or difficult‐to‐study predators, such as those with large home ranges and those that move vast distances over short amounts of time, leaving gaps in our knowledge of trophic interactions in many systems. Migratory raptors represent one such group of predators where detailed diet studies have been logistically challenging. To address knowledge gaps in the foraging ecology of migrant raptors and provide a broadly applicable tool for the study of enigmatic predators, we developed a minimally invasive method to collect dietary information by swabbing beaks and talons of raptors to collect trace prey DNA. Using previously published COI primers, we were able to isolate and reference gene sequences in an open‐access barcode database to identify prey to species. This method creates a novel avenue to use trace molecular evidence to study prey selection of migrating raptors and will ultimately lead to a better understanding of raptor migration ecology. In addition, this technique has broad applicability and can be used with any wildlife species where even trace amounts of prey debris remain on the exterior of the predator after feeding.

Migrating raptor counts: the need for sharing objectives and field protocols, and the benefits of using radar

ABSTRACTCapsule: Raptor migration attracts the interest for different reasons, but not all raptor counts achieve the goal of repeatability through the use of standardized field protocols, and this does not allow comparisons of data to be made across years and sites.Aims: We analysed migrating raptor count activities in Italy to verify the interest on this phenomenon by identifying organizers of such counts, and we ascertained the use of a minimum repeatable field protocol (MRFP), and the implications of using it or not. Moreover, we tested the use of radar to support field monitoring.Methods: We analysed 298 migrating raptor counts carried out between 1984 and 2016 by considering characteristics of raptor counts at migratory bottlenecks (number of years covered, use of MRFP, numbers of raptors counted, etc.). In addition, we analysed two case studies using radar to evaluate the effectiveness of raptor counts.Results: There is a growing interest in raptor migration, as well as an increasing use of a MRFP, although differences between counts emerged, probably due to the different aims of the promoting organizations. At sites not using MRFP, more raptors were counted than at other sites, probably because of a greater bias in the data collected. Radar is able to clarify the water-crossing behaviour of raptors at coastal sites, and allows the proportion of birds passing undetected by observers to be evaluated. It also provides data on the spatial density of migrants across a sample area.Conclusions: The use of MRFP is important to harmonize data collection and is necessary to allow comparisons across years and sites. Moreover, the use of radar can be recommended for optimizing raptor monitoring schemes.

&lt;b&gt;Mapping the conflict of raptor conservation and recreational shooting in the Batumi Bottleneck, Republic of Georgia&lt;/b&gt;

Illegal use of natural resources threatens biodiversity and often leads to conservation conflicts between affected parties. Such a conflict is emerging in the Batumi Bottleneck in the Republic of Georgia, where every autumn more than one million migrating birds of prey funnel above a handful of villages, and where thousands of these birds fall victim to illegal shooting. As a first step towards resolving this conflict, utilizing semi-structured interviews, we map the goals and opinions of relevant stakeholders associated with raptor migration in the bottleneck. Our results show that most stakeholders, except some local hunters, are on common ground considering the shooting unacceptable, but articulate different preferences concerning a solution, which hinged on institutional and enforcement issues. The hunters expressed a wide spectrum of responses concerning their involvement and motivation in raptor shooting, the role and importance of hunting in their lives, and preferred mitigation actions. The most urgent issues to be addressed via conservation actions are the wide-scale lack of awareness of the conflict, the potential loss of species, and the risk of conflict escalation.

Observations of Spring Raptor Migration at Palenque, Mexico

Observations of Spring Raptor Migration at Palenque, Mexico

Autumn Raptor Migration in Yellowstone National Park, 2011–2015

Raptors are wide-ranging, vagile avian predators whose populations can be difficult and costly to monitor on their breeding or winter range. However, monitoring raptors during their annual northbound or southbound migration is a cost-effective and efficient alternative to time-intensive, single-species breeding surveys. In 2010, we observed numerous Swainson’s Hawks (Buteo swainsoni) and Red-tailed Hawks (Buteo jamaicensis) migrating through the Hayden Valley in central Yellowstone National Park, prompting an investigation into raptor migration patterns in the park. Our objectives were to monitor annual autumn raptor migration in Hayden Valley from 2011 to 2015 and to determine the relative role of this undocumented migration site by comparing our observations to simultaneously collected migration data from three other sites in the Rocky Mountain Flyway. From 2011 to 2015, we observed 6441 raptors of 17 species across 170 d and 907 h of observation. Red-tailed Hawks, Swainson’s Hawks, and Golden Eagles (Aquila chrysaetos) accounted for 51% of the total individuals observed over five years. Overall counts from Hayden Valley were comparable to counts from the three migration sites in the Rocky Mountains, although abundance of individual species varied by site. Data from this study suggest that Hayden Valley may serve as a stopover site for migrating raptors and presents an opportunity for future research. By improving our understanding of where raptors migrate and the characteristics of stopover areas in the Rocky Mountains, land managers may develop effective strategies for protecting raptor populations and habitat from threats including development and climate change.

Morphological changes in American kestrels (Falco sparverius) at continental migration sites

Many American kestrel (Falco sparverius) populations are declining across North America. Potential causes include mortality from reduction in food availability, a changing climate, habitat degradation, an increase in avian predators, disease, and toxins. We analyzed American kestrel count and banding data from seven raptor migration sites throughout North America with at least 20 years of migration data. We used count data to determine the year at which the kestrel population began a significant decline and then used banding records to determine whether body mass and wing chord declined after this point. We found reductions in kestrel body mass at three sites and reductions in kestrel wing chord at five sites. Our results indicate declines in body size at the majority of sites are consistent with the hypotheses that food availability, impacts of a changing climate, or predation risk may be contributing to population declines.

Raptor migration in an oceanic flyway: wind and geography shape the migratory route of grey-faced buzzards in East Asia.

Flapping flight is relatively costly for soaring birds such as raptors. To avoid costly flight, migrating raptors generally avoid flying over water. As a result, all but one of the global raptor migration flyways are largely over land. The East Asian oceanic flyway for raptors is the exception. Raptor species using this flyway migrate by island-hopping, flying over open ocean for distances of up to 300 km between islands. We used satellite telemetry data for grey-faced buzzards Butastur indicus, a species that dominates the southern part of the flyway, to investigate the geographical and atmospheric factors responsible for the suitability of this flyway for raptor migration. Using a combination of least-cost path analysis and a step selection function, we found that the occurrence of numerous islands and also suitable wind support along the oceanic flyway are responsible for route selection in grey-faced buzzards. These results confirm the role of islands, but also wind, in shaping the East Asian oceanic flyway of long-distance raptor migration.

Invited Commentary: Fifty Years of Raptor Research

This review examines the main developments that have occurred over the past 50 years in our understanding of three aspects of raptor biology: (1) natural factors that limit breeding densities; (2) influences of toxic chemicals; and (3) movements and migrations. Early evidence indicated that raptor breeding densities were limited naturally by the availability of either prey or nest sites, whichever was in shortest supply in the area concerned. More recent evidence has shown that predation can have additional influence, with larger raptors and owls limiting the numbers of smaller ones to below what food or nest sites would permit. In addition, it has become apparent that some migratory raptors, like other migratory birds, can be limited in their migration and wintering areas to levels below those that conditions in breeding areas would permit. As many raptor populations have recovered from the effects of organochlorine pesticides, attention has switched to other limiting agents, including lead (from ammunition), which is currently preventing California Condors (Gymnogyps californianus) from establishing self-sustaining populations in the wild, and anti-inflammatory veterinary drugs, which have caused massive declines in Asia vultures (Gyps spp.). The development of radio-tracking enabled studies of the local movements of individual raptors, providing new information on territories and ranging behavior, while satellite-based tracking has revealed the migration routes, wintering areas, and behavior of hundreds of individual birds.

Increase in Numbers and Potential Phenological Adjustment of Ruby-throated Hummingbirds (Archilochus colubris) during Autumn Migration at Hawk Mountain Sanctuary, Eastern Pennsylvania, 1990–2014

ABSTRACT Global change can affect several aspects of bird biology, including population size and migration timing. We used count data collected during 25 years (1990–2014) at Hawk Mountain Sanctuary, a raptor migration watch-site in eastern Pennsylvania, to investigate population changes in Ruby-throated Hummingbirds (Archilochus colubris) and the timing of their autumn migration, in light of ongoing climate change. Hummingbird numbers increased significantly from 1990–2014. The first 5%-, 50%-, 95%- and average passage dates of hummingbirds over this time indicated an earlier passage, with the first 5% passage-date shifting earlier significantly. Passage duration (number of days between 5% and 95% of the flight) remained relatively constant from 1990–2014. In light of similar shifts in timing of spring passage of this species, our results suggest that Ruby-throated Hummingbirds may be shifting the timing of their migratory cycle.

Rare case of an adult male Montagu’s Harrier Circus pygargus over-summering in West Africa, as revealed by GPS tracking

Over-summering within or near the African wintering range by immature, non-breeding individuals occurs regularly in several species of long-distance migratory raptors, yet the extent of over-summering in Africa by adult birds remains unclear. In this study, we describe a case of an adult Montagu’s Harrier over-summering in Africa, as revealed by GPS tracking. By relating detailed knowledge of the bird’s movements to remotely sensed environmental data (normalized difference vegetation index, NDVI), we show that over-summering in this case was likely related to an exceptionally difficult breeding season the previous year rather than an effect of adverse weather conditions encountered during the winter or a failed attempt to migrate. Various factors are discussed as potential driving forces behind the bird’s intra-African movements. Finally, we relate the documented case to a large number of North European Montagu’s Harriers studied by telemetry and show that over-summering in Africa by adult individuals is indeed a rare event.

Species Composition, Timing, and Weather Correlates of Autumn Open-Water Crossings By Raptors Migrating Along the East-Asian Oceanic Flyway

Abstract Raptor migration rarely involves long-distance movements across open oceans. One exception occurs along the East-Asian Oceanic Flyway. We collected migration data at two terrestrial hawkwatch sites along this flyway to better understand open-ocean movements along this largely overwater corridor. At the northern end of the Philippines, at Basco on the island of Batan, we recorded 7587 migratory raptors in autumn 2014. Near the southern end of the Philippines, at Cape San Agustin on the island of Mindanao, we recorded 27,399 raptors migrating in autumn 2012. Chinese Sparrowhawks (Accipiter soloensis) were the most common raptors observed, making up approximately 89% and 92% of total records for Basco and Cape San Agustin, respectively. The Grey-faced Buzzard (Butastur indicus) was the second most common raptor migrant, accounting for 8% of the total counts at both watch sites. The migration period was about 1–2 wk earlier at Basco, the more northerly site, than at Cape San Agustin. Overwater flight...

Understanding the Relationship Between Northern Bobwhite Mortality and Raptor Migration in South Texas (Poster Abstract)

The fall and spring migration routes of numerous raptor species converge in the Rio Grande Plains ecoregion of Texas resulting in a high seasonal diversity and abundance of raptors. Raptors are believed to be an important source of mortality for northern bobwhite (Colinus virginianus). Because of the economic importance of bobwhites and the high concentration of raptors in south Texas, we investigated the relationship between bobwhite mortality and raptor abundance. Our objectives were to document raptor diversity and abundance, correlate bobwhite mortality with raptor abundance, and correlate ambient temperature with raptor abundance.We monitored radiomarked bobwhites (n = 164) biweekly during September–February 2000–01 in Brooks County, Texas. We conducted raptor surveys bimonthly during October–February 2000–01 between 1100 and 1500 hours along a 24-km road. We documented a total of 96 bobwhite mortalities. Raptors accounted for 16 % of the mortalities, with 43% by mammals, 13% unknown predator, and 28% hunter. We observed a total of 310 raptors, representing 15 identified species. Red-tailed hawk (Buteo jamaicensis) and white-tailed hawk (Buteo albicaudatus) comprised a large percentage (38%) of the raptors observed. A weak correlation (r = - 0.11) existed between raptor abundance and total bobwhite mortality. A stronger correlation (r = 0.86) existed between Accipter abundance and raptor depredation of bobwhites. We detected a relatively strong negative correlation (r = -0.65) between raptor abundance and ambient temperature. Our limited data suggest that general raptor abundance may not be a strong indicator of actual bobwhite mortality, and that it may be erroneous to infer that a high abundance of raptors results in a high mortality of bobwhites. Understanding the relationship between bobwhite and raptors will involve determining species-specific migration timing and numbers.

Annual Variation in Northern Bobwhite Survival and Raptor Migration

We estimated survival of radio-marked northern bobwhite (Colinus virginianus) on a managed prairie site in northeast Mississippi during 2 disparate winters (15 Sep-14 Apr 2000–2002). We retrospectively examined factors that may influence bobwhite survival. Pooled survival rates differed substantially between years (S 1⁄4 0.03 6 0.02 in 2000–2001 and S 1⁄4 0.36 6 0.16 in 2001–2002). Regional relative abundance of 3 species of raptors thought to be important predators of bobwhite was greater during 2000 compared to 2001 based on kriging of Christmas Bird Count (CBC) data. We demonstrate an approach for characterizing annual variation in spatial distribution of migratory raptors and suggest that annual variation in local winter predator context may be useful for explaining annual variation in winter survival of local bobwhite populations.

A comparison of migratory strategies of partial migratory raptors from Germany

The phenomenon of partial migration in birds in which some individuals of a population are migratory while others stay in the breeding area is of increasing scientific interest. The strategies of partial migratory raptors from Central Europe are, however, unclear for most species. We analysed ring re-encounter data of Eurasian Sparrowhawks Accipter nisus and Common Buzzards Buteo buteo ringed in Germany in terms of distances and directions between ringing and re-encounter sites. We investigated possible differences between sexes (only in Sparrowhawks) and age classes, as well as effects of ringing region, seasonal weather (in the form of North Atlantic Oscillation indices) and long-term temporal changes (including climate change) on migratory strategies by means of generalized linear models. We found that, in both species, migration is mostly conducted by juveniles, although migratory adults were also found. In general, male Sparrowhawks tend to migrate less than females. Sparrowhawks from different regions showed no differences in migration activity and no responses to long-term temporal changes. They did not respond to seasonal weather either. Buzzards showed strong responses to winter weather (‘winter escapes’) predominantly in highland regions, and a reduction of migratory intensity probably due to global warming. We compared our results to ones for Common Kestrels Falco tinnunculus and found most similarities between Kestrels and Sparrowhawks.

Short-toed snake eagles Circaetus gallicus (Gmelin, 1788) (Aves: Accipitridae) approaching a water barrier show reverse direction of migration

We investigated the directions of migration (reversed vs. expected) of raptors approaching a geographical strait in relation to local wind conditions, time of day, flock size and location of the observation post (coastal zone vs. inland zone). Fieldwork was conducted during autumn migration in 2011, 2012 and 2013 at a migratory bottleneck located in the southernmost part of the Italian Peninsula (Calabrian Apennines), using four watch points. In this area, migrating birds face the narrowest water surface between continental Italy and Sicily, the Strait of Messina. The only species showing substantial reverse migration was the short-toed snake eagle (Circaetus gallicus). In particular, eagles, mostly first calendar year (cy) birds, showed this behavior when passing closer to the coast (5 km inland of the Strait of Messina). Our results could reflect the reluctance of these birds to head south when approaching this relatively short stretch of sea, even before reaching the coastline. This behavior could be evidence of the strong selective pressure, which would have led to the evolution of the extremely detoured flight path of birds breeding in Italy.