Age-dependent migration strategy in honey buzzards Pernis apivorus tracked by satellite

Six adult and three juvenile honey buzzards Pernis apivorus were radio-tracked by satellite during autumn migration from southwestern Sweden. All adults crossed the Mediterranean Sea at the Strait of Gibraltar and continued across the Sahara desert to winter in West Africa, from Sierra Leone to Cameroon. Analysing three main steps of the migration, (1) from the breeding site to the southern Mediterranean region, (2) across the Sahara and (3) from the southern Sahara to the wintering sites, the adults changed direction significantly between these steps, and migrated along a distinct large-scale detour. In contrast, the juveniles travelled in more southerly directions, crossed the Mediterranean Sea at various places, but still ended up in the same wintering areas as the adults. Average speeds maintained on travelling days were similar for the two age groups, about 170 km/day in Europe, 270 km/day across Sahara and 125 km/day in Africa south of Sahara. However, as the adults used fewer stopover days en route, they maintained higher mean overall speeds and completed migration in a shorter time (42 days) than the juveniles (64 days). Although the juveniles set out on more direct courses towards the wintering grounds, they did not cover significantly shorter distances than the adults, as they tended to show a larger directional scatter between shorter flight segments. The results corroborate previous suggestions that adult and juvenile honey buzzards follow different routes during autumn migration, and that the birds change migration strategy during their lifetime. While juveniles may use individual vector orientation, social influences and learning may be of great importance for the detour migration of adults. The remarkable and distinct age-dependent shift in migratory route and orientation of the honey buzzard provides a challenging evolutionary problem

Can vector summation describe the orientation system of juvenile ospreys and honey buzzards? - An analysis of ring recoveries and satellite tracking

Juvenile bird migrants are generally believed to use a clock-and-compass migratory orientation strategy. According to such a strategy migrants accomplish their migration by flying a number of successive flight steps with direction and number of steps controlled by an endogenous programme. One powerful way of testing this is by comparing predictions from a model of such a strategy with observed patterns. We used data from ringing and satellite-based radio telemetry to investigate the orientation system of juvenile ospreys (Pandion haliaetus) and honey buzzards (Pernis apivorus) migrating from Sweden to tropical west Africa. The ring recoveries showed a much larger scatter in the orientation of ospreys than of honey buzzards, but there was only a slight such difference in the satellite tracks. These tracks of individuals of both species were rather straight with a high directional concentration per step. The honey buzzard. data showed a close fit to a simple vector summation model, which is expected if birds follow a clock-and-compass strategy. However, the osprey data did not fit such a simple model, as ring recoveries showed a significantly greater deviation at short distances than predicted on the basis of long distance data. Satellite tracking also indicated less concentrated orientation on short distances. The pattern observed for the osprey can generally be explained by an extended vector summation model, including an important element of pre-migration dispersal. The existence of extensive dispersal in the osprey stands in contrast to the apparent absence of such dispersal in the honey buzzard. The explanation for this difference between the species is unclear. The model of orientation by vector summation is very sensitive to the existence of differences in mean direction between individuals. Assuming such differences, as tentatively indicated by the satellite tracking data, makes simple compass orientation by vector summation inconsistent with the distribution of ring recoveries at long distances, with a high proportion of misoriented birds falling outside the normal winter range

Migration counts at Falsterbo in the autumn of 2006

Räkningar i Naturvårdsverkets regi bedrevs mellan 1 augusti och 20 november. Den övervägandemajoriteten av arterna räknades medan åldern bestämdes för rovfåglar och vissa andra grupper.Dominerande västvindar och god ungproduktion ligger sannolikt bakom den förhållandevis högatotalsumman. Flertalet rovfåglar räknades i relativt höga antal trots att ungfågelsandelen endastvar genomsnittlig. Uppgången fortsätter för flertalet ökande arter medan det finns indikationer påatt trenden vänt för en del fågelarter som minskat långsiktigt. Slående var den totala avsaknadenav invasionsarter

Differential migration in raptors

Differences between ages, sexes and populations in the timing of migration at Falsterbo, Sweden and in the choice of wintering site among raptors in Scania were investigated. Competition and dominance is the best primary explanation to the results. In most cases the more competitive adults winter north of juveniles and the larger females north of males. Moult strategy is probably one of the most important reasons why females in general migrate south earlier than males, and juveniles ahead of adults, in some short-distance migrants. The proportion of adults and juveniles in Swedish raptors leaving via Falsterbo is calculated. The concentration rate is generally highest in species with a more southerly distribution and in raptors primarily using thermal migration. With a few exceptions juveniles are more concentrated at Falsterbo than adults. An analysis of the long-term systematic migration counts from Falsterbo, shows that in all raptors where the Swedish population trends are reasonably well known from other studies, the fluctuation in numbers is very well reflected in the autumn figures from Falsterbo. The proportion of juvenile birds among the migrants gives a good measure of the annual reproduction in several species. By separating Common Buzzards with a white plumage (mainly with a relatively southerly breeding range) at Falsterbo it was possible to demonstrate some differences in migration pattern compared to darker and more northerly birds. Studies of three species of skuas on the Russian tundra demonstrated interesting differences in the proportion of dark phase. While no dark phase birds seem to occur in Long-tailed Skua the proportion was close to 8 % dark birds in Pomarine Skua all the way from Norway to Wrangel Island. This strongly indicates a gene flow all along the transect in this nomadic species. In Arctic Skua on the other hand there was an abrupt change from 64 % dark phase on the Kola Peninsula to an almost complete dominance of light birds all along the Russian tundra. Satellite transmitters constitute an invaluable tool enabling detailed studies of the migration strategies of individual birds. Thirteen Ospreys were tracked on autumn migration from Sweden to the wintering grounds in tropical Africa. The average speed for the whole journey was 174 km/day. This is similar to migration speeds in other raptors studied by satellite telemetry, but considerably faster than results from ringing recoveries. There was large individual variation with few major differences between sexes and ages. The birds made between 0 and 4 stopovers along the way. Migration took place only during the light hours and the Ospreys spent about nine hours in active flight on travelling days

Migration counts at Falsterbo in the autumn of 2002

BAKGRUND De standardiserade höst räk ning ar nai Na tur vårds ver kets regi påbörjades hösten1973 som en del i deras fågelmonitoring. Frånoch med 2001 ansvarar undertecknad för dessaräk ning ar. Från detta år påbörjas räkningarnaden 1 au gus ti (i stället för 11 augusti), främstför att er hål la en mer fullständig täckning avrov få gel sträck et. Vidare artbestäms lommar,fi sk/sil ver tär nor, och korsnäbbar, vilket ej skettti di ga re. I mån av tid åldersbestämdes storkar,sång sva nar, prutgäss, rovfåglar, vissa vadare,labbar, dvärg mås, silltrut och tärnor för atterhålla ett mått på årets ungproduktion.Åren 1986-2000 har höststräcket av rov fåg laröver halvön specialstuderats (Kjellén 2001 ochtidigare). Räkningarna har då skett på sam masett som höstarna 2001-2002. En jäm fö rel semellan denna serie och de standardiseraderäk ning ar na visar en signifi kant korrelationför samt li ga rovfåglar. Jag har därför valt att räkna upp de standardiserade siffrorna från1973-1985 med den genomsnittliga skillnadenunder femtonårsserien 1986-2000. Mer detaljerbe träf fan de omräkning av siffror för olika arterpre sen te ras av Kjellén (2002)

Migration counts at Falsterbo in the autumn of 2008

Räkningar i Naturvårdsverkets regi bedrevs mellan 1 augusti och 20 november. Den övervägande majoriteten av arterna räknades medan åldern bestämdes för rovfåglar och vissa andra grupper. Sammantaget en bra säsong där totalsumman ligger väl över genomsnittet. Uppgången fortsätter för flertalet långsiktigt ökande arter medan listan med signifikant minskande arter sakta krymper något. Även om den genomsnittliga ungfågelsandelen låg något under medel uppträdde flertalet rovfåglar talrikt, med exempelvis nya rekordnoteringar för röd glada och sparvhök. Lågtryckspassager i augusti gav höga antal av adulta vadare och tärnor. Flera tropikflyttande tättingar räknades i relativt höga antal och den milda senhösten borgade för goda siffror för flertalet kortflyttare. Bland invasionsarterna registrerades måttliga rörelser av främst större hackspett, sidensvans, stjärtmes, blåmes, nötkråka och gråsiska

Migration counts at Falsterbo in the autumn of 2008

Räkningar i Naturvårdsverkets regi bedrevs mellan 1 augusti och 20 november. Den övervägande majoriteten av arterna räknades medan åldern bestämdes för rovfåglar och vissa andra grupper. Sammantaget en bra säsong där totalsumman ligger väl över genomsnittet. Uppgången fortsätter för flertalet långsiktigt ökande arter medan listan med signifikant minskande arter sakta krymper något. Även om den genomsnittliga ungfågelsandelen låg något under medel uppträdde flertalet rovfåglar talrikt, med exempelvis nya rekordnoteringar för röd glada och sparvhök. Lågtryckspassager i augusti gav höga antal av adulta vadare och tärnor. Flera tropikflyttande tättingar räknades i relativt höga antal och den milda senhösten borgade för goda siffror för flertalet kortflyttare. Bland invasionsarterna registrerades måttliga rörelser av främst större hackspett, sidensvans, stjärtmes, blåmes, nötkråka och gråsiska

Sträckfågelräkningar vid Falsterbo hösten 2005

Räkningar i Naturvårdsverkets regi bedrevs mellan 1 augusti och 20 november. Den övervägandemajoriteten av arterna räknades medan åldern bestämdes för rovfåglar och vissa andra grupper.Dominerandevästvindar och god ungproduktion ligger sannolikt bakom den förhållandevis högatotalsumman. De flesta rovfåglarna räknades i relativt höga antal medan ungfågelsandelen varnågotlägre än 2004. Uppgången fortsätter för flertalet ökande arter medan det finns indikationerpå att trenden vänt för en del fågelarter som minskat långsiktigt. Relativt få invasionsarter passerade,men höga antal noterades för sidensvans, domherre och gråsiska

Population trends in Swedish raptors demonstrated by migration counts at Falsterbo, Sweden 1942-97

The autumn migration of raptors at Falsterbo, Sweden has been studied since the early 1940s, and from 1973 standardized counts were made. Here we present data for 15 species over a 39-year period from 1942-97. These are discussed in the context of available information on population trends in Sweden and neighbouring countries. Although annual numbers and concentration rate vary considerably between species, population changes are very well reflected in the migration figures from Falsterbo. Most raptors showed stable populations at a fairly high level during the 1940s, hilt a marked decline was already obvious in White-tailed Eagle Haliaeetus albicilla and Peregrine Falcon Falco peregrinus. During the 1950s and 1960s, a more or less steep decline occurred in most species. Four species started to increase during the 1960s, but the real change came during the 1970s. At that time, decreased human persecution and a reduction ill the effects from pesticides resulted ill a general increase in Scandinavian raptors, with only Honey Buzzard Pernis apivorus continuing to decrease. The increases continued during the 1980s, but in the 1990s many raptors seem to have reached stable numbers or to have started to decline again. Two species, Marsh Harrier Circus aeruginosus and Montagu's Harrier C. pygargus show a positive trend through the study period. Numbers of Northern Harrier Circus cyaneus, Rough-legged Buzzard Buteo lagopus and Eurasian Kestrel Falco tinnunculus stabilized during Me 1980s and show a clear decline since then, most probably due to a general lack of rodent peaks in Northern Scandinavia since 1982. Most species of raptors scent to he doing reasonably well at the moment, brit a continuous decline in Honey Blizzard and Common Buzzard Buteo buteo is disturbing, and is possibly due to declining proportions of old deciduous forest and grazed meadows in Scandinavia. Since a general census programme of birds of prey does not exist in Sweden, the migration counts at Falsterbo is the best general method of monitoring population changes