Satellite tracking of the Short-toed Eagle migration
On our webpages we repeatedly raised the question of studying migration of birds of prey using satellite telemetry. This method has pluses and minuses compared to traditional ringing, colour marking etc. The review makes an attempt to reveal, at least partly, potential opportunities of the satellite telemetry on the example of studying migrations of the Short-toed Eagle.
Some researchers and birdwatchers oppose the direct installation of satellite transmitters on birds basing on ethic reasons. Their main argument: until the absence of negative impacts is proved the method cannot be used in researches. However, it is not an easy task to receive such proofs. It would require control observations of certain types of transmitters and certain bird species and further processing of results with the use of statistical analysis to minimize as much as possible the effect of random factors. Firstly, the sample of birds should be large enough for such processing, which means installation of a lot of transmitters, secondly we must conduct parallel observations of the control bird group without transmitters attached. But exactly the latter is impossible since nowadays there are no other effective methods designed for long-term tracking of birds in their natural environment. In addition, installation and monitoring of even a single transmitter is still fairly expensive, not to mention their dozens. This fact puts restrictions on the use of this method in favour of its opponents.
However, the value of this method is undoubted, as already mentioned in our reviews on this subject. We would like to demonstrate its effectiveness on the example of tracing migratory movements of the Short-toed Eagles in the researches, periodically held in different countries. In these studies, satellite transmitters were attached both to adults (Meyburg et al. 1998; Darawshi 2009) and under-yearlings of the Short-toed Eagles (Pavon et al. 2010). By now more data are obtained for autumn migration of the Short-toed Eagles from France (Meyburg et al. 1996; Meyburg et al. 1998), Israel (Darawshi 2009) and Spain (Pavon et al. 2010), and less data – for spring migration. The reasons of the latter are that the probability of signal loss increases with time due to wearing or damages of a transmitter’s harness, unanticipated failures in its work or the bird’s death. Apart from identification of the precise route of migration, the velocity of birds, their dependence on weather and other factors, there are also other questions can be answered with satellite tracking. The most intriguing of them: how do juveniles distribute on breeding areas when reach maturity?
Reproductive success for the Short-toed Eagle is now defined quite precisely due to long-term monitoring programs carried out in different countries. For instance, in France over the last eight years, since 2002, there have been recorded successful nest departures of 1,059 juveniles of Short-toed Eagles. At present we can say that the breeding success in Europe is averagely equal to 0.5-0.7 fledglings per a territorial pair (Ivanovsky 2002; Bakaloudis et al. 2005; Joubert 2010; Petretti 2008). Our studies conducted in the north of Ukraine for the last seven years in 10 breeding areas bring similar results. It is also known that the Short-toed Eagles reach the reproduction age when they are 3-5 years old. However, several issues still remain unsolved. In particular: how do juveniles distribute; whether this productivity of population enough for its stable existence or at the expense of this productivity the stability of other less successful populations is also supported; whether at these parameters the further expansion of the species range to the north and east is possible; what happens if breeding success decreases in a particular area; to what extent and for how long its decrease will not lead to the reduction in the population size. The answers are essential to know for providing timely measures for conservation of the species in some regions. Of course, ideally, all possible measures must be taken everywhere and always, but reality dictates its own terms: available resources should be directed in advance to the area where they are most needed, where the situation is about to be critical. Unfortunately, ringing and color marking, as practice shows, cannot give clear answers to these questions. Moreover, the application of these methods is very laborious and requires marking of a large number of birds, and this means increasing disturbance along with low probability of receiving recoveries. In addition, the color marking is also a subject to harsh criticism because of the significant artificial changes in visual perception of marked animals by other birds and humans.
The method of satellite telemetry is assigned to contribute to revealing mysteries of the population dynamics through attachment of transmitters to young birds. On the one hand, the probability of their death is higher than adults, i.e. the effectiveness of this expensive method is certainly reduced, but on the other hand the adaptability of young Short-toed Eagles, just left the nest, is far superior to that of their parents. In the Ukrainian Polesye, for example, after only 1-1.5 months since the moment of leaving the nest, they become completely independent and ready to travel to Africa. If we assume that the installed transmitter, despite all the design tricks, really has a negative impact on a bird life activity, it is more for adults than for juveniles, which are better able to adapt to it.
At present it is conducted the satellite tracking of several young Short-toed Eagles equipped with transmitters in 2008 (Pavon et al. 2010) and 2009 in Province of Alicante in the south-east of Spain, and two birds from the south of Italy (Basilicata). For the latter, continuously updated maps with records of migratory movements are available at the website of the Regional Park of Gallipoli Cognato. According to Italian colleagues the transmitters are designed for 3-4 years of work. Thus, there is a chance to get the first data on the territorial distribution of the Short-toed Eagle. By now we have received confirmation of some pre-existing assumptions: that, at least, Spanish Short-toed Eagles 1 year after birth may not return to Europe but stay summering in Africa, and also concerning the direction of migration of these birds from southern Italy.
Breeding sites of the Short-toed Eagles in Ukraine are located much further from the wintering grounds than of those birds for which satellite telemetry has been already applied. Therefore the results of tracking migrations of our birds would naturally lead to increased interest among researchers. Satellite telemetry will also help to answer the main question: if, year after year, the number of the fledged juvenile Short-toed Eagles in Polesye is declining how much time we will have at our disposal to identify the causes and improve the situation? Take into account it is not just a species at the top of the trophic pyramid and the last link of the food chain. It is a raptor, feeding on raptors: more sensitive indicator of the ecosystem state is hard to find…
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Bakaloudis D. E., Vlachos C. G., Holloway G. J. Nest spacing and breeding performance in Short-toed Eagle Circaetus gallicus in northeast Greece // British Trust for Ornithology, Bird Study. – 2005. – 52. P. 330–338.
Darawshi S. The ecology of the Short-toed Eagle (Circaetus gallicus) in the Judean Slopes, Israel: Graduate Thesis Research report to R.S.G – 2009. – 17 p.
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Meyburg B.-U., Meyburg C., Pacteau C. Migration automnale d’un Circaète Jean-le-Blanc Circaetus gallicus suivi par satellite // Alauda. – 1996. – 64 (3). P. 339-344.
Meyburg B.-U., Meyburg C., Barbraud J. C. Migration strategies of an adult Short-toed eagle Circaetus gallicus tracked by satellite // Alauda. – 1998. – 66 (1). P. 39-48.
Pavón D., Limiñana R., Urios V., Izquierdo A., Yáñez B., Ferrer M. & de la Vega A. Autumn migration of juvenile Short-toed Eagles Circaetus gallicus from southeastern Spain // Ardea. – 2010. – 98 (1). P. 113–117.
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