Autumn migration of Ospreys from two distinct populations in Poland reveals partial migratory divide

Dariusz Anderwald,Łukasz Czajka,Sławomir Rubacha,Michał Zygmunt,Paweł Mirski

doi:10.1186/s40657-021-00281-6

Abstract

BackgroundLong-term ringing and telemetry studies show that the Osprey (Pandion haliaetus) is a broad-front migrant following different migratory flyways, depending on the geographical location of their breeding populations. We have investigated two distinct and declining populations of Osprey in Poland, separated by only a few hundred kilometres, and hypothesised they may exhibit two different migration routes. We followed mortality causes, comparing them between migration and stationary phases of annual cycle, as well as between two distinct populations.MethodsNineteen Ospreys, both juveniles and adults, were equipped with GPS loggers in 2017–2020 in two populations in western and eastern Poland and followed on their autumn migration. We calculated the distance they covered on the migration, number of stopover days, migration duration, daily distances covered and departure dates to compare them between age and sex classes and between the eastern and western populations.ResultsOspreys from the western and eastern populations showed a partial migratory divide. While the first migrated through a western flyway, the second followed a central flyway, resulting in crossing the Mediterranean Sea in distant passes that affected the distance covered. Annual mortality reached at least 67% in juveniles and at least 57% in adults.ConclusionsWe showed that two distinct Osprey populations in Poland revealed a partial migratory divide, with one covering greater distances over sea and deserts over the central flyway. This might affect individual survival rates and contribute to a steeper decline in one of the populations. In order for this to be confirmed, more individuals still have to be followed.

Highlights

Long-term ringing and telemetry studies show that the Osprey (Pandion haliaetus) is a broad-front migrant following different migratory flyways, depending on the geographical location of their breeding populations
Migrations are considered the most dangerous part of the annual cycle and mortality at that time may exceed that of the breeding and wintering periods even six-fold (Klaassen et al 2014)
Migration routes Out of 19 Ospreys tagged with GPS tags, we recorded the autumn migration of 12:5 adults (1 for 2 seasons) and 7 juveniles (Table 1)

Summary

Introduction

Long-term ringing and telemetry studies show that the Osprey (Pandion haliaetus) is a broad-front migrant following different migratory flyways, depending on the geographical location of their breeding populations. We followed mortality causes, comparing them between migration and stationary phases of annual cycle, as well as between two distinct populations. Migrations are considered the most dangerous part of the annual cycle and mortality at that time may exceed that of the breeding and wintering periods even six-fold (Klaassen et al 2014). A great body of studies have shown that Ospreys can migrate in broad-front. A study by Østnes et al (2019) can serve as a good example of such a flexible migration pattern, where juveniles dispersed in different directions and crossed the Mediterranean Sea at known bottlenecks and through open sea. Individuals from western Europe (United Kingdom, Germany, Norway and Sweden) were shown to most frequently use (but not exclusively) the western-most Mediterranean passage through the Iberian Peninsula (Alerstam et al 2006; Klaassen et al 2011; Mackrill 2017; Meyburg et al 2018)

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