Are Western European ospreys (Pandion haliaetus) shortening their migration distances? Evidence from trends of the wintering population in the Iberian Peninsula

Abstract

Here, we provide evidence that the number of Western European ospreys wintering in the Iberian Peninsula has been increasing over nearly two decades due to a reduction of the migration distances. We compared trends in wintering and breeding populations of ospreys in the Iberian Peninsula and western Europe, respectively, and we provide a detailed description of the present distribution and the numbers of ospreys wintering in the Iberian Peninsula. Observations of the species were collected as a citizen science project in January 2017. Based on the long-term data series from the Andalusian region (2004–2016) and from the Bay of Cadiz site (2000–2016), we estimated temporal trends in the population size of the ospreys wintering in these areas. Trends in the western European breeding population were derived from counts of ospreys migrating over the Strait of Gibraltar conducted by volunteers (1999–2016). All the trends were estimated by fitting a linear regression to the logarithm of the annual counts. For quantifying the origin of ospreys wintering in the Iberian Peninsula, we collected 204 confirmed field sightings of wintering ospreys in Spain and 155 in Portugal. We showed that the number of wintering ospreys has been increasing in southern Spain over the last 16 years. The magnitude of this increase is similar to the rate of change observed in the ospreys breeding across Western Europe. Recoveries of ringed birds in the Iberian Peninsula during winter indicate a reduction in the migration distances of Central and Northern European ospreys, making these birds winter at higher latitudes more than before. According to our results, this reduction in the migration distance was fairly uniform among different breeding populations in western Europe, but it did not affect all age classes equally, with juvenile birds more prone to winter at higher latitudes compared to adult birds. Our results showed that the overall number of ospreys which are shortening their migration distance, now over 3% of the total breeding population estimated for western Europe, is on an upward trend.