Body condition of long-distance migrant birds from the Eastern Baltic before and after a population decline

Abstract

According to standardized bird trapping for banding purposes the population trends of Wrynecks (Jynx torquilla Linnaeus, 1758), Lesser Whitethroats (Sylvia curruca Linnaeus, 1758), Tree Pipits (Anthus trivialis Linnaeus, 1758), and Red-backed Shrikes (Lanius collurio Linnaeus, 1758) over 45 years in the Courish Spit, Eastern Baltic, indicate two periods, an initial population growth and a subsequent significant decline. All results, including the already published data on the Barred Warblers, indicate that 5 species of long-distance trans-Saharan migrants experienced a significant decline in numbers since the mid-1970s. Ringing protocol included determination of age and taking biometric measurements, the wing length as a proxy of structural size, and body mass. During the period of population decline, neither wing length, nor body mass of birds changed. None of the indicators of population productivity (clutch size, average brood size, breeding success) did not change, either. Presumably this may mean that the reasons for the population decline of the species are related to factors affecting birds on migration route or in wintering areas, and not on their breeding grounds. It is known that in catastrophic droughts on the African continent, the quality of habitats deteriorates, and competition for food leads to an increase in the mortality rate of wintering birds.