Biometrics of Razorbills Alca torda staying during the nonbreeding season in the Polish Baltic zone

Włodzimierz Meissner,Szymon Bzoma,Joanna Drozdowska

doi:10.34080/os.v23.22573

Abstract

Biometrical analysis of Razorbills Alca torda captured or drowned in fishing nets in the Polish economic zone of the Baltic Sea is based on measurements of 98 individuals collected in the non-breeding period (October–April) in 2006–2013. These birds are assumed to represent the breeding population of the Baltic Sea as recoveries show that very few other birds visit that area. Bill depth and wing length of juveniles were much smaller than in older birds which suggests that the final size of these measurements is not achieved during the first winter but continue to increase as the birds grow older. In older birds only total head length and bill length were significantly larger in males than in females. Mean bill depth of Razorbills from the Polish Baltic zone fits well to clinal variation from south-west to north of the European part of the breeding range. However, mean wing length was much larger than expected according to the regression, which might result from differences in measuring technique. On the other hand it cannot be excluded that the Baltic population may actually have longer wings.

Highlights

Remarkable variability in biometrics among Razorbills Alca torda has attracted attention of ornithologists at least since the 1940s resulting in several publications on geographical (e.g. Salomonsen 1944, Jones 1990, Barrett et al 1997) and sexual differences in linear measurements (e.g. Anker-Nilssen et al 1988, Jones 1988, Wagner 1999, Grecian et al 2003)
Data on wing length and bill depth of males and females analysed in this study were pooled for the analysis, i.e. fitting of the data to the geographical cline in size of Razorbills
Most adult Razorbills from the Baltic breeding colonies stay year around in the Baltic Sea and only some immatures move to Skagerrak, Kattegat and the North Sea (Bønløkke et al 2006, Fransson et al 2008)

Summary

Introduction

Remarkable variability in biometrics among Razorbills Alca torda has attracted attention of ornithologists at least since the 1940s resulting in several publications on geographical (e.g. Salomonsen 1944, Jones 1990, Barrett et al 1997) and sexual differences in linear measurements (e.g. Anker-Nilssen et al 1988, Jones 1988, Wagner 1999, Grecian et al 2003). Breeding populations of Razorbills in Europe are divided into two groups. The proportion of larger and smaller individuals was often used in assessing the origin of oiled birds collected after ship accidents (Jones et al 1985, Anker-Nilssen et al 1988, Camphuysen & Leopold 2004, Heubeck et al 2011). Clinal increase in size of Razorbills from the south-western towards north-eastern part of the breeding range is observed in Europe, with the exception of Baltic birds which appear to be much larger than would be expected from their geographical position (Barrett et al 1997). At least partly these data were taken from museum specimens and were corrected by adding a constant value of 2 mm to wing length and 1 mm to bill depth (Barrett et al 1997)

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