Breeding success in Greater White-fronted Geese in 2009/10 – a progress report
by Kees Koffijberg
In the past decade, increasing efforts have been made to assess age ratios among flocks of wintering geese on the European continent. Today, especially for Greater White-fronted Geese Anser a. albifrons large samples are collected by a small team of dedicated volunteer observers throughout major staging areas in western Europe. During winter 2009/2010, more than 213,000 White-fronts were individually aged. Total flock size out of which these samples were taken is even higher, about 268,000 birds. This represents a good proportion (22%) of the overall population, which size was estimated at 1,200,000 birds during the last Goose Specialist Group meeting in Sweden, autumn 2009 (Fox et al. in press).
Overall, 14.6% first-year birds were counted. Throughout the wintering range small differences occurred (Figure 1). The most remarkable outlier is represented by the UK-wintering (European) White-fronts, that had 26.2% juveniles among their flocks. Even if this a small sample, it is known from previous years that flocks staying on the western fringe of the wintering range, like UK, usually consist of more juveniles than those observed at the core wintering sites in e.g. The Netherlands and Germany. Unfortunately, we do not have data from Belgium, that during the cold winter of 2009/2010 probably will have had larger numbers than usual, and also is expected to have a higher amount of juveniles in its wintering population. On the lower end of the range were samples from Mecklenburg-Vorpommern in NE-Germany, with 11.2% first-year birds. Many of the smaller differences between single sites will probably partly be the result of differences in flock sizes sampled. Usually, juvenile proportion decreases with flock size. Hence, large concentrations sampled in e.g. The Netherlands or the Lower Rhine area in Germany often show below-average age ratios compared to sites with smaller and more scattered flocks, that usually support a higher number of successful families and/or larger broods.
In many flocks, also brood size was assessed. However, these data are less well distributed over the wintering range since 58% of the sample of 3489 assessed broods originates from only two sites, both from the Dutch and German section of the Lower Rhine Area. Brood size ranged from 1-6 young per (successful) pair. Average brood size recorded was 1.67 young per pair. Most broods consisted of one (57%) or two (27%) young.
The amount of juveniles in 2009/10 was among the lowest recorded so far and the data fit well in the recent pattern of poor breeding seasons in White-fronts (Figure 2). Since the beginning of the 1990s reproductive output in Greater White-fronted Geese has declined considerably. Until 1991 on average 31% juveniles were observed. After 1991 on average only 20% were first-year, after 2000 even 17%.
Besides Greater White-fronted Geese, also a large number of Tundra Bean Geese Anser fabalis rossicus was aged in 2009/2010. In a sample of 59,154 individuals collected in Sweden, Germany and The Netherlands 8.9% juveniles were counted. However, there were large differences between the three countries with 5.2% in Sweden, 10.% in Germany (mainly Thüringen) and 15.1% in The Netherlands. This implicates that also Tundra Bean Geese show increasing juvenile percentages when going towards the western edge of the wintering range. However, extensive samples are not available from previous years, so it is unknown if this phenomenon is specific to 2009/2010 (a very cold winter) or does fit in a more general pattern. Similar to Greater White-fronted Goose, also reproduction in Tundra Bean tends to decline in recent years (based on data collected in The Netherlands).
Thanks to Y. & T. Albada, H. Castelijns, B. Coenen, F. Cottaar, H. Ernst, G. Gerritsen, T. Heinicke, F. Hustings, E. Klunder, C. Kowallik, J. Kramer, H. Kruckenberg, J. Laber, J. Mooij, R. Oosterhuis, L. Schilperoord, R. Steinbach, R. Strucker, D. Tanger, K. Veldkamp, B. Voslamber, J. Vreehen & Wildfowl & Wetlands Trust for spending many hours in the field and suffering the cold to assess juvenile percentages and brood sizes.