Seksratio van overwinterende Bruine Kiekendieven Circus aeruginosus in Nederland: enige overwegingen

Small numbers of Marsh Harriers have been wintering in The Netherlands for decades. At present, the only wintering site of importance is situated in the southwestern section of the country, where numbers increased to reach annually between 50 and 160 in the 2000s (Castelijns & Castelijns 2008). Sex and age identifications of Marsh Harriers wintering at this site (plus immediate surroundings) involved two separate datasets, i.e. observations made at roosts and during daytime. The age- and sex ratios found were compared to the records submitted to www.waameming.nl, a popular website where thousands of birders deposit their observations. To calculate a reliable sex ratio of wintering Marsh Harriers, we need to know how many age- and sex-specific records are submitted (as a proportion of the total number of records submitted), and whether these are correct and/or biased. Regarding the latter: many observers do not differentiate between juvenile and female (inputting one or the other), whereas sexing juveniles is difficult and often impossible. The proportion of unspecified Marsh Harriers (age nor sex) varied between 0% (daytime observations Zeeland), to 12% (roosts) and 61% (waameming.nl). These widely differing proportions already hint at problems with ageing and sexing: especially the categories ‘female’ and ‘juvenile’ are likely to imply a combination of females and juveniles (of either sex, but probably mostly females in the setting of winter in The Netherlands). Consequently, the sex ratio cannot simply be equated as males/males+females. Contrarily, to calculate the sex ratio, just using positively identified males and females cannot result in biologically meaningful statistics, because males are more likely to be identified (and reported, as adult and subadult males are scarce in wintertime) than females. Considering the identification hurdles, it seems that ages and sexes scored during roost counts cannot be used to calculate sex and age ratios (Table 2). Daytime observations are preferred for such calculations, because observation conditions are then so much better. Depending on whether ‘juveniles’ and/or ‘unknown’ are implicitly categorised as females or not, the resulting sex ratio (% male) then varies between 17.6% (juveniles = females) and 6.3-6.9% (juveniles and aged nor sexed birds = females). This sex ratio does credit to the impression of fieldworkers in The Netherlands that most wintering Marsh Harriers that are neither aged nor sexed are juveniles, and that most juveniles are females. This impression needs testing (photographs and captures).