De bijdrage van tweede-kalenderjaars-vrouwtjes aan het broedsucces van Torenvalken Falco tinnunculus rond Zwolle

In 1999, some 70 nest boxes for Eurasian Kestrels were supplied in a study plot of 15,000 ha near Zwolle (central Netherlands, 52°31’N, 6°06’E), of which 20% is urban, 40% suburban and industrial and 40% mainly grassland. Nest boxes were checked regularly, to obtain data on clutch size, brood size and breeding success, and to measure, weigh and ring chicks and capture adults. The latter were aged by means of rings (when ringed as nestling) or by means of plumage characteristics (2 calender year birds – mainly females in this study, as they were captured in the nest box – show a diagnostic moult and ditto pattern of outermost rectrices; birds older than 2cy were lumped in the category >2cy). The local population slightly increased in 1998-2005, but much of this increase could be contributed to outbreaks of Common Vole Microtus arvalis in 2000 and 2004. In 2002-05, the proportion of 2cy-females varied between 25.0 and 36.8% (on average 31.7%). Onset of laying in 2cy-females was 1-20 days later than in >2cy-females (on average 5.5 days), and mean clutch size varied between 5.4-5.9 in >2cy-females and 4.7-5.4 in 2cy-females annually (Table 2). However, losses of eggs and chicks were highest in >2cy-females (on average 32.6%), and much lower in 2cy-females (13.2%). Overall, 2cy-females produced 32.7% of the total reproductive output, in line with their proportion in the local population (31.7%). The reasons for this atypical breeding performance (normally, 2cy-females show a poorer breeding success than older females) are not known. It is speculated that adult females occupy the better territories (in terms of food supply and nest sites), hence are more often involved in territorial conflicts (resulting in egg losses). Also, twice the mates of >2cy-females were bigamous, in both cases resulting in the loss of one of the two nests. And finally, the synchronous peak of fledging in Starlings Sturnus vulgaris is more profitable to be exploited by late-laying Kestrels (i.e. 2cy-females) than by early breeding Kestrels; this is particularly important in years with a poor vole supply.