Twee bijzondere broedgevallen bij de Torenvalken Falco tinnunculus van het Lauwersmeer: een dwerglegsel en uitgesteld broeden

In addition to previous findings of runt clutches of Kestrels in the Lauwersmeer area in the northern Netherlands (in 2006 and 2008), another clutch with small eggs was recorded in 2011. It was found in the same nestbox where a 6-year old female had produced two runt clutches (first and repeat) in 2008 (but normal clutches in 2009 and 2010). Four out of seven eggs were <75% of mean egg volume (hence runts by definition); the other three <80% of mean egg volume (means based on data from Lauwersmeer and the literature). The female was not captured, but may have been the same as the one in 2006 and 2008 given the rarity of runt clutches. If so, mean egg volumes for the runt clutches had slightly increased over time, i.e. 7.5 cm3 (39.9% of overall mean) in 2006, 9.1 cm3 (46.6%) in 2008, and 14.6 cm3 (72.2%) in 2011. Delayed incubation was expected in a Kestrel pair in 2016: the first nestbox check on 14 April held a female but no eggs yet, with 3 fresh eggs on 6 May and a completed clutch of 5 eggs on 31 May. Start of laying must have been on 1-2 May. Hatching was therefore expected between 28 May and 1 June, but a nest visit on 22 June showed 5 chicks of which the oldest was only 7 days of age (based on wing length: 34 mm, instead of the expected 140-145 mm). The discrepancy of two weeks is thought to have been the result of a delay in the onset of incubation, possibly caused by a cold wave (perhaps in combination with poor rodent supply).