Broedgedrag van Wespendieven Pernis apivorus onderzocht door temperatuurmeting in de nestkom

The breeding strategy of European Honey-buzzards was studied by inserting a thermistor probe in the nest cup (Tinytag and Squirrel used, with an accuracy of 0.1 °C and 0.05°C respectively). Temperature in the nest cup and ambient temperature were recorded simultaneously, in 1997 each 30-45 seconds (3 June-8 July), in 1998 each 4 minutes (minimal temperature in nest cup) or 7 minutes (ambient temperature) (29 May-20 June), and in 1999 each 8 minutes (29 May-8 July). The sensor was put in the central part of the nest cup directly underneath the eggs. As the sensor was not in contact with the brood-patch, or slightly hidden underneath leaves, the recorded temperatures do not reflect real egg temperatures. However, significant changes in temperature are indicative of nest attentiveness. Installing probes and changing data logs took 40-60 minutes per visit, including the tree climb. On days in 1997 when data logs were replaced, substantial drops in nest cup temperatures were recorded, indicating prolonged intervals of non-brooding (Fig. 1, Table 1). It took 1.5 hours after installment of the probe on 3 June before normal brooding temperatures were regained (Fig. 1a); during this period very high temperatures were occasionally measured, presumably because the unprotected eggs were sunlit (hyperthermia). Even longer intervals of non-brooding were recorded after nest visits to swap data logs on 17 June (5.5 hours; Fig. 1b) and 27 June (>5 hours; Fig. 1c). In between, several other irregular brooding intervals were visible, the causes of which remained unknown. On 29 May 1998, when the probe was installed, the nest cup was lined but eggs were not yet laid. In the afternoon of June 2nd, nest cup temperature started to increase for the first time, indicating onset of laying and incubation (Fig. 2). A long non-brooding interval of 3.5 hours was last recorded in the afternoon of 3 June (Fig 2a). From then on, few extremes in temperatures were recorded; following such intervals, temperatures in the nest cup took 1-2 hours to recover from hypothermia (Table 1). Nest temperatures in 1999 showed few variations, apart from irregularities during the first 8 days (start of incubation) and drops in temperature of short duration (15-30 minutes each). The 30 minute-drop in temperature on 5 July around midnight may have coincided with the hatching process (Fig. 3). Despite sometimes prolonged intervals of absence of the parents (up to almost 6 hours), resulting in eggs being subjected to hypothermia and – less often – hyperthermia, all eggs hatched and all chicks fledged. Nevertheless, it is suggested to restrict the number and duration of nest visits as much as possible.