Video observations at nests of Honey-buzzards in Schleswig-Holstein

Abstract

We analyzed data from two nests, each with two nestlings aged about three weeks until after fledging. Videos (with audio at one nest) and some photos were taken by day and by night, with some gaps due to technical issues. All young fledged. Both females spent the nights on the nests until the nestlings left to adjacent branches. Females fed the young until they were 45 to 53 days old. Males continued to feed for another four to five days. After a further one to three days, the young came to the nests for the last time.

At both nests 77 % of the food items were wasp combs, the rest were frogs and nestling passerines, most of them thrushes, and a single bumblebee nest. The young obviously preferred wasp grubs and seemed to treat birds indifferently whereas, at least in one nest, they disliked frogs.

Males supplied most of the prey items, with 71 resp. 81 % of all food items, with frogs and birds being overrepresented in comparison to females. It is assumed that males tend to switch to alternative prey, when, during the summer, wasp nests become less available. Males that defend territories tend to stay closer to the nest whereas females, by extending their home ranges, can stick to wasp nests, their preferred prey.

The nestlings were remarkably good-natured toward their siblings. If they struggled in an attempt to secure a food item they concentrated on the object, not on their rival. A nestling took up a twig and rearranged it (nest building behaviour) for the first time at nearly four weeks old. Scratching in the nest bowl, an indication of digging behaviour, occurred at an age of 32 to 39 days. At an age of 37 to 41 days the nestlings visited branches adjacent to the nest. Later, when the young temporarily left the surroundings of the nest, the adults would melodiously call them to take over prey.

Three adults were repeatedly observed to probe deep in the nest bowl by day and by night, thereby turning the head, seemingly pursuing and eating insect larvae (wasp grubs, maggots, beetle larvae?) which they may have heard moving in the nest construction.

Gender roles may differ between pairs. The same is true for food provided for the nestlings. Its composition can be recorded reliably by video observation, whereas traditional methods, like sampling prey remains from the nest, may give biased results.