Ongava’s social spiders and Gabar goshawks

Although Gabar Goshawks are the commonest birds of prey (or raptor) on Ongava, finding their nests is not that easy. And each time we find one, questions get raised about their nesting habits. Like most small raptors, their nests are built of sticks arranged in rather scruffy platforms in the upper branches of trees. Clutches of 2 or 3 eggs are laid in shallow bowl-like depressions in the nests, and that is where the chicks are raised until they can fly. The most extraordinary thing about Gabars is their addition of cobwebs of social spiders to their nests. These tiny spiders of the genus Stegodyphus build their own conspicuous nests on the margins or tops of trees. Spiders are carried to the Gabar nests with the silk, and they soon multiply and spin cobwebs that cover the stick platforms.

Two major questions: what do the Gabars gain by adding the cobwebs, and do they intentionally add live spiders? Four hypotheses have been suggested:

1. the cobwebs are used to line the nest so that the eggs rest on soft material,

2. the silky cobwebs help to strengthen the nests so they are less likely to fall apart during strong wind,

3. the cobwebs help to disguise or hide the nests,

4. and the cobwebs trap insects that may bother the nestlings, such as blowflies, mosquitoes, and biting ants.

The first explanation would not require that live spiders be added to the nests, while the other three would only be viable if spiders were present to add silk to their new-found homes. These hypotheses were described some 31 years ago (Henschel JR, Mendelsohn JM & Simmons R, 1991). To our knowledge no one has tested them or added new explanations. Support for the first, ‘nest lining’ idea comes from occasional observations of Dark and Pale Chanting goshawks also lining their nests with wads of social spider cobwebs. The two goshawks are closely related to Gabars, suggesting that they share some evolved predilection for padding their nests with soft linings. And Gabars also use other soft material to line their nests, such as wool, the nests of penduline tits, feathers, rags and lichen.

A typical Stegodyphus nest on a branch at the top of a mopane on Ongava (left). The spiders are colourful but less than 4 millimetres in length (right).

Adult female Gabar incubating on a nest partly covered by Stegodyphus cobwebs.

A Gabar chick barely a week old in the same nest in Ongava, showing the padding of Stegodyphus nests.

Reference: Henschel JR, Mendelsohn JM & Simmons R. 1991. Is the association between Gabar Goshawks and social spiders Stegodyphus mutualism or theft? Gabar 6: 57-60. Read the article here.

Photos: John Mendelsohn and Angus Middleton