Sunday 19 July 2009

The sunspot owners: Speckled Wood butterflies

Male Speckled Wood perched on his sunny spot
The Speckled Wood (Pararge aegeria) butterfly is largely a new millennium butterfly in East Yorkshire. Probably due to the warming climate in the British Isles, the range of this butterfly is gradually increasing north. I saw the first Speckled Wood in Hull in a disused cemetery with large trees and lush undergrowth in 2005. This is a woodland and hedgerow species, not often found feeding on flowers. Adults feed mainly on tree canopies on honeydew, a lovely name for the sugar-laden aphid and scale insect sticky secretions. The most obvious individuals are males, which defend sunlight spots on the woodland floor. They rest on exposed, sunny leaves and inspect any butterfly or insect that passes by. Other butterfly species are briefly checked before the males come back to their resting place, females are courted, and intruding Speckled Wood males that come down from the tree canopy are chased in a brief spiral flight. This territorial behaviour was investigated in a classic paper by Nick Davies in woodland near Oxford. He visited the woodland daily and carried out lots of simple experiments with Speckled Wood individuals marked with colour spots. This way he was able to describe in detail the movements of each individual butterfly. Individual males spent all day in a sunny spot in the woodland floor, fluttering around and perching in it, and following faithfully the sunny spot as it moved though the day on the woodland floor. One of the most interesting findings of this study is that every time an intruder entered a sunny spot, and was involved in a 'spiral flight' with the owner, the intruded ended up retreating to the tree canopy and the resident took its position in the sunny spot. In all the spiral flights observed, over 200, the resident always won regardless of his condition. Davies concluded that the spiral flight is not a 'fight' but a courteous exchange of pleasantries: "to put it anthropomorphically, the owner says 'I was first here and the intruder says 'Sorry, I didn't know there was anybody occupying the sunspot, I'll retreat back to the canopy'. The reason the intruder comes down into the canopy is that it doesn't know the spot is taken. Davies carried out some experiments in which he introduced a male into an already occupied sunspot without the resident male noticing (he remarks he failed frequently, as the resident male seems to be always on the lookout for intruders). In the five occasions he succeeded, the spiral flight escalated and was more lengthy than usual, as both males regarded themselves as the territory owners. Only a few seconds of residency were enough for them to consider themselves the spot owners. Males were competing for the sunny spots and the tree canopy males readily occupied empty spots. The reason for this competition is that females often visit these sunny spots and therefore, males defending a sunspot are more likely to mate. The males on the canopy do not defend a territory but patrol the trees up and down in search of the rarer females. 
Female Speckled Wood
Male perched on his sunny spot

No comments: