A selection from Stockholm University publication database

• Bird attacks on a butterfly with marginal eyespots and the role of prey concealment against the background

  Martin Olofsson, Jakobsson Sven, Christer Wiklund.

  Small eyespots on butterflies have long been thought to deflect attacks and birds are presumptive drivers selecting for these patterns. However, evidence of this function is still ambiguous. Marginal eyespots typically consist of a UV-reflective white pupil, surrounded by one black and one yellowish ring. We have recently shown that blue tits attack such eyespots, but only under low light intensities with accentuated UV-levels. An increased salience of the eyespots relative to the rest of the butterfly probably explains this result. Possibly, a background against which the butterfly is concealed may deceive birds to making similar errors. We therefore presented speckled wood butterflies provided with eyespots (or controls without eyespots) to blue tits against two backgrounds, oak- and birch bark. Results show that (i) eyespots, independent of background, were effective in deflecting attacks, (ii) the time elapsed between a bird’s landing and attack was interactively dependent on background and whether the butterfly bore an eyespot and (iii) the speed at which a butterfly was attacked predicted the outcome, with faster birds being more prone to errors than slower birds. This underscores a speed-accuracy tradeoff in the predators and that background plays a role in the defensive qualities of marginal eyespots.

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• Information from the geomagnetic field induce changes in corticosterone secretion in a migratory bird

  Ian Henshaw (et al.).

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• No compensatory fuelling due to late autumn migration in the Garden Warbler Sylvia borin

  Robert Stach (et al.).

  Birds migrating late in the migration season may need to compensate for the late departure by increasing migration speed. To increase migration speed late migrants should depart from stopovers along the route with larger fuel loads than early migrants. Both higher migration speeds and increasing fuel loads with the progress of the season have been reported in the literature. Here we test if Garden Warblers (Sylvia borin) show different fuelling strategies when captured on migration in the early or late part of autumn migration and given unlimited access to food. We also included a group of birds that were captured early in the season but held under a light regime with shorter day lengths to simulate thirty days advancement in time. We found no difference in maximum body mass between the groups and all groups reached fairly large fuel loads (mean: 39.2 % of lean body mass). Maximum fuel load was also strongly correlated with fuel deposition rate and this may suggest that Garden Warblers migrate at high speed during the entire season, which leaves little room for increasing speed later in the season.

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Show all publications by Sven Jakobsson at Stockholm University