Does Kleptoparasitism by Glaucous-winged Gulls limit the reproductive success of Tufted Puffins?

Auk, The, Oct 2001 by St Clair, Colleen Cassady, St Clair, Robert C, Williams, Tony D

Good weather caused a decrease in kleptoparasitism frequency. That effect may have been mediated through improved visibility and foraging success experienced by surface-- feeding gulls under those conditions. Similarly, poor visibility increases kleptoparasitism rates in a raptor guild, presumably through its effects on host concentration and food availability (Temeles and Wellicome 1992). Gulls may also have avoided kleptoparasitism in good weather because good visibility made it more difficult to ambush returning puffins (Furness 1978) or because it was correlated with wind direction. Northwest winds that typically accompanied good weather on Triangle Island may have precluded hovering on the updraft of the southeast facing observation slope. Although we observed puffins on a northwest facing slope to test that idea, too few puffins returned to be able to compare gull activity.

When kleptoparasitism rates were tallied spatially, it appeared that gulls concentrated their activity where puffin burrow density-- and thus number of returning puffins-was greatest, an effect found also in Herring Gulls (Larus argentatus) and Atlantic Puffins (Hudson 1985). Kleptoparasitism rates also declined with increasing slope angle, as reported for other puffins (Nettleship 1972, Wilson 1993). That may have occurred because shallow slopes occurred at higher elevations where puffins had to fly farther over land to reach their burrows and taller grass there made it harder to see their burrows. Or it may be because puffins have more difficulty lifting off from shallow slopes once they have been attacked (Nettleship 1972). There was no evidence that elevation alone was important to kleptoparasitism frequency, although that may be important for aerial kleptoparasites where it affects chase distance to the sea (Furness 1978).

We found only weak predictors of kleptoparasitism success. Success increased during foul weather and at higher elevations. Although foul weather did not influence rates of kleptoparasitism, success may have been more likely then due to poorer visibility (sensu Furness 1978). Puffins, notable for their high wing-loading (Harris 1984), may have also had more difficulty maneuvering during foul weather when the wind tended to blow upslope (i.e. from the southeast). Gull success may have increased at higher elevations because it corresponded to a greater distance from the sea (making it more difficult for puffins to escape) or because it was correlated with shallower slopes that may favour gulls for a number of reasons (above).

Effect of kleptoparasitism on puffin fledging success.-Despite the association between heightened kleptoparasitism rates and lower chick survival in the second year, it did not appear that kleptoparasitism affected fledging success directly. Among attributes we measured, only inaccessibility was a significant determinant of puffin fledging success, conferring higher success at lower elevations with steeper slopes. Grid-specific kleptoparasitism rates did not influence fledging success and, to the extent that a difference existed at all, it was in the opposite direction; kleptoparasitism rates were higher at the grid squares with higher chick survival, presumably because food deliveries were also more frequent there. Height of the grass surrounding the burrows, which might help to conceal landing puffins, was similarly unimportant.