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

ABSTRACT.-Interspecific kleptoparasitism (food stealing) occurs in many seabird species and can sometimes significantly affect host individuals and populations. We investigated effects of kleptoparasitism by Glaucous-winged Gulls (Larus glaucescens) on a population of Tufted Puffins (Fratercula cirrhata) in which almost all chicks die before fledging in some years. Rates of kleptoparasitism were estimated during two seasons and compared with several spatial and temporal factors that have been predicted to influence kleptoparasitism rates both within and among seasons. Kleptoparasitism rates increased from 2% in the first year to 19% in the second, whereas puffin fledging success declined precipitously from 81% in the first year to 11% in the second. Within years, kleptoparasitism rates increased with number of puffins returning with fish, decreased during fair weather, and were unaffected by tidal height. Spatially, those rates increased with puffin burrow density and declined with angle of the nesting slope. Gull kleptoparasitism success increased during foul weather and at higher puffin nesting elevations. Despite those associations, survival of puffin chicks was not influenced by kleptoparasitism activity near their nests although it increased with a combination of low elevation and high slope at the nesting burrow. Neither kleptoparasitism nor predation by gulls were exceptional relative to other seabird colonies and they were unlikely to cause reproductive failures that characterize puffins at that site. Other factors, particularly food shortages at sea, merit further attention in explaining those failures. Received 12 November 1999, accepted 27 March 2001.

INTERSPECIFIC KLEPTOPARASITISM is relatively common in seabirds (Brockmannn and Barnard 1979, Furness 1987), but frequency and effects of that behavior vary both among and within species. Gulls, with their opportunistic foraging strategies, are frequent kleptoparasites (Duffy 1982, Barnard and Thompson 1985), and puffins, which nest in highly concentrated colonies and return at predictable intervals conspicuously carrying their prey, epitomize suitable kleptoparasitism hosts (sensu Brockmann and Barnard 1979). Thus, kleptoparasitism by gulls (Family Laridae) and closely related skuas (Family Stercorariidae) has been well-studied in Atlantic puffins (Fratercula arctica), but effect of that behavior on puffin reproductive success varies. Nettleship (1972) reported that piracy, and associated avoidance of gulls by puffins, can severely limit puffin food deliveries and, ultimately, reproductive success. However, other researchers suggested that kleptoparasitism by gulls only exacerbates extrinsic food shortages (Harris 1984, Furness 1987), found that it affects puffins only where they nest in low densities (Harris 1980), or discounted its effect on puffin populations altogether (Evans 1975, Pierotti 1983, Hudson 1985, Rice 1985). For some seabirds, predation by gulls on host eggs or chicks may impose a more significant limitation on reproductive success (Gilchrist et al. 1994, Becker 1995, Castilla 1995) and that has resulted in culling programs at several seabird colonies (Thomas 1972, Blokpoel and Spaans 1991). However, those programs have sometimes been initiated in the absence of a causal link between gull behavior and host population success (Harris and Wanless 1997).

Despite considerable work on kleptoparasitic effects on Atlantic Puffins, almost nothing is known of the interactions between gulls and the Pacific-dwelling Tufted Puffin (Fratercula cirrhata), for which there is some indirect evidence of a negative effect by gulls. Populations of Glaucous-winged Gulls (Larus glaucescens) have increased several-fold in recent decades along the southwestern coast of British Columbia (Verbeek 1986, Reid 1988) and increasing gull populations may generally destabilize kleptoparasitic (Furness 1987) and predatory (Russell and Montevecchi 1996) relationships. Concurrently, the population of Tufted Puffins on the provincial Ecological Reserve of Triangle Island, British Columbia (50 deg 52'N, 129 deg 05'W), the stronghold of that species in Canada (Vermeer 1979), has exhibited almost complete reproductive failures in more than half of its recent breeding seasons (Vermeer et al. 1979, Rodway et al. 1990, C. C. St. Clair pers. obs.). Those failures may be linked to periodic scarcity of sandlance (Ammodytes hexapterus), a favoured prey of Tufted Puffins (Vermeer et al. 1979). However, several other factors, including kleptoparasitism and predation by Glaucous-- winged Gulls, may also be important (G. Gilchrist and I. Jones pers. comm.). Although gulls frequently attack puffins on Triangle Island (Vermeer 1979), there has so far been no attempt to quantify extent or effect of that behavior.

Studies of other kleptoparasitic relationships suggest that population-level effects of kleptoparasitism are mediated through a variety of factors that influence both rate and success of individual kleptoparasitic events. From those studies, several predictions can be made. First, gull activity may track number of puffins returning with fish to equalize per puffin risk of being attacked (Rice 1987). That correlation may also occur because gulls follow diel cycles of puffin activity attacking most often in the morning when puffin returns are likely to be highest (Vermeer 1979). Second, gulls may not track puffin activity well (Hudson 1985), and may instead be affected by meteorological or tidal conditions that influence gull foraging success directly (e.g. Furness 1987, Belisle and Giroux 1995), or by stormy weather when surface feeding is difficult. Third, poor visibility may also make it easier for gulls to ambush incoming puffins (Furness 1987). Finally, gull activity and success may vary with nesting-slope topography, increasing on shallower slopes where puffins are more vulnerable (Nettleship 1972, Wilson 1993), and either decreasing (Grant 1971) or increasing (Furness 1978) at higher elevations according to puffin escape tactics. We examined relationship between Glaucous-winged Gulls and Tufted Puffins nesting on Triangle Island to (1) quantify frequency and nature of kleptoparasitism on adults and predation on chicks, and compare those to other kleptoparasitic relationships; (2) identify factors responsible for variation in rates and success of kleptoparasitism; and (3) assess potential effect of kleptoparasitism on puffin reproductive success by measuring chick survival.