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

Effects of kleptoparasitism on puffin success.-As a measure of effect of gulls on puffins, we monitored the growth and fledging success of a sample of puffins from the observation grid. In 1995, we located an active burrow (i.e. one containing a chick, adult, or warm egg) early in the chick-rearing period as near as possible to each of 41 grid flags, marked its entrance with a small flag, dug an access hole to the nesting chamber and covered it with a wooden shingle and sod, and then returned every 10 days to record chick survival. In 1996, we repeated that procedure with 32 of our original burrows, replaced inactive burrows, and added a burrow at most grid flags for a total of 79 burrows. We considered puffin chicks to have fledged if they were at least 219 g with a wing length of 131 mm when they were last measured because this size exceeds the 95% confidence interval of 45 dead (presumably starved) chicks found on Puffin Rock in the 1995 season. In 1996, too few puffins fledged for meaningful comparisons, so we used measurements taken on 19 July as a cut-off date to compare chick survival. On that date roughly half the chicks had perished (the remainder were two to three weeks old), therefore using that cut-off point maximized the power of our statistical comparisons of chick survival.

Because slope topography and burrow position potentially influence kleptoparasitism rates and puffin fledging success, we quantified several slope attributes at grid flags and monitored burrows. At each grid flag, we measured the direction of the steepest downslope and then applied trigonometry to the vertical and horizontal distances between gridmarked points to determine elevation and slope-angle at each flag. Elevations were quantified relative to the highest point on the southwest end of the ridge. At each burrow, we measured distance and height difference (for elevation) to the nearest grid flag, height of the tallest hairgrass on each of four sides surrounding the entrance (a measure of exposure), slope (by triangulation over 10 m horizontal), and number of puffin burrows within a 5 m radius. Later we also tallied number of kleptoparasitism events referenced to each grid square as a more direct measure of effects of kleptoparasitism activity on puffin fledging success.

RESULTS

Frequency of kleptoparasitism and predation.Chases were proportionately more common than attacks in both years (Table 1; Likelihood ratio chi^sup 2^ > 6.0, df = 1, P 6.4, df = 1, P 6.7, df = 1, P 6.0, df = 1, P

Observations of marked gulls suggests that frequency and success of kleptoparasitism is highly variable among individuals. Number of events performed by each individual ranged from 0 to 87 (mean = 27.7 /- 33.7, n = 7) per year. Individual success rates ranged from 0 to 25% for chases (mean = 6.4 /- 10.1, n = 6) and 0 to 33% for attacks (mean = 14.3 /- 16.2, n = 6).

In addition to kleptoparasitizing adults, gulls also targeted puffin offspring by walking around on the nesting slope and probing in burrows. In 1995, we saw only two chicks consumed in that way. In the first case, a gull stabbed repeatedly at the ground above a burrow, tearing a hole in the burrow ceiling, then pulled out the live puffin chick from within and swallowed it whole. In the second case, we saw a gull eating a puffin chick, but could not tell if the chick was initially alive and where it had come from. In 1996, chick consumption by gulls was much more common, but it may have represented scavenging more than predation. We saw 28 puffin chicks being eaten by gulls; four of those were dead when discovered by the gull, one was alive and actively tried to escape, but we could not tell whether the others were initially alive or dead. Four chicks were pulled from their burrows, eight were already outside a burrow on the ground, and the others (16) were first observed in the bill of a gull. Other species may also prey on puffin offspring. One fledging puffin chick was apparently killed by a Peregrine Falcon (Falco peregrinus) (identifiable by the characteristic head and wing remains), we saw one egg dug out and eaten by a Common Raven (Corvus corax) and a giant race of deer mice (Peromyscus maniculatus triangulatus) or Townsend's vole (Microtus townsendi cov)ani) may have been responsible for the tooth marks and predated eggs we found in puffin burrows near the observation slope in 1996. In 1995, we saw gulls eat two other puffin eggs and one was taken directly from a burrow. On one occasion, we saw a gull apparently kill an adult puffin after grabbing it by the head and shaking it.