Moonlight avoidance behavior in Leach's Storm-Petrels as a defense against Slaty-backed Gulls

-Diurnal activity patterns of Leach's Storm-Petrels (Oceanodroma leucorhoa) and Slaty-backed Gulls (Larus schistisagus) were investigated. The petrels reduced activity in moonlight in May and June when the predation rate by gulls was relatively high. Petrel activity levels were inversely correlated with light intensities and the corresponding risk of predation by the gull. This suggests that nocturnal activity and moonlight avoidance by the petrel in its colony are an effective defense against diurnal predators. Activity synchronization of the petrel was most marked during the full moon, further supporting the predatoravoidance hypothesis. Received 15 October 1984, accepted 27 April 1985. MANX Shearwater (Puffinus puffinus; Harris 1966), Leach's Storm-Petrel (Oceanodroma leucorhoa; Harris 1974), Fork-tailed Storm-Petrel (0. furcata; Harris 1974, Boersma et al. 1980), and Cassin's Auklet (Ptychoramphus aleuticus; Thoresen 1964, Manuwal 1974) are strictly nocturnal in their colonies and are less active on moonlit nights than on dark nights. Cody (1973) discussed the nocturnal activity of alcids as a defense against diurnal predators. Gross (1935), M. P. Harris (1966), and S. W. Harris (1974) also suggested that nocturnal procellariiforms are vulnerable to diurnal predatory gulls on moonlit nights. However, the relationship between daily activities of procellariiforms and predation risk has not been studied. I studied Leach's Storm-Petrels and Slatybacked Gulls (Larus schistisagus) on Daikoku Island. Their activity patterns are described and the nocturnal behavior of the petrels in the colony is discussed as predator avoidance. STUDY AREA AND METHODS The colony. -The study was conducted on Daikoku Island (42? 52'N, 144? 52'E), Akkeshi, Hokkaido, between late April and early October 1982. The island is 6.1 km in circumference and treeless, with the exception of birch (Betula ermanii) groves in the ravines. Leach's Storm-Petrel (the only petrel breeding on the island) nests in the interior parts of the island, which is covered with Artemisa montana and Urtica platyphilla. Abe et al. (1972) estimated that there were 1,070,000 breeding pairs of petrels, but a more recent estimate is 415,000 (Watanuki 1985b). About 3,500 pairs of Slaty-backed Gulls nested on maritime slopes, which are covered with Calamagrostis langsdorffii, isolated rock stacks, and cliff ledges. A few pairs of Blacktailed Gulls (L. crassirostris) nested on these sites. The Slaty-backed Gull is an important predator of adult petrels on the island. Although Jungle Crows (Corvus macrorhynchos) excavated petrel burrows and ate adults, eggs, and chicks, predation by the small crow population (13 pairs) was not significant. Activity patterns. -Observations of flying birds were made from a blind set on top of a headland about 25 m above sea level. For 5 min every 30 min, I counted all birds passing an imaginary 20 x 30-m plane oriented vertically with reference to the cliff face opposite the headland. Two 6-volt electric lights, one set horizontally and the other about 450 upward at the blind, lit the plane facing toward the sea from the lower corner to the opposite side. This reduced the effect of the light on landing birds approaching from the sea. If all the landing petrels were attracted instantaneously to the lights, the number of petrels flying through the lights would increase during the observations. However, the number was rather constant during 5-min observations (Fig. 1). A few petrels and gulls flew circularly in the lights; these were excluded from the data. Observations on moonlit nights showed that Slaty-backed Gulls did not avoid the lights. No differential response to the lights by gulls and petrels was assumed. Observations started in daylight and lasted 24 h (n = 19 days). Data taken during a day with dense fog (13-14 May) were excluded from the analysis because of low visibility. Data of 27-28 April were included in those of May. Light intensity was measured by a lux-meter (? 10%) set horizontally and was divided into four classes: dark (0 lux), moonlight with no cloud cover ( 5 x 104 lux). Time of sunrise, sunset, moonrise, and moonset were from the astronomical tables for Kushiro, about 40 km west of Daikoku Island. 14 The Auk 103: 14-22. January 1986 This content downloaded from 207.46.13.75 on Fri, 08 Jul 2016 05:57:15 UTC All use subject to http://about.jstor. org/terms January 1986] Moonlight Avoidance in Leach's Petrel 15