184 AUSTRALIAN Field Ornithology 2010, 27, 184–185 Silver Chroicocephalus novaehollandiae Harassing Foraging Yellow-billed Spoonbills Platalea flavipes

MICHAEL CONNOR 19 Pamela Grove, Lower Templestowe, Victoria 3107 (Email: [email protected])

Summary. Silver Gulls Chroicocephalus novaehollandiae harassed foraging Yellow-billed Spoonbills Platalea flavipes to gain access to their feeding locations. The interaction between the appears to be intermediate between strict commensalism and true kleptoparasitism.

On the afternoon of 26 August 2009, Yellow-billed Spoonbills Platalea flavipes, usually in groups of three to five, were foraging in the pond system to the north of Paradise Road, in the Western Treatment Plant south-west of Melbourne, Victoria (38°S, 144°34′E). The were all foraging in the same way, moving steadily forward and sweeping their bills from side to side in the manner described by Vestjens (1975) as ‘slow sweeping’. As was evident from the regularity with which birds momentarily interrupted their sweeping and swallowed, prey items were plentiful. As they fed, the Spoonbills were accompanied by several Silver Gulls Chroicocephalus novaehollandiae, each of which regularly hovered above and behind the head of a foraging Spoonbill. The response of Spoonbills singled out in this way was usually to take evasive action, taking several rapid steps, with wings partly raised, to a position several metres away where they immediately resumed feeding. However, once, a Spoonbill held its ground, turned its head towards the and opened its bill wide in what appeared to be a threat display. When a Spoonbill moved off, the Gull harassing it often alighted on the surface of the water at the point where the Spoonbill had been feeding. While on the water the Gulls did not appear to search actively for prey, and no Gull was observed to pick anything off the surface. Despite the apparent unproductiveness of their harassing behaviour, the Gulls then took off and resumed pestering the Spoonbills. Associations between feeding spoonbills and several other species have been recorded previously. Russell (1978) observed Great Egrets Ardea alba and Snowy Egrets Egretta thula increasing their foraging efficiency by following feeding Roseate Spoonbills Platalea ajaja in a marsh in Costa Rica. A similar ‘beater-follower’ association between African Spoonbills P. alba and Little Egrets Egretta garzetta was reported by Connor (1979). Feeding associations have also been observed between Eurasian Spoonbills Platalea leucorodia and Little Egrets, Great Egrets, cormorants, pelicans and Black-headed Gulls Chroicocephalus ridibundus (Otto Overdijk, Secretary, International Spoonbill Working Group, in litt. 23 September 2009). In the latter case, the Black-headed Gulls reportedly flew above spoonbills that were feeding on shrimps, diving on free-swimming shrimps disturbed by the foraging spoonbills but not attempting to steal any prey caught by the spoonbills. All of these interactions or associations recorded between spoonbills and other bird species appear to be of a commensal nature. However, this was not the case at the Western Treatment Plant. Here the gulls appeared to be deliberately harassing the spoonbills, placing themselves in positions that the spoonbills found too threatening to ignore; this behaviour seemed specifically designed to cause the VOL. 27 (4) CONNOR: december 2010 Silver Gulls Harassing Yellow-billed Spoonbills 185 spoonbills to vacate their immediate feeding area and give the gulls unhindered access to any prey disturbed and brought close to the surface of the water by the foraging spoonbills. There are similarities between this behaviour and that employed by Silver Gulls harassing foraging Bar-tailed Godwits Limosa lapponica (Taylor et al. 1996) by following them and thereby causing them to spend more time being vigilant and taking evasive action. However, the gulls following the godwits showed more aggression than those attending the spoonbills at the Western Treatment Plant: they often rushed at godwits that were probing repeatedly in a particular location or were pulling a worm from the substrate. As in the case of the spoonbills, however, once the target bird had been displaced from its feeding location the gull usually remained there, searching for any scraps of food left behind by the departing godwit. The behaviour of the Silver Gulls harassing the Yellow-billed Spoonbills appears to be intermediate between strictly commensal behaviour (like that reported by Overdijk for Black-headed Gulls feeding in association with Eurasian Spoonbills) and true kleptoparasitism (as typified by the attempted stealing of food by a from an Australian Pelican Pelecanus conspicillatus described by Weston 1992). Silver Gulls have also been recorded kleptoparasitising Lesser Crested Terns bengalensis and Crested Terns T. bergii around tern nesting colonies (Hulsman 1976). Kleptoparasitism of shorebirds by gulls has also been observed: Black-headed Gulls have been reported kleptoparasitising Northern Lapwings Vanellus vanellus (Barnard & Thompson 1985), and Weston (1992) mentioned Silver Gulls kleptoparasitising oystercatchers Haematopus spp.

I thank referees Kevin Wood and Rohan Clarke for their helpful suggestions.

References Barnard, C.J. & Thompson, D.B.A. (1985), Gulls and Plovers: The Ecology and Behaviour of Mixed-species Feeding Groups, Croom Helm, London. Connor, M. (1979), ‘Feeding association between Little Egret and African Spoonbill’, Ostrich 50, 118. Hulsman, K. (1976), ‘The robbing behaviour of terns and gulls’, Emu 76 143–149. Russell, J.K. (1978), ‘Effects of interspecific dominance among egrets commensally following Roseate Spoonbills’, Auk 95, 608–610. Taylor, I.R., Taylor, S.G. & Larmour, G.N. (1996), ‘The effect of food stealing by Silver Gulls novaehollandiae on the foraging efficiency of Bar-tailed Godwits Limosa lapponica’, Emu 96 234–239. Vestjens, W.J.M. (1975), ‘Feeding behaviour of spoonbills at Lake Cowal, NSW’, Emu 75, 132–136. Weston, M.A. (1992), ‘Unusual kleptoparasitism by Silver Gull toward Pelican’, Geelong Naturalist 29, 23.

Received 8 December 2009 