Many ground-nesting bird species have been observed incubating foreign objects in their nests (Mellink 2002). Pine cones, golf balls, guano, hermit crabs, mammalian bones and many more objects have all been recorded in bird nests (Knight & Erickson 1977; Mellink 2002; Langlois et al. 2012). A study by Conover (1985) showed that 10% of Ring-billed Gull Larus delawarensis and 6% of California Gull L. californicus nests contained a foreign object. These foreign objects were more similar in size and shape to the gull eggs than to randomly selected pebbles. Little Terns Sterna albifrons have been observed incubating egg-like pebbles for several weeks on two separate occasions at a breeding colony in Kilcoole, Co. Wicklow, Ireland (Farrelly 1990). Several hypotheses have been put forward to explain this behaviour. Coulter (1980) suggested that foreign objects may act as an incubation stimulus. Many tern and gull species, unlike other birds, have three brood patches not one (Gochfeld 1976; Reid 1987). Coulter (1980) conducted incubation studies of several species of gulls suggesting that birds incubating three eggs sat for longer periods of time, resettled less frequently, have a shorter incubation period and a higher hatching success than birds on either fewer eggs or more. He speculated that gulls are highly adapted for three-egg clutches whereby the foreign objects provide an important stimulus for incubation behaviour when there are less than three eggs (Beer 1961; Beer 1965; Baerends et al. 1970; Coulter 1980). Alternatively, the mistaken-food hypothesis suggests that a predatory bird brings a foreign object back to the nest, regurgitates the object and then proceeds to incubate it, mistaking it for one of its own eggs (Sugden 1947; Twomey 1948). A third hypothesis is that the foreign object is mistaken by the bird for one of its own eggs and rolled into the nest, in this scenario the foreign object is likely to have originated from close to the nest (Conover 1985).

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