Seabird Group Seabird Group

Common Terns Sterna hirundo incubating Common Garden Snail shells Helix aspersa on Rockabill Island

Andrew Power 1*, Stephen Newton2 and Ian O’Connor1

* Correspondence author. Email:

1Marine and Freshwater Research Centre, Galway-Mayo Institute of Technology, Dublin Road, Galway, H91 T8NW, Ireland;

2BirdWatch Ireland, Unit 20, Block D, Bullford Business Campus, Kilcoole, Co. Wicklow, Ireland.

Full paper


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).


Baerends, G. P., Drent, R. H., Glas, P. & Groenewold, H. 1970. An ethological analysis of incubation behaviour in the Herring Gull. Behaviour 17(Suppl.): 135–235.

Beer, C. G. 1961. Incubation and nest building behaviour of Black-headed Gulls. 1: Incubation behaviour in the incubation period. Behaviour 18: 62–105. [Crossref]

Beer, C. G. 1965. Clutch size and incubation behaviour in Black-billed Gulls (Larus bulleri). Auk 82: 1–18. [Crossref]

Buckley, P. A. & Buckley, F. G. 1972. Individual egg and chick recognition by adult Royal Terns (Sterna maxima maxima). Animal Behaviour 20: 457–462. [Crossref]

Burke, B., Keogh, N. T., Power, A. & Newton S. F. 2016a. Mother knows best? Interspecific fostering between Roseate Terns Sterna dougallii and Common Terns Sterna hirundo on Rockabill Island, County Dublin. Irish Birds 10: 288–289.

Burke, B., Kinchen-Smith, D., Somers, S. & Newton, S. F. 2016b. Rockabill Tern Report 2016. Unpublished BirdWatch Ireland Seabird Conservation Report.

Conover, M. R. 1984. Occurrence of supernormal clutches in the Laridae. Wilson Bulletin 96: 249–267.

Conover, M. R. 1985. Foreign objects in bird nests. Auk 102: 696–700.

Coulter, M. C. 1980. Stones: an important incubation stimulus for gulls and terns. Auk 97: 898–899.

Farrelly, P. 1990. Little Terns incubating a stone. Irish Birds 4: 561.

Gochfeld, M. 1976. Intraclutch Egg Variation: The uniqueness of the Common Tern’s Third Egg. Bird-Banding 48: 325–332.

Harrison, C. 1975. A Field Guide to the Nests, Eggs and Nestlings of British and European Birds. Collins, London.

Knight, R. L. & Erickson, A. W. 1977. Objects incorporated within clutches of the Canada Goose. Western Birds 8: 108.

Langlois, L. A., Murböck, K., Bulla, M. & Kempenaers, B. 2012. Unusual incubation: Longbilled Dowitcher incubates mammalian bones. Ardea 100: 206–210. [Crossref]

Marshall, N. 1943. Factors in the Incubation Behavior of the Common Tern. Auk 60: 574–588. [Crossref]

McKeon, C., Miley, D., Somers, S. & Newton, S. 2017. Rockabill Tern Report 2017. Unpublished BirdWatch Ireland Seabird Conservation Report.

Mellink, E. 2002. Pseudo-eggs of Brown Sula leucogaster and Blue-footed S. nebouxii Boobies in the Gulf of California, Mexico. Marine Ornithology 30: 43–44.

Nisbet, I. 1997. Female Common Terns Sterna hirundo eating mollusc shells: evidence for calcium deficits during egg laying. Ibis 139: 400–401. [Crossref]

Pfleger, V. & Chatfield, J. 1983. A guide to snails of Britain and Europe. The Hamlyn Publishing Group Ltd., London.

Reid, W. 1987. Constraints on clutch size in the Glaucous-winged Gull. Studies in Avian Biology 10: 8–25.

Saino, N. & Fasola, M. (1993). Egg and nest recognition by two tern species (Sternidae Aves). Ethology, Ecology and Evolution 5: 467–476.

Sugden, J.W. 1947. Exotic eggs in nests of California Gulls. Condor 49: 93–96. [Crossref]

Twomey, A. C. 1948. California Gulls and exotic eggs. Condor 50: 97–100. [Crossref]