Courtship feeding in breeding Great Cormorants Phalacrocrorax carbo sinensis?

Maarten Platteeuw

On 10 April 1996 it was observed how a male Great Cormorant Phalacrocorax carbo sinensis at a nest in the Oostvaardersplassen colony, The Netherlands, voluntarily vomited about 4 or 5 specimens of Ruffe Gymnocephalus cernuus on the edge of the nest that was occupied by its apparent female partner. The female took the fishes one by one and swallowed them. Courtship feeding between breeding partners is a well-known and extensively described phenomenon among various species of colonial fish-eating birds (e.g. terns) and usually quite resembles the way in which in later stages the chicks are being fed. In this case, however, at least the feeding itself was very much unlike the way Great Cormorant nestlings come by their food, eating it directly from their parents’ gular pouch. Moreover, as far as I know, any similar behaviour has never before been described for breeding Great Cormorants (e.g. Cramp & Simmons 1977, Kortlandt 1995) and also extensive observations in recent years of breeding birds at Oostvaardersplassen, mainly aimed at ring-reading, have only once revealed a similar observation (M. Zijlstra, pers. comm.).

Particularly at the start of the breeding season, it may very well pay off for well-established breeding pairs to adopt some sort of courtship feeding, since at this stage nests abandoned by both male and female tend to be robbed by settling neighbours. Moreover, a female bird, being smaller and slightly weaker and having to prepare for laying, can certainly do with some extra food without the need to undertake a costly foraging flight to get it. A conservative estimate of the extra energy intake of four 10-12 cm large Ruffe, as observed, would amount to about 46-85 g of fresh fish mass, which is approximately 12-22% of a daily ration at this time of year (cf. Voslamber 1988, Platteeuw & Van Eerden 1995). Four of such feedings a day could provide a female with a sufficient amount of food to stay in the colony all day and save herself the costly foraging flight. It is likely that this kind of behaviour will only pay off in pairs that know each other well (e.g. in older birds of well-established colonies) and develop in situations where travelling distances between colony and main fishing grounds are relatively large. Particularly when comparative observations are carried out in colonies of varying antiguity and varying distances to fishing areas, it may be interesting to look more systematically into the incidence of this 'courtship feeding' behaviour in Great Cormorants or even other cormorant species.

References

Cramp S. & Simmons K.E.L. 1977. The birds of the western Palearctic. Vol. I. Ostrich to ducks. Oxford University Press, Oxford.

Kortlandt A. 1995. Patterns of pair-formation and nest-building in the European Cormorant Phalacrocorax carbo sinensis. Ardea 83: 11-25.

Platteeuw M. & Van Eerden M.R. 1995. Time and energy constraints of fishing behaviour in breeding Cormorants Phalacrocorax carbo sinensis at lake IJsselmeer, The Netherlands. Ardea 83: 223-234.

Voslamber B. 1988. Visplaatskeuze, foerageerwijze en voedselkeuze van Aalscholvers Phalacrocorax carbo in het IJsselmeergebied in 1982. [in Dutch with English summary]. Flevobericht No. 286. Rijksdienst voor de IJsselmeerpolders, Lelystad.

Maarten Platteeuw, Rijkswaterstaat RIZA, P.O.Box 17, NL-8200 AA Lelystad, the Netherlands