Ardea
Official journal of the Netherlands Ornithologists' Union

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Nelson J.B. (1967) Colonial and cliff nesting in the Gannet. Compared with other Sulidae and Kittiwake. ARDEA 55 (1-2): 60-90
The aims of the paper are to demonstrate the Gannet's (Sula bassana) adaptations to cliff nesting; to compare these with those of the Kittiwake (Rissa tridactyla) and to analyse the factors associated with the Gannet's specialized habitat preference, dense nesting, and marked territorial aggression. The separate factors involved in the Gannet's choice of habitat (mainly cliffs of small islands) and their advantages are discussed. The many ways in which Gannets are adapted to cliff nesting, anatomically, phylogenetically and behaviourally, are described. The numerous behavioural adaptations are compared in a table with those of the Kittiwake (E. Cullen 1957) as an example of convergent evolution and much of the behaviour is more fully described in the text. Adaptations fitting with the cliff habitat include the colouration of young, which in their juvenile plumage are much darker than those of any other member of the family. It is suggested that the function of their dark plumage may be to provide maximum contrast with that of adults, hence minimizing the stimulus value of the young as objects releasing attack by adults, particularly the male parent. Rejecting the idea that Gannets are forced to nest densely and compete for sites because these are in short supply, it is suggested that the adaptive value of dense nesting and its associated aggression is that it aids synchronization of breeding by means of increased social stimulation. Evidence for such an effect is given. The value of synchronized breeding in terms of (a) allowing adults to exploit a seasonally abundant food supply, hence achieving an extremely high breeding success and producing young with extensive fat reserves and (b) producing young at the most favourable time for post fledging survival, is supported by evidence. Finally, the rest of the family are considered in relation to their food supply, density and synchronization of breeding and the results appear consistent with the requirements of the hypothesis advanced for the Gannet. Most boobies do not breed densely, and with non-seasonal and erratic food supplies, they breed asynchronously and suffer extensive loss of young by starvation. The Peruvian, and to some extent the Blue-footed Boobies are special cases and their situations are explained. They, nevertheless, show local group synchronization as an effect of social stimulation where this is probably not of great adaptive value.


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