Incubation and Nest-Building Behaviour of Black-Headed Gulls Ii : Incubation Behaviour in the Laying Period

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Abstract
(i) Gulls in the laying period show incubation responses towards eggs in the nest but the pattern differs from the incubation pattern shown by gulls in the incubation period; the birds in the laying period rise and settle more often, complete relatively fewer of their settling sequences, sit for less of the available time, and the partners spend more time together on the territory. (ii) By adding two eggs to nests in the one-egg stage, and by comparing the behaviour of birds in the one-egg stage with birds with clutches of one egg in the incubation period, it is shown that part of the difference between the behaviour of birds in the laying period and birds in the incubation period can be attributed to the difference in clutch sizes but that this factor does not account for all the difference. (iii) Experiments with cylindrical models at the two stages of the laying period indicate that, at this time, the gulls are not as sensitive to the sharp corners of the models as they are in the incubation period; birds sitting or trying to sit on the oddly shaped "eggs" did not rise and resettle as often in the laying period as they did in the incubation period. (iv) It is suggested that during the laying period there is progressive increase in the tendency to sit on the nest and eggs coupled with increase in sensitivity to the shapes of the objects sat on. (v) The males perform more settlings and sit for more time than do the females during the laying period but both partners spend about equal amounts of time near the nest at this time. (vi) The proportion of eggs warm to the touch steadily increases during the laying period. (vii) Defeathering of the brood-patches progresses during the laying period. (viii) The tendency to return to the nest in the face of fear stimulation increases as the laying period advances. (ix) Increase in effective incubation during the laying period accounts for the differences between laying and hatching intervals. Functional aspects of this are discussed. (i) Gulls in the laying period show incubation responses towards eggs in the nest but the pattern differs from the incubation pattern shown by gulls in the incubation period; the birds in the laying period rise and settle more often, complete relatively fewer of their settling sequences, sit for less of the available time, and the partners spend more time together on the territory. (ii) By adding two eggs to nests in the one-egg stage, and by comparing the behaviour of birds in the one-egg stage with birds with clutches of one egg in the incubation period, it is shown that part of the difference between the behaviour of birds in the laying period and birds in the incubation period can be attributed to the difference in clutch sizes but that this factor does not account for all the difference. (iii) Experiments with cylindrical models at the two stages of the laying period indicate that, at this time, the gulls are not as sensitive to the sharp corners of the models as they are in the incubation period; birds sitting or trying to sit on the oddly shaped "eggs" did not rise and resettle as often in the laying period as they did in the incubation period. (iv) It is suggested that during the laying period there is progressive increase in the tendency to sit on the nest and eggs coupled with increase in sensitivity to the shapes of the objects sat on. (v) The males perform more settlings and sit for more time than do the females during the laying period but both partners spend about equal amounts of time near the nest at this time. (vi) The proportion of eggs warm to the touch steadily increases during the laying period. (vii) Defeathering of the brood-patches progresses during the laying period. (viii) The tendency to return to the nest in the face of fear stimulation increases as the laying period advances. (ix) Increase in effective incubation during the laying period accounts for the differences between laying and hatching intervals. Functional aspects of this are discussed.

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