I. Descriptions are given of the elements of the behaviour performed by Black-headed Gulls on the nest during the time between laying of the third egg and hatching of the first egg. 2. With three eggs in the nest it is found that the majority of settling sequences are complete, the majority of sitting spells are relatively long, and the eggs are covered for almost all of the available time. Also most of the relatively long sitting spells are preceded by complete settling sequences and most of the short sitting spells are preceded by incomplete settling sequences; most of the complete settling sequences lead to long sitting spells and most of the incomplete settling sequences lead to short sitting spells. 3. The frequency of settling, the proportion of complete settling sequences, and the total amount of time spent sitting, are used in comparing the behaviour in different situations. It is found that more or less than three eggs in the nest, eggs of abnormal shape or size, fixed eggs, disturbed nest material, scaring stimuli near the nest, presence of the mate, and the early stage of the sitting bout, produce incubation behaviour which differs from the standard pattern; the birds rise and settle more often, spend less total time sitting, and, with the exception of the last two cases, complete less of their settling sequences. On the other hand different times of day, different stages of the incubation period, differences in weather conditions, eggs of abnormal colour or markings were not associated with differences in the incubation pattern in my observations. 4. It is concluded that stimuli relayed from the brood-patches are particularly important in determining the pattern of settling and sitting behaviour. 5. Some problems connected with the use of the concepts of incubation "drive" and incubation "tendency" are discussed. 6. Stimuli to the feet and legs probably play a part in the release of shifting. 7. Quivering while sitting is probably stimulated by disturbed or "novel" contact between brood-patches and eggs. I. Descriptions are given of the elements of the behaviour performed by Black-headed Gulls on the nest during the time between laying of the third egg and hatching of the first egg. 2. With three eggs in the nest it is found that the majority of settling sequences are complete, the majority of sitting spells are relatively long, and the eggs are covered for almost all of the available time. Also most of the relatively long sitting spells are preceded by complete settling sequences and most of the short sitting spells are preceded by incomplete settling sequences; most of the complete settling sequences lead to long sitting spells and most of the incomplete settling sequences lead to short sitting spells. 3. The frequency of settling, the proportion of complete settling sequences, and the total amount of time spent sitting, are used in comparing the behaviour in different situations. It is found that more or less than three eggs in the nest, eggs of abnormal shape or size, fixed eggs, disturbed nest material, scaring stimuli near the nest, presence of the mate, and the early stage of the sitting bout, produce incubation behaviour which differs from the standard pattern; the birds rise and settle more often, spend less total time sitting, and, with the exception of the last two cases, complete less of their settling sequences. On the other hand different times of day, different stages of the incubation period, differences in weather conditions, eggs of abnormal colour or markings were not associated with differences in the incubation pattern in my observations. 4. It is concluded that stimuli relayed from the brood-patches are particularly important in determining the pattern of settling and sitting behaviour. 5. Some problems connected with the use of the concepts of incubation "drive" and incubation "tendency" are discussed. 6. Stimuli to the feet and legs probably play a part in the release of shifting. 7. Quivering while sitting is probably stimulated by disturbed or "novel" contact between brood-patches and eggs.