THE BREEDING BIOLOGY OF THE GANNET SULA BASSANA ON THE BASS ROCK, SCOTLAND
Authors: Nelson J. B. 
Published: April 2008

The Bass colony is increasing—in 1962 there were 5,350–5,700 breeding pairs; 1,340–1,430 pairs of non‐breeders with nests or sites (mainly pairs in their season before first breeding) and 2,000–2,500 “club” birds without nest or site. 15% of nests were occupied by both birds of a pair at the time of the count.

Oldest males return to the colony in January, followed by experienced females, considerably later by young adult‐plumaged birds, immature birds later still, and the few one year‐olds that return usually not until May and June. Mid‐cliff sites are the first to be re‐colonized each year.

Gannets usually breed in their fifth year and there is some evidence that females breed earlier than males.

The characteristics of Gannet nests and sites are described. Nests function in raising the egg and young above the morass of the breeding colony, and reach a density of about 2.3 per square metre. Nests are demolished and their positions changed more often than might be suspected. The extremely strong social tendency which causes Gannets to establish their sites amongst or very close to existing breeders probably is the factor ensuring high density and this contributes to synchronization of laying.

Egg laying is analysed. Experienced pairs forming an isolated group of 20 nests began laying later and showed less synchronization than two other groups of the same size but from the middle of a dense mass, probably due to the greater social stimulation experienced by the non‐isolated groups. The date for first and median eggs was also earlier in larger than smaller groups in the same year. The effect of density as distinct from group size is also discussed. Early eggs are mainly laid on cliff or cliff‐edge sites and in large nests.

Different groups within the colony produced the median egg within 2–3 days of the end of April each year. In the fullest documented group the mean date was also constant from year to year and closely approached the median, implying a considerable degree of synchronization within the gannetry as a whole. Laying in the observation colony became progressively earlier in successive years, probably due to recovery from previous disturbance. Nevertheless, individual females tended to lay in a fixed position each year with relation to the mean for the group.

Increasing age of the female causes earlier egg laying and heavier eggs for up to at least five years.

It is suggested that the survival value of seasonally synchronized laying in the Gannet is maximum utilization of a seasonally dependable and abundant food supply for the production of young with the optimal chance of post‐fledging survival. The spread of laying acts as an insurance against possible adverse conditions. There is a considerable reserve of unutilized breeding capability within the colony (adult non‐breeders, a pre‐maturity period longer than physiologically necessary for egg production, and a one‐egg clutch when in fact two young can easily be reared).

The mean of 393 Gannet eggs was 104.5 gm. (range 81–130). Eggs constitute about 3.4% of adult female weight and lose 9–13% in weight during incubation.

Replacement laying of the invariable one‐egg clutch takes 6–32 days. The mean incubation period was 43.6 days. Male incubation spells averaged 35.6 hours; female's 32.0 hours. Copulation ceases immediately after egg laying.

During three seasons, 82% of eggs laid in the observation colony hatched. Inexperienced pairs hatched 62.6% of eggs laid; experienced pairs hatched 86%. Some of the processes of incubation and chick rearing depend on the maturation of innate abilities and not on experience Inexperienced breeders do not seem inferior to experienced ones in finding enough food for their young.

Parental care of the new chick is described; the pterylosis of the chick is figured. A summarized account of plumage development is given.

Food fish and chick feeding are described. The average frequency of feeds throughout a continuous two‐day watch was 2.7 feeding bouts per chick per day. Adult fishing trips usually took 7–13 hours and the estimated fishing range is over 100 miles, and possibly up to 400 miles, from the breeding colony. Despite this, 15% of daylight hours are spent by the pair together at the nest in addition to the constant guarding of the chick by one or other.

Gannet young have a very compressed growth period compared with boobies and fledge at 3,100–4,100 gm., after a steady growth uninterrupted by periods of starvation or arrested development after an average 90 days.

92.3% of all eggs which hatched in 500 nests in the observation colony during three years of the study gave fledged young. Excluding inexperienced birds, there was no difference in the fledging success of eggs laid at different dates in the breeding season, in accordance with the proved abund‐ ance of food. However, post‐fledging survival is probably higher among young fledging at the peak period (first half of September) than later and the few relevant ringing returns tend to support this.

Breeding success at the small colony at Bempton was less compared with groups from the Bass for the years 1961–3.

Causes of chick loss before and during fledging are discussed. They are unimportant compared with the great mortality in the first year of life after fledging.

The adaptive significance of black plumage in the juvenile Gannet probably lies in reduced attack‐releasing qualities of such plumage on the male parent.

The Gannet alone in the Sulidae produces young which leave the nest with large fat deposits, and which are not fed at all by the parents after fledging. This is possibly another result of adaptation to using a seasonally abundant food supply to the maximum.

The present Gannet population increase is discussed in relation to cessation of human predation, the possible impetus given by a temporary but large increase in pelagic fish during the war, but also the overall steady downward trend in fishing returns since the early part of this century. One cannot explain the steady and considerable rise in Gannet numbers only in terms of increased food supply. The fact that, at a time of population expansion and obviously favourable conditions, Gannets are still far from utilizing their full recruiting powers, needs investigating further. It may be partly due to the relative slowness of evolutionary change in a long‐lived species with slow population turn‐over, if the Gannet has evolved its characteristics in response to an environment different from the present one.