The breeding biology of Guillemots Uria aalge on the Isle of May over a six year period

The breeding of Guillemots was studied in five areas of different breeding density and habitat type on the Isle of May in 1981-86. Prior to 1981 numbers were increasing at 5 6° per annum but during the study the rate of increase slowed down and from 1983 to 1986 numbers were fairly constant. Adult survival was high, with a mean minimum annual adult survival of 930% (s.e. = 03). Observations in 1986 suggested that the percentage return of colour-marked immature birds was low, with only l-6% and 5.5% of second and third year birds being seen. We suggest that poor recruitment was responsible for the levelling off in numbers at the colony.
The timing of laying was constant from year to year in 1981-85 but was later in 1986. It was significantly and inversely related to sea temperature the previous March. There was a consistent ranking in median laying dates amongst the areas, with area 1 (the highest density of birds) always earliest. However, there was no significant difference in synchrony between the areas. Overall breeding success was high (0–71-0-82 young fledged per pair). There was no consistent ranking of breeding success with breeding density, habitat type or laying synchrony.
The only aspect of Guillemot biology which changed significantly was the daily food intake of a chick which approximately halved during the study period. However, this reduction in food intake had no detectable effect on either the weight of chicks with wing lengths greater than 60 mm or the amount of time off-duty breeders spent at the site. Both of these parameters were still consistent with conditions being favourable in 1986.