- The paper is divided into two parts. In part I, I tried to answer the question ‘at what densities are guillemots most productive?’ Breeding
success was highest in Dense groups and lowest in Sparse groups.
- Breeding success was not related to the median laying date of groups but was related to synchrony of laying within groups. Dense groups had a
shorter spread of laying than Sparse groups, and a shorter spread on laying minimized the number of birds at the beginning and end the
season whose breeding was out of phase with the rest.
- Birds which were out of phase were more vulnerable to gull predation. In Dense groups several guillemots lunging at a gull were able to deter
it, but in a Sparse group a gull could readily displace a guillemot and steal its egg or chick.
- Birds in Dense groups spent more time sleeping and less time Alarm-bowing than birds in Sparse groups, indicating that they were less nervous
and spent less time looking for predators. Increased nervousness and less tenacious incubation in Sparse groups may have facilitated gull
attacks. Some individuals occasionally behave in an apparently altruistic manner, and protect other birds’ chicks and attack predators.
- The second question I tired to answer was ‘why are relatively few guillemots on Skomer breeding at high densities?’ The Skomer guillemot
population has shown a marked decline during the present century. It is assumed that adult mortality was high during the decline, thus
reducing the density of breeding groups and leaving them open to gull predation.
- Site tenacity is highly developed in guillemots, and new breeders tend to attach themselves to existing groups. Because Medium density groups
are more productive than Sparse groups, Medium density groups have persisted. Guillemots apparently exhibit a sort of behavioural
conservatism, preventing a reduced population from coalescing into a few Dense groups.
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