Recruitment dynamics of a resident passerine: dippers Cinclus cinclus in Scotland

Abstract

1. This thesis presents the results of a population ecology study of the Dipper Cinclus cinclus in the western Ochil Hills, Tayside and Central Regions, Scotland between April 1985 and May 1988.

2. Particular attention was given to factors affecting juvenile survival between fledging and recruitment to the breeding population. These included investigation of the variation and significance of body size, plumage colour. dispersal distance, home range acquisition, dominance status and autumn body condition.

3. Overwinter survival was higher in adults than juveniles. Juvenile females had a greater overwinter survival and recruitment rate than juvenile males. Few body size measures were consistently associated with overwinter survival,though juvenile females with longer wings and tarsi tended to survive better.

4. Males had higher plumage brightness scores than females and, within sexes, adults were brighter than juveniles. Overall, survival overwinter and recruitment were not related to plumage brightness.

5. A laboratory test arena was developed for assessing dominance relations in small groups of temporarily captive birds. Social status between age and sex classes was correlated with plumage brightness. Within age classes, plumage brightness was a significant predictor of status in adults, but body size was more important in juveniles.

6. Females settled farther from their natal sites than males; most of this dispersal was completed soon after independence. The relationship between dispersal and dominance is discussed and a model developed.

7. Autumn population density was manipulated locally in a series of experimental juvenile introductions. Numbers rapidly returned to initial levels, though earlier released individuals persisted for longer. About 20% of introduced birds recruited, mainly higher status males.

8. Body composition of a small sample of birds collected between September and April is described. Lipid stores were greatest in winter and least in spring. A method for measuring pectoralis muscle thickness was developed using an ultrasound-based technique.

9. Condition indices derived from "ultrasound" measurements on live birds were used to evaluate the importance of protein reserves in overwinter survival. Males in good condition in autumn were more likely to recruit but no trend was apparent in females.

10. Two periods of high juvenile losses were identified: post-independence and late autumn. Predation could only be implicated in the former. The agent of late autumn losses was not proven but probably involved territorial intolerance and the consequent exclusion of subordinates to fringe habitats. It is concluded that density-dependent changes in mortality, related to dominance and mediated via dispersal, caused Dipper numbers tobe matched to available resources, principally food and breeding territories.