Consequences of Competitive Asymmetry in Broods of the Black Guillemot Cepphus grylle

I examined natural variation in the reproductive strategy of the black guillemot, with particular emphasis on the consequences for nestlings. Data were collected from a population of c. 65 breeding pairs on the Holm of Papa Westray, northern Scotland. Temporal differences in reproductive strategy are likely to reveal the reproductive constraints most pertinent to a species, exhibit how reproductive costs are manifest, and highlight the potential trade-offs selected to maximise reproductive success in the face of such costs. Constraints appeared to operate both at the egg production and chick rearing stages, with annual and seasonal effects influencing egg size, chick growth and survival. However, females did not alter the allocation of resources between eggs, nor the degree of hatching asynchrony, suggesting that females maintained the level of competitive asymmetry within the brood, despite these constraints. The relationship between egg size and breeding success was investigated. Chick hatching size and quality were positively related to egg size, but I found no effect of egg size on chick growth or survival. Within clutches, first-laid eggs (a-eggs) were larger than second- laid (b-eggs), but egg-size disparity decreased with decreasing a-egg size. Thus females producing large eggs (i.e. potentially higher quality females) appeared to be striving for within- clutch egg size disparity, inferring that such a difference might be adaptive in the context of sibling competitive asymmetry. Paradoxically, however, egg-size disparity exhibited no relationship with hatching asynchrony. Egg size was a significant predictor of hatching success in the b-egg, suggesting that egg viability decreases once a certain minimum size was attained. I also examined breeding success in relation to natural variation in hatching asynchrony. At all levels of hatching asynchrony, a-chicks attacked their siblings more frequently than vice versa. Consequently, a-chicks in asynchronous broods grew faster, reached higher asymptotic weights and were more likely to survive to fledging than b-chicks. No such differences were evident between siblings in S3aichronous broods, despite a-chick aggressive dominance in these broods. Overall, broods hatching with a two-day hatching interval achieved the highest breeding success. I investigated food amount and competitive asymmetry as potential proximate cues for sibling aggression. Parental provisioning rates were experimentally manipulated in broods comprising a range of hatching intervals over a twelve-hour period. Aggression became evident only after parental provisioning rates were experimentally reduced. When parental provisioning resumed, adults did not increase their feeding rate to compensate for the induced food deficit and the result of sibling rivalry was a change in the allocation of parental deliveries from one of equality to one in favor of the dominant chick. Food deprived chicks from synchronous broods were more aggressive than those from asynchronous broods, suggesting that one benefit of hatching asynchrony in the black guillemot is to establish an efficient competitive hierarchy among siblings which minimizes the need for costly aggressive interactions. Nonetheless, dominance was always established by the chick hatching from the first-laid egg, suggesting that factors in addition to size disparity are important in establishing competitive hierarchies. These results provide the first evidence that short-term food shortage per se acts as an initial trigger for aggression, yet also reveal that the aggressive response is complicated by factors associated with hatching and laying order. Before the functional significance of hatching asynchrony within any avian species can be resolved, it is first necessary to determine the proximate mechanisms controlling the pattern of hatching. To date, most studies have tacitly assumed that hatching patterns are effected predominantly by parental incubation behaviour. I compared incubation periods of male and female black guillemot (Cepphus grylle) embryos to ascertain whether development rates are a function of embryo sex and, if so, the effects of clutch sex-composition on hatching pattern. Chick sex was determined using a molecular DNA technique based on the CHD gene. Laying date and egg mass had no significant effect on incubation period, but eggs containing male embryos developed significantly faster than those containing females. The onset of incubation in relation to clutch completion is variable in black guillemots. Thus, in mixed-sexed clutches where the first-laid embryo is male, hatching asynchrony was attained regardless of the incubation regime employed. These results clearly show that mechanisms in addition to incubation behavior are important in establishing avian hatching patterns. I also demonstrated that pre-laying maternal allocation varied according to progeny sex and brood sex composition. Furthermore, survival probability was found to depend not only on the sex of the individual and its position in the laying/hatching sequence, but also on the sex of its sibling. Females appeared to respond to this gender related mortality by dramatically skewing the frequency of brood sex compositions in favour of those less prone to mortality