Egg production in the thick-billed murre (Uria lomvia) and razorbill (Alca torda) - a life-history perspective

Hipfner, J. Mark (2000) Egg production in the thick-billed murre (Uria lomvia) and razorbill (Alca torda) - a life-history perspective. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A key tenet of life-history theory is that costs of reproduction lead to physiological and evolutionary trade-offs among fitness components. Although avian egg production was a key topic in the development of life-history theory, the significance of egg production costs within the life histories of birds remains poorly understood. Two aspects of egg production that have received considerable attention are variation among females in their capacity to renest following clutch loss, and variation in the size of egg they lay. I examined these aspects of egg production in two pelagic seabirds, Thick- billed Murres Uria lomvia and Razorbills Alca torda. It is generally thought that egg production costs are relatively low in pelagic seabirds that lay single-egg clutches, but age- and date-specific patterns in renesting capacity and egg size suggest that significant constraints on egg production operate in these birds. -- The proportion of Thick-billed Murres that renests following egg loss declines with the date of loss. However, early-laying females that had their eggs removed continued to renest until late in the laying period. First and replacement eggs were similar in crude composition, but replacement eggs had low protein content. Despite this, replacement eggs were no less likely to hatch than were first eggs, and chicks from replacement eggs were no less likely to survive to nest departure, and to recruitment age (4-5 years). I conclude that variation in the egg-production capacity of females that lose their eggs early and late (probably age/experience effects) drives the seasonal declines in renesting rates. Capable females will relay until late in the laying period because, for these birds, the potential fitness payoff from a replacement egg is similar to that from a first egg. It remains to be determined whether there are survival costs associated with the production of replacement eggs for female Thick-billed Murres. -- Thick-billed Murre eggs vary considerably in size, and egg size affects offspring performance: chicks from large eggs have their wing feathers grow more quickly than do those from small eggs. One hypothesis often invoked to explain the existence of variation in egg size in the face of expected directional selection for large eggs is that the optimal egg size varies with environmental conditions; this hypothesis predicts that benefits of hatching from a large egg will be magnified when feeding conditions are unfavourable. I tested this by comparing between colonies that experience favourable (Coats Island) and unfavourable (Digges Island) conditions, using an experimental egg-switching protocol. Contrary to prediction, the effect of egg size on wing feather growth was no greater at Digges Island than at Coats Island. -- This effect of egg size on wing-feather growthhas not been detected in other birds, suggesting that it might reflect adaptations to the unique intermediate developmental strategy employed by murres and Razorbills. To test this hypothesis, I examined the effect of egg size on post-hatching development in the Razorbill using the same egg-switching protocol. Results clearly supported the hypothesis: as in Thick-billed Murres, large-egg Razorbills experienced enhanced early wing-feather growth. There are a number of adaptations in the development of intermediate alcids that might explain why this egg-size effect is readily detectable in these birds. As there was no evidence for a trade-off between egg size and provisioning in either Thick-billed Murres or Razorbills, the existence of considerable female-specific variation in egg size remains unexplained.

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