Size-related life history traits:: insights from a study of snow petrels (Pagodroma nivea)

1. Body size is commonly tied to major life history traits in many animals. The main problem with studies on this topic is that the range of body sizes within species is generally too small to produce enough variation in traits for quantitative analysis. The snow petrel shows considerable differences in body size with structural body size varying by approximate to 1.6-fold. This high variability in size provides a good opportunity for examining how life history traits are related to body size intraspecifically. 2. We studied the breeding phenology, foraging parameters and chick growth in relation to body size in two populations of snow petrel in Antarctica. We also investigated the relationship between body size and distance separating colony from the edge of the pack ice using morphometric data from 16 breeding stations. 3. The largest snow petrels laid eggs approximate to 2-fold larger and hatched hatchlings approximate to 1.9-fold heavier than smallest ones. During the chick rearing period the smallest adults made longer foraging trips (approximate to 70 h) and fed their chick less frequently than the largest ones (approximate to 40 h). Meal size was positively related to adult body size. 4. Chicks raised by large parents grew more rapidly, reached higher peak mass and asymptotic size than chicks raised by small parents. Chick body size at fledging was correlated to the size of their biological parents. A sample of 20 chicks from parents that differed in body size were swapped. Body size of swapped chicks at fledging was not correlated to their foster parent size. Swapped chicks tended to resemble their biological parents in size but no significant relationship was found. 5. These results provide evidence of intraspecific variability in several ecological variables and life history traits linked to body size. The difference in foraging trip duration between large and small birds was not only because of a lower flight speed or a higher metabolic rate per mass unit of smaller birds and suggests that small birds had lower feeding efficiency or fed on more distant areas. This was reinforced at the population level by a negative correlation obtained between body size and distance between colony and the edge of the pack ice for 16 breeding stations. 6. The results suggest that body size is probably in the main genetically determined but that there is a significant environmental component in fledgling body size.