The quantity and quality of food available within the foraging area set important constraints for chick-rearing birds, but responses to low quality are not well understood.
This study explored the potential for parent birds to adjust quantity (feeding rate) and quality (energy content) in chick provisioning, by studying Common Guillemots Uria aalge on Stora Karlsö, Baltic Sea, predominantly utilizing Sprat Sprattus sprattus, during conditions of high food quantity but reduced food quality. Quality is central to reproductive success in this single-prey loader. From the chick's perspective, provisioning rates should be increased to compensate for low food quality and to fulfil its growing needs with increasing age. However, the high energy cost of flying in Guillemots makes it important for parent birds to minimize commutes to feeding areas.
Provisioning parameters were recorded during three dawn-to-dusk watches each breeding season from 2005 to 2013, when clupeids, presumably Sprat, constituted 98% of chick diet. Generalized additive mixed models showed that both feeding rate and size of clupeids (a proxy for energy content) varied between years and changed non-linearly with chick age, but that there was no change within breeding seasons. Chick age and year explained 36% of the variation in feeding rate but only 2% of the variation in the size of clupeids in chick diets.
We conclude that parent birds tried to adjust both feeding rate and prey size, but were less successful with the latter. A strong negative correlation was found between annual feeding rates and size of clupeids, evaluated as the differences relative to the baseline year, and adjusted for the effects of chick age. Although the differences between years were small, the relationship indicates a compensation mechanism that does not seem to impact adult survival, and by which increased feeding rates can partly counteract reduced chick energy intake when food quality is low.
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