Effect of food shortage on the physiology and competitive abilities of sand martin (Riparia riparia) nestlings.

We examined developmental and behavioural plasticity of sand martin (Riparia riparia) nestlings hand-reared under laboratory conditions. We created six broods of six 4-day-old nestlings and randomly assigned them to one of the two following feeding regimens, each lasting for 3 days: (1) all nestmates fed a similar, limited amount of food (FR nestlings). This simulated synchronous hatching under conditions of food restriction. (2) Half the brood were food-restricted (FR/AL nestlings), and half were fed ad libitum (AL nestlings), as in asynchronously hatched broods with differential food allocation. Under both regimens, food restriction resulted in a reduction in body mass, intestinal mass, pectoral muscle mass, fat reserves, body temperature and resting metabolic rate (RMR). However, it simultaneously triggered a significant increase in intestinal uptake rates of L-proline and locomotor activity, quantified as frequency of crawling into the artificial nest tunnel by individual nestlings. Locomotor activity and intestinal uptake rates of L-proline by FR nestlings were higher than those of FR/AL young, while body temperature and RMR of FR nestlings were lower. We conclude that food-restricted nestlings responded actively to food shortages by upregulating their gut function, reducing the energy costs of maintenance and increasing locomotor activity. These behavioural and physiological responses were strongest in broods of similar-sized FR nestlings, which can be interpreted as an escalation of sibling competition. Thus, developmental and behavioural plasticity may be an important factor in the evolution of sibling rivalry.