Horizontal prey distribution determines the foraging performance of short- and long-billed waders in virtual resource landscapes

When waders gather in mixed-species flocks to feed on benthic prey, differences in morphological traits, foraging strategies and prey selection may allow different species to optimize their energy intake while reducing competition. As the effect of the fine-scale spatial distribution of resources on energy intake is unknown, we simulated the foraging performance of two types of waders with contrasting body plans and foraging strategies in a variety of virtual mudflats with different horizontal and vertical prey distribution patterns. Although larger, longer-billed individuals had higher energy intake rates, smaller individuals with shorter bills maintained higher prey capture rates by relocating if prey was insufficiently available. Shorter-billed individuals struggled more to meet their energetic demands because they selected smaller prey items and had a more limited capacity to metabolize energy from food. Being able to catch larger, high-quality prey offered a competitive advantage for longer-billed individuals, which could be a driving force for the evolutionary lengthening of bills in waders. Interestingly, their performance was more affected by the horizontal than by the vertical prey distribution. Quantifying prey distributions may help explain how some wader species can co-occur in the field and why some wetland areas are unattractive as foraging areas. The work confirms that the foraging performance of larger, longer-billed birds is not limited by bill size per se, but by the energetic trade-offs associated with the ability to catch larger prey items more efficiently, increased handling times and higher absolute energy costs. These trade-offs may become particularly important in landscapes where prey is scarce and spatially patchy.