Reinterpretation of gizzard sizes of red knots world-wide emphasises overriding importance of prey quality at migratory stopover sites

The size of digestive organs can be rapidly and reversibly adjusted to ecological circumstances, but suchphenotypic flexibility comes at a cost. Here, we test how the gizzard mass of a long-distance migrant, thered knot (Calidris canutus), is adjusted to (i) local climate, (ii) prey quality and (iii) migratory fuellingdemands. For eight sites around the world (both wintering and stopover sites), we assembled data ongizzard masses of free-living red knots, the quality of their prey and the local climate. Using an energeticcost–benefit approach, we predicted the gizzard size required for fastest fuelling (net rate-maximization,i.e. expected during migration) and the gizzard size required to balance daily energy budgets (satisficing,expected in wintering birds) at each site. The measured gizzards matched the net rate-maximizingpredictions at stopover sites and the satisficing predictions at wintering sites. To our surprise, owing to thefact that red knots selected stopover sites with prey of particularly high quality, gizzard sizes at stopoversand at wintering sites were nevertheless similar. To quantify the benefit of minimizing size changes in thegizzard, we constructed a model incorporating the size-dependent energy costs of maintaining and carryinga gizzard. The model showed that by selecting stopovers containing high-quality prey, metabolic rates arekept at a minimum, potentially reducing the spring migratory period by a full week. By inference, red knotsappear to time their stopovers so that they hit local peaks in prey quality, which occur during thereproductive seasons of the intertidal benthic invertebrates.