Time-stressed Bar-tailed Godwits exploit novel prey during migratory fuelling

Long-distance migratory birds breeding in the Arctic face challenges as earlier snowmelt shifts the timing of vital resources. To keep up with these advancements, birds will need to modify their migration. Higher fuelling rates during stopover have been identified as a critical mechanism for accelerating migration, but this requires adequate food availability. The sexually dimorphic Bar-tailed Godwits Limosa lapponica taymyrensis, migrating between West Africa and Siberia, have been observed shortening their fuelling time in the Wadden Sea. Currently, the mechanisms and limitations of this phenological adjustment are unclear. To understand their scope for adaptation to climate change, we studied Bar-tailed Godwits in two main fuelling areas in the Wadden Sea to investigate (1) sex-based differences in prey selection, (2) habitat selection in relation to prey density and (3) energy intake-rates. Visual observation and fine-scale tracking revealed sex-specific selection for the two study areas. Droppings and resource selection analyses showed that females, the larger and longer-billed sex, primarily consumed and selected areas with deeply burrowed lugworms (Arenicola sp.) and ragworms (Nereis spp.), whereas males almost exclusively consumed and selected areas with shallow-living mud shrimps (Corophium spp.). The latter has not been documented as a primary fuelling resource in the well-studied Bar-tailed Godwits. Long-term survey data suggest an increasing reliance of Bar-tailed Godwits on mud shrimps. We estimate that at least 10% of the global male African L. l. taymyrensis population now relies on mud shrimps. Nevertheless, estimated intake rates suggest that males can (barely) achieve sufficient intake rates on a mud shrimp diet, but there is limited leeway to further accelerate migration. We suggest that a recent phenological shift in mud shrimp abundance allows Bar-tailed Godwits and other waders to newly exploit this resource. However, our study raises questions about whether novel prey enables birds to mitigate the effects of Arctic warming by fuelling faster. Our findings illustrate how food-related limitations on refuelling rate may pose a challenge for populations that need to keep up with advancing Arctic springs.