Black-Tailed Godwits Eat Animal and Plant Material at a Key Non-Breeding Site at the Overlap of Two Asian Flyways.

Abstract

Dietary knowledge of migratory birds along their migration routes is fundamental for understanding ecological and evolutionary aspects of bird migration, how migrants select and repeatedly use specific non-breeding areas, accumulate fuel for migration and breeding, and assess carrying capacity at key sites in the migratory network. Local changes in food availability and foraging habitat have been shown to contribute to population declines in migratory shorebirds. Nevertheless, the diet of many migratory bird populations remains unknown at key migratory sites. We address this knowledge gap for Black-tailed Godwits (Limosa limosa) by exploring their diet in the Ganges-Brahmaputra-Meghna delta in the Bay of Bengal, a key non-breeding site overlapping the Central Asian and East Asian-Australasian flyways. We assessed how benthic prey availability changed during their non-breeding period based on 600 mud samples collected from 120 foraging locations between October 2018 and March 2019. In mid-January 2019, we determined godwit diets by analyzing undigested remains of 15 feces and 15 pellets collected from roosting areas, and compared dietary composition with benthic prey availability from samples collected concurrently. Polychaetes and oligochaetes were the most abundant benthic groups in the mud, but we did not find them in the droppings. We observed intra-seasonal changes in abundance and body size of bivalvia, decapoda, and gastropoda, which were the main animal foods found in droppings and mud samples, and were positively selected by godwits based on availability compared to other benthic prey. We also found plant materials, rice kernels, and rhizomes of Porteresia coarctata, in one-third of the droppings. This finding extends previous evidence of godwits' herbivory in the East-Atlantic flyway to Asian flyways. Scaling up our efforts to systematic monitoring could help address conservation questions, such as the impact of shellfish harvesting on the carrying capacity of the world's largest river delta for shorebirds.