Stopover by migrant Montagu's Harriers in the Thar Desert is determined by vegetation greenness and grasshopper abundance but not locust outbreaks

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Food availability is an important criterion for migrant birds' decision to stay or leave a stopover site. This can vary depending on the conditions at the site. The Thar Desert of the Indian subcontinent is a major stopover area for birds wintering further south in the region. The desert is also prone to stochastic outbreaks of desert locusts that often coincide with the stopover time of migrating birds. Between 2017 and 2022, we tracked nine tagged Montagu's Harriers (Circus pygargus) using PTT and GPS-GSM transmitters to determine if the stopover duration at stopover sites in the desert is influenced by the availability of migratory locusts or resident grasshoppers and the strategies the birds employ, such as, foraging over a wide area or multiple sites to ensure refuelling during stopover. The mean stopover duration during the 2019 peak locust outbreak was 12 days compared to 18 days in other years. Birds did not track locust occurrence deliberately and preferred to stopover at the same sites each year, irrespective of locust outbreaks. The stopover duration at stopover sites appears to be linked to arrival date, vegetation greenness (NDVI) and marginally to grasshopper abundance. We conclude that the stochastic availability of locusts at stopover sites may provide only an opportunistic source of abundant food for migrant harriers and does not seem to affect their stopover duration. Instead, they choose a more stable strategy of relying on resident grasshoppers, less prone to strong fluctuations in abundance and distribution at the stopover sites.