Nesting niche partitioning between two sympatrically breeding Chlidonias Tern species revealed by remote sensing

Sympatrically breeding avian species may have similar environmental requirements for nesting sites, resulting in interspecific competition. It may be reduced by partitioning resources in space or/and time, allowing relatively stable coexistence in the shared habitat. Here, we investigated nesting niches of sympatrically breeding, Black Terns (BT, Chlidonias niger) and Whiskered Terns (WT, Chlidonias hybrida) in Druzno Lake (Poland) in 2024. We compared nesting site proximity characteristics between both studied species at two different spatial scales using remotely sensed from drone surveys: (contribution of water and vegetation) and indices derived from satellite imagery: Normalized Difference Vegetation Index (NDVI) as a proxy for vegetation density, and Normalized Difference Water Index (NDWI) as a proxy for open water contribution. Both species partitioned location (they bred in separate locations) and resources within common breeding ground. BT's nests were situated significantly closer to the lake shore (mean ± SD 102.25 ± 62.47 m) and further away from each other (mean ± SD 22.22 ± 14.81 m) compared to WTs (mean ± SD 257.24 ± 109.64 m & 11.24 ± 10.67 m, respectively). BTs, unlike WTs prefer to nest in areas with higher open water contribution and lower vegetation density. The nesting niches of BT were wider than those of WT, suggesting that BT use more diversified habitats for breeding.