Data from: Diet dichotomy between two migrant seabirds breeding near a high Arctic polynya

High Arctic polynyas are predictable areas of open water, which offer long-distance migrant seabirds a reliable source of food during a period when they have to replenish and accumulate energy for reproduction. Investigating the interaction between species nesting sympatrically in the vicinity of polynyas should provide insights into the role that such oceanographic features play for pre-breeding seabirds. We used stable isotopes (δ13C and δ15N) to compare the diet of two ground-nesting seabirds, Sabine’s gull (Xema sabini) and Arctic tern (Sterna paradisaea), nesting on an island adjacent to a recurring polynya in the Canadian high Arctic in 2008 and 2009. We show that unlike Arctic terns, the diet of Sabine's gulls appears to include a non-negligible amount of terrestrially-derived prey during early incubation, and that overall both species segregate their dietary niche during pre-laying and early incubation.

高北极冰间湖（polynyas）是一类可预测的开阔水域，可为长途迁徙海鸟在需要补充、积累繁殖所需能量的关键阶段，提供稳定的食物来源。对冰间湖附近同域筑巢的物种间相互作用开展研究，有助于阐明这类海洋学特征对繁殖前期海鸟的生态功能。本研究于2008年与2009年，在加拿大高北极地区一处毗邻周期性冰间湖的岛屿上，选取两种地面筑巢海鸟——萨宾鸥（Sabine’s gull，*Xema sabini*）与北极燕鸥（Arctic tern，*Sterna paradisaea*）——作为研究对象，通过稳定同位素（stable isotopes，δ¹³C与δ¹⁵N）分析比较二者的食性组成。研究结果表明：与北极燕鸥不同，萨宾鸥在孵化初期的食谱中包含了占比不容忽视的陆生源性猎物；且整体而言，两种海鸟在产卵前期与孵化初期均存在食性生态位分化。