Foraging segregation of two congeneric diving seabird species breeding on St. George Island, Bering Sea

Nobuo Kokubun, Takashi Yamamoto, Nobuhiko Sato, Yutaka Watanuki, Alexis Will, Alexander S. Kitaysky, Akinori Takahashi
2016 Biogeosciences  
<p><strong>Abstract.</strong> Subarctic environmental changes are expected to affect the foraging ecology of marine top predators, but the response to such changes may vary among species if they use food resources differently. We examined the characteristics of foraging behavior of two sympatric congeneric diving seabird: common (<i>Uria aalge</i>: hereafter COMUs) and thick-billed (<i>U. lomvia</i>: hereafter TBMUs) murres breeding on St. George Island, located in the seasonal sea-ice region
more » ... al sea-ice region of the Bering Sea. We investigated their foraging trip and flight durations, diel patterns of dive depth, and underwater wing strokes, along with wing morphology and blood stable isotope signatures and stress hormones. Acceleration–temperature–depth loggers were attached to chick-guarding birds, and data were obtained from 7 COMUs and 12 TBMUs. Both species showed similar mean trip duration (13.2<span class="thinspace"></span>h for COMUs and 10.5<span class="thinspace"></span>h for TBMUs) and similar diurnal patterns of diving (frequent dives to various depths in the daytime and less frequent dives to shallow depths in the nighttime). During the daytime, the dive depths of COMUs had two peaks in shallow (18.1<span class="thinspace"></span>m) and deep (74.2<span class="thinspace"></span>m) depths, while those of TBMUs were 20.2<span class="thinspace"></span>m and 59.7<span class="thinspace"></span>m. COMUs showed more frequent wing strokes during the bottom phase of dives (1.90<span class="thinspace"></span>s<sup>−1</sup>) than TBMUs (1.66<span class="thinspace"></span>s<sup>−1</sup>). Fish occurred more frequently in the bill loads of COMUs (85<span class="thinspace"></span>%) than those of TBMUs (56<span class="thinspace"></span>%). The <i>δ</i><sup>15</sup>N value of blood was significantly higher in COMUs (14.5<span class="thinspace"></span>‰) than in TBMUs (13.1<span class="thinspace"></span>‰). The relatively small wing area (0.053<span class="thinspace"></span>m<sup>2</sup>) of COMUs compared to TBMUs (0.067<span class="thinspace"></span>m<sup>2</sup>) may facilitate their increased agility while foraging and allow them to capture more mobile prey such as larger fishes that inhabit deeper depths. These differences in food resource use may lead to the differential responses of the two murre species to marine environmental changes in the Bering Sea.</p>
doi:10.5194/bg-13-2579-2016 fatcat:v6lv23ugzzfodbgqd4n677m5xq