Pacific Common Eider (Somateria mollissima v-nigrum) Nest Records, Yukon-Kuskokwim Delta, Alaska, 1991-2004

We searched for Pacific common eider nests on foot and sometimes with the aid of trained dogs, beginning shortly after spring breakup (late May–early Jun) through the end of the first week of incubation (early–mid Jun) each year. During the search period, we covered most of the available (vegetated) nesting habitat within each study site (average area of vegetated habitat = approx 9 km2/per study site). We defined a nest as ≥1 egg in a bowl with at least some evidence of down. We also included nests discovered incidental to other activities in our sample.

We mapped each nest on aerial photos and recorded coordinates of the nest location using Global Positioning System units. In addition, we marked each nest with a small flag or wooden lathe placed 5-10 m north of the nest. We recorded clutch size, incubation stage, female status (flushed, present, near nest, or absent), abundance of down, and condition of nest (laying, incubating, flooded, or depredated) at each visit. We used egg candling (Weller 1956) and floating (Westerkov 1950) to determine incubation stage, predict hatch date, and assess egg viability. We monitored partial predation by numbering all eggs with permanent markers and monitoring their presence or absence at each nest visit. After each visit, we covered the nest with down to conceal eggs from avian predators. We revisited nests every 5–7 days until hatch or failure.

To determine egg fates, we visited all nests either on the day of hatch, or within 5 days post-hatch, and used the remaining nest contents (membranes, depredated egg shells) or presence of ducklings in the bowl to assess final fates.