Population Plots at Two Seabird Colonies in Lower Cook Inlet, Alaska

FIELD: Population plots were established on each colony in the 1990s to count nesting seabirds and track productivity (Piatt 2002). Seabird counts from the 1990s (Piatt 2002) were tabulated together with count data since 2016. We used distinguishing physical features of these plots to serve as landmarks and repeat these historical counts. To compare to historical data, we collected counts on 10 population plots at Gull Island and 8 at Chisik/Duck Island. Plots included both nesting Common Murres and Black-legged Kittiwakes. Starting in 2016, population counts were estimated using photos of the plots, which were taken multiple times during the summer nesting seasons. In the original version of this data release (version 1.x) a table of total colony counts was included. Each record was the total count (sum of all plot counts) from a colony on a single survey day. The totals were derived wholly from counts of the individual plots. The number of birds on individual population plots should not be compared to other plots, as the plots all differ in size and habitat for nesting birds.

FIELD: During this study, breeding phenology was difficult to assess due to multiple years of breeding failure. Mid-incubation appeared to occur at dates similar to those in the past, but initiation of fledging was often relatively late, if any chicks were fledged at all. Birds that fledged may have been from replacement clutches, and they fledged so late in the season that most adults had already abandoned the colony. To compensate for this protracted time-window which would have created a bias in our analysis, we used only population plot count dates within five days of the range of historic mid-incubation to start-of-fledge dates for kittiwakes and murres, a slightly different subset of dates than those reported in (Arimitsu et al. 2021). We believe the updated estimates more accurately represent the breeding population. Range of historic mid-incubation to start-of-fledge dates for kittiwakes: Chisik: 19 June – 28 July; Gull: 22 June – 28 July Range of historic mid-incubation to start-of-fledge dates for murres: Chisik: 21 July – 25 August; Gull: 24 July – 24 August

LAB: Individual photos were mosaicked together for each plot using Adobe Photoshop. Landmarks were located to help define each plot and outlines were digitized in GIS. All birds within each plot were counted.

LITERATURE CITED: Piatt, J.F. 2002. Response of seabirds to fluctuations in forage fish density. USGS Final Report to Exxon Valdez Oil Spill Trustee Council and Minerals Management Service. Anchorage, Alaska. 406 pp.