The objective of this dataset was to assess the diet of black-legged kittiwakes (Rissa tridactyla) in the Canadian Arctic using stomach content analysis and stable isotope analysis from liver tissues of black-legged kittiwakes collected near Mittimatalik (Pond Inlet), Nunavut, and Qikiqtarjuaq, Nunavut, in 2021. This resulted in two datasets: 1) Black-legged kittiwake diet data, which includes the date the bird was dissected, the bird ID number, the order, family, species (if known) of the prey item, the otolith number (if applicable), the status of the otolith (full, tip, greater than half), and the width (mm) and length (mm) of the otolith (if applicable); and 2) Stable isotope values of black-legged kittiwakes in the Canadian Arctic, which includes the location collected, year collection, common and scientific name of the species, sample ID number, age (if known), stable carbon (δ¹³C) and nitrogen (δ¹⁵N) values, and atomic C:N. 

Black-legged kittiwakes were collected in areas where birds were actively feeding near Qikiqtarjuaq (n = 20) and Mittimatalik (n = 19), Nunavut, in late August 2021 by Environment and Climate Change Canada and Inuit collaborators. The nearest colony (i.e., likely breeding origin) for birds collected near Qikiqtarjuaq is Akpait National Wildlife Area, and for Mittimatalik, the nearest colonies are Niaqunnguut (Cape Graham Moore) and Cape Hay. Carcasses were frozen within a few hours of collection and shipped to Acadia University in Nova Scotia, Canada, for dissection. These samples were collected as a part of a long-term monitoring program and are being used in a suite of other studies (e.g., contaminant analyses). Unused portions of each specimen were stored in the National Wildlife Specimen Bank in Ontario, Canada for future research. During dissection, the gastrointestinal tract (proventriculus and gizzard) was removed, re-frozen and shipped to McGill University, Quebec, Canada, for dissection. Each gastrointestinal tract was slit longitudinally along the length of the tract and all contents were flushed into a glass beaker using tap water. Beaker contents were then separated into glass petri dishes, and all diet items visible to the naked eye were removed and preserved in 70% isopropyl alcohol until identification and measurements.

Sagittal otoliths were photographed and identified to species, and then length (measured along an axis parallel to the sulcus) and width were measured (mm) using the software ImageJ (https://imagej.net/ij/). Fragmented otolith pieces were matched with their other halves when possible, and if not, length was estimated using the known width in the following equation (for Arctic cod only).

Otolith length (mm) = (2.8481×otolith width (mm))-0.9768

For all other prey species (i.e., invertebrates), samples were identified to the lowest taxonomic level possible. To avoid an overestimation of invertebrate species, we counted the number of whole invertebrates and/or the number of heads to provide a minimum abundance for each sample.

To examine stable carbon (δ¹³C) and nitrogen (δ¹⁵N) values in liver tissue, livers were removed at the time of dissection and sent to the National Wildlife Research Centre in Ottawa, Canada. There, samples were homogenized using chemically cleaned (triple hexane and acetone rinsed) Polytron shafts (SOP-TP-PROC-07I), then frozen at -40°C until freeze drying. Stable isotopes were analyzed at the Ján Veizer Stable Isotope Laboratory at Ottawa University (https://www.uottawa.ca/research-innovation/isotope/services). All stable isotope ratios (¹³C/¹²C, ¹⁵N/¹⁴N) are expressed in per mil (‰) standard delta notation (δ) relative to international standard using the following equation: δX = [(Rsample/Rstandard) -1] x 10^-3, where X is either δ¹³C or δ¹⁵N. Instrument accuracy was determined using international standards IAEA-N1, IAEA-N, USGS-40 and USGS-41 for δ¹³C and IAEA-CH-6, NBS-22, USGS-40 and USGS-41 for δ¹⁵N, where the mean difference from the certified values was 0.1 and 0.02‰ for δ¹³C, 0.1 and 0.01‰ for δ¹⁵N. The precision, determined as the standard deviation of replicate analyses within each standard, was <0.1*‰* for δ¹³C and <0.1*‰* for δ¹⁵N.