Table 1.

Sampling sites in the Baltic Sea Area where data on habitat, population density, and nest substrate were collected

Sampling site	Geographic location	ICES	Abb.	Salinity	Salinity class	Temp. (°C)		Latitude	Longitude	2012		2013		2014
				PSU		Early	Late			Early	Late	Early	Late	Early	Late
Kristineberg	Bohuslän, Western Sweden	23	KR	22	High	16.1	19.1	58°24′ N	11°46′ E	T/S/A		T/S	T	T/S/M
Vrinners	Jutland, Kaløvig, Denmark	22	VR	20	Intermediate		19.6	50°14′ N	10°30′ E		A
Kerteminde	Funen, Great Belt, Denmark	22	KE	17	Intermediate	18.1	19.0	55°44′ N	10°65′ E		T/S/A			T/S/M
Poel Island	Bay of Mecklenburg, Germany	22	IP	12	Intermediate	15.1	18.2	53°99′ N	11°48′ E		T/S/A	T/S	T/S	T/S/M	T
Gotland	Gotland Basin, Sweden	28	GO	7	Low	16.6	17.5	57°78′ N	18°94′ E	T/S/A		T/S	T/S	S/M	T
Tvärminne	Archipelago, Southern Finland	29	TV	5	Low	13.5	15.2	59°82′ N	23°14′ E	T/S/A		T/S	T/S	T/M	T/S

Notes: Indicated are: sampling sites, geographic locations, international council for the exploration of the sea (ICES) subdivisions, abbreviations for sites (Abb.), salinity (PSU) and salinity classes, mean surface water temperature pooled over 3 consecutive years (2012–2014) early and late during the breeding season, and coordinates (latitude and longitude). Data collection of different variables vary between populations, year, and time points during the breeding season: T (transect data collected), S (body size measurements), M (morphometric analyses), and A (mating assay conducted). Blanks denote missing data.