The endangered continental Black-tailed Godwit (Limosa limosa limosa) is a migratory ground-nesting wader breeding in a wide variety of open, wet habitats across Europe. Conservation research has concentrated on the causes of population decline, but we know surprisingly little about whether any resources limit local breeding populations and, if so, whether these are resources for the adults or the chicks. We collected data from 63 key breeding sites in five countries across Europe to test whether, after correcting for differences in surveyed areas, the size of Godwit breeding populations was related to environmental variables (vegetation biomass, soil moisture) or food resources for adult birds (soil invertebrates) or chicks (vegetation dwelling arthropods) measured during different times of the reproductive cycle. We found the number of Godwit territories to be positively related to arthropod abundance during the chick-hatching period. We found additional, weaker support for a positive relation between Godwit territory numbers and the abundance of soil-dwelling invertebrates (mostly earthworms) at clutch laying, but not at chick-hatching. These relationships were observed across countries, while we found little support for relationships within countries, possibly due to the smaller range in conditions that exist within countries. Both vegetation growth and soil moisture weren’t related to Godwit territory numbers. Our results suggest that food abundance for chicks, and to a lesser extent adult birds, are key factors determining the size of local Godwit breeding populations. Conservation management aiming to enhance local Godwit populations should therefore consider the impacts of management strategies on the arthropod prey of chicks.

We collected data in 63 sites in France, the Netherlands, Poland, Estonia and Finland. All sites were areas potentially hosting breeding Godwits, as indicated by survey information from the years before this study was conducted. Site selection aimed to include locations varying widely in environmental conditions both within countries and between countries to incorporate as much variation as possible in variables potentially explaining Godwit breeding numbers and representing all habitat types used by Godwits for breeding. We investigated how the total arthropod and soil-dwelling invertebrate abundance, vegetation biomass and soil moisture in two separate periods were related to the number of Godwit territories per site. As this gave a better model fit than directly using territory densities, we used as the main response variable the number of territories while statistically accounting for differences between sites in the area surveyed. Additionally, it allowed us to examine whether area-territory number relationships differed between countries. Because it is unknown if adult Godwits select a breeding area because it confers benefits to themselves or to their chicks during the rearing period we ran analyses with variable estimates at the estimated clutch-laying and chick-hatching dates.