Breeding numbers collected in 12 common tern Sterna hirundo colonies in the Firth of Forth, Scotland, along with sprat landings data for the area, were used to investigate how the dynamics of a shared prey resource may affect different colonies in a region. Between 1969 and 2010, breeding numbers fluctuated much more at individual colonies than across the region as a whole, with the largest colonies showing opposite trends, suggesting relocation by birds. This indicates that data from individual colonies may be less useful than regional numbers when using seabirds as indicators. Tern breeding numbers in the region were reduced when the sprat stock ( Sprattus sprattus) collapsed in the early 1980s after targeted fishing, but recovered during recent decades when the stock was unfished. This should be considered for reopening the Firth of Forth sprat fishery, as well as in the management of other shared prey stocks.