Determining how resource use affects a species’ demography is important, especially in habitats that are being altered by anthropogenic land-use change. If changes result in species consuming resources of reduced quality, their demographic traits may be adversely affected. Generalist species are useful when investigating changes in resource availability, as they can switch to alternatives if their preferred food becomes unavailable. For species that can forage on marine and terrestrial resources, it is often not known whether a switch from marine to terrestrial resources will have negative demographic consequences. The herring gull Larus argentatus is a widespread generalist that opportunistically forages within marine and terrestrial habitats that are increasingly altered by humans. We determined marine and terrestrial resource use of gulls from 8 colonies over 2 years across south-west Scotland and Northern Ireland, using pellets and stable isotope analysis of chick feathers, which gave comparable results. Herring gulls in the study region used very little marine offshore resources; however, birds from colonies located in areas with sheltered coastlines, which provide abundant and diverse marine food from the intertidal zone, foraged more on intertidal resources. In contrast, colonies closer to built-up areas used more terrestrial resources. Herring gulls raised larger broods in colonies where they consumed a higher proportion of intertidal resources. Therefore, when generalist species switch to alternative resources available to them within their foraging range, this may come at a cost of lower breeding success.