This study examines the responses to human presence of the most abundant shorebird species in an important coastal migration staging area (Los Lances Beach, southern Spain). Long‐term census data were used to assess the relationship between bird abundances and human densities and to determine population trends. In addition, changes in individual bird behaviour in relation to human presence were evaluated by direct observation of a shorebird resident species. Our results show that a rapid increase in the recreational use of the study area in summer has dramatically reduced the number of shorebirds and gulls which occur, limiting the capacity of the site as a post‐breeding stop‐over area. In addition, the presence of people at the beach significantly reduced the time that resident species spent consuming prey. Negative effects of human presence on bird abundance remain constant over the study period, showing no habituation to human disturbance in any of the studied species. Moreover, although intense human disturbance at Los Lances Beach occurs mainly in summer, the human presence observed is sufficient to have a negative impact on the long‐term trends of a resident shorebird species. The impacts of disturbance detected on shorebirds and gulls may be reversible through management actions that decrease human presence. We suggest a minimum distance of 80 m for any track or walkway from those areas where shorebirds are usually present, particularly during spring and summer, as well as an appropriate fencing in the most sensitive area.

Summary: The Balearic Shearwater Puffinus mauretanicus is considered one of the most threatened seabirds in the world, with the breeding population thought to be in the range of 2,000–3,200 breeding pairs, from which global population has been inferred as 10,000 to 15,000 birds. To test whether the actual population of Balearic Shearwaters is larger than presently thought, we analysed the data from four land-based census campaigns of Balearic Shearwater post-breeding migration through the Strait of Gibraltar (mid-May to mid-July 2007–2010). The raw results of the counts, covering from 37% to 67% of the daylight time throughout the migratory period, all revealed figures in excess of 12,000 birds, and went up to almost 18,000 in two years. Generalised Additive Models were used to estimate the numbers of birds passing during the time periods in which counts were not undertaken (count gaps), and their associated error. The addition of both counted and estimated birds reveals figures of between 23,780 and 26,535 Balearic Shearwaters migrating along the north coast of the Strait of Gibraltar in each of the four years of our study. The effects of several sources of bias suggest a slight potential underestimation in our results. These figures reveal the urgent need to reformulate the population viability analysis for the species, and then if necessary reconsider its conservation status.