The effect of local change in habitat quality on populations of migratory species

1. Habitat deterioration is a major problem world-wide as a result of processes such as change in land use, introduced species, human disturbance and exploitation of food supplies. Many studies have shown that habitat change can have considerable effect on the numbers of individuals using a site. For migratory species, however, the consequences for the total population cannot be deduced from local studies. 2. For a migratory species, the change in total population size ΔN, as a consequence of habitat change in the wintering area, can be calculated from ΔN = LMγd′/(b′ + d′), where γ is the expected proportional change in the number of birds using a site as a result of the habitat change, L is the area affected, M is the density of individuals using the site prior to habitat change, b′ is the strength of the per capita density-dependent breeding output, and d′ is the strength of the per capita density-dependent winter mortality. Similarly the consequences of habitat change in the breeding area can be calculated from ΔN = LMγb′/(b′ + d′). 3. The same approach can be used for predicting the consequences of improvements in habitat quality. 4. A worked example is given to illustrate how this approach could be used to predict the consequences for the total population of changes in the food supply of oystercatchers within one estuary. 5. There is a need for more measures of γ, the expected proportional change in the number of birds using a site as a result of various forms of habitat deterioration, and the strengths of density dependence.