How do crop types influence breeding success in lesser kestrels through prey quality and availability? A modelling approach.
In the middle of the 20th century, the colonial lesser kestrel Falco naumanni experienced a marked decline in its western palaearctic breeding range. Although this decline has been attributed to changes in land use influencing breeding success through lowering the abundance and quality of prey, a quantification of these effects has not yet been undertaken. To study how these two key factors influence breeding success, an individual-based model, which translates the hunting performance of each adult foraging trip into nestling growth and enables the effect of crop types (i.e. cereals, sunflower, grasslands, olive groves), on the breeding success of the lesser kestrel to be quantified, was developed and tested during 1999-2001 in Spain. Both the collected field data and published inferential and experimental studies were used to parameterize and validate the growth model. Model results demonstrated that adult provisioning rates provide very little information on the relation between prey availability and breeding success. On the other hand, the model indicated that small differences in prey abundance among crop types cause large differences in breeding success, highlighting the importance of crop composition in the vicinity of lesser kestrel colonies. Mean prey biomass is an even more important influence on breeding success. Our results indicated a minimum mean prey size of 0.6 g, below which colonies become population sinks. Synthesis and applications. We developed an individual-based model that simulates the growth in body mass of a lesser kestrel brood as a function of prey abundance and prey size. These two key factors define a parameter space in which the conservation status of breeding colonies can be evaluated. We applied the model to six lesser kestrel breeding colonies with different mean prey size and different agricultural land use influencing prey abundance. Our model suggests that the habitat quality of two of these colonies is sufficient for population persistence, while management actions to increase habitat quality are required in the remaining four colonies. An increase in the availability of prey-rich habitats such as field margins would achieve this goal without imposing too great a cost on the local farming community.