1.3. Prey choice and timing of breeding

According to the life history theory, animals tend to increase their genes in future generations by maximising their reproductive output, measured as the number of offspring surviving until reproductive age (Williams 1966). This can be reached by making the right decisions in the different breeding stages. Firstly, the predator has to get extra energy to produce eggs and rear its young by choosing its prey properly. This can be achieved by searching for prey that give the most energy/time used for its provision. The main prediction of this optimal foraging theory states that predators should continue consuming the main prey if available, irrespective of the abundance of secondary prey (Pyke et al. 1977, Pyke 1984, Stephens & Krebs 1986). Predators that have a short searching time for prey compared to handling time should specialise in their foraging and generalise in the opposite situation (Reynolds et al. 1988). Therefore goshawks should specialise in spring and hunt mostly grouse and generalise in summer when more easily catchable bird are available. Secondly, predators should adjust their breeding so that a maximum number of nestlings reach independence (Lack 1954). On the other hand, the value of this decision is not manifested until the first breeding attempt of the offspring. Therefore, the survival of offspring during the first year, being lowest then, largely determines the value of parental choices. Birds can time their breeding period so that the highest demand for food in the nestling phase coincides with the highest supply (Lack 1954), or they can start as early as possible and give the offspring time to prepare for harsh winter conditions. This might be profitable in raptors, whose style of life, hunting, demands a lot of experience (Perrins 1970, Newton 1986, Village 1990).