Emerging properties on the individual level: modelling the reproduction phase of the European robin Erithacus rubecula

It has proven difficult to integrate more than a limited number of internal as well as environmental factors governing the reproductive success of birds into a simulation model. We developed an individual based model of the reproductive phase of the European robin Erithacus rubecula extending the given possibilities. A priority value driven activity scheduling mechanism permits the inclusion of a variety of behavioural as well as physiological aspects in relation to the local environmental situation represented by a grid map, food availability and microclimate data sets. The life history patterns of the individual robins emerge as a result of the interaction of their behavioural repertoire, activity selection mechanism, temperature and food availability. The model integrates energy-budgets, activity scheduling on the individual level and reproductive success as well as spatial distribution patterns on the integration level of the population in a coherent way. One of the main aims of the model was to identify constraints occurring during the reproductive phase. We analysed the intensity and extent of critical situations during the time when the males have to feed the young at the utmost possible level and at the start of breeding in late spring. Investigation of the effects of a possible climate change in Northern Germany emphasises the very critical timing of reproductive activities in relation to temperature and caterpillar emergence. These effects are detectable on the individual level more precisely than on the population level. They are levelled out by averaging the state of the whole population. Similarly, a simpler model basing on differential or difference equations would not suffice to explain the reproductive success as the result of the diverse interrelated influences and decision schemes.