Reproductive effort strategies in passerine birds : the role of body state
State-dependent life history theory predicts a trade-off between an individual's state and reproductive effort. The identification and effects of key state variables, however, have not been explored empirically in depth. Although there are some studies which have provided evidence that state-dependent behaviour indeed occurs, the bulk of this work is neither experimental, nor related to reproduction. Furthermore, the quantitative effects of manipulations of specific state variables are invariably unknown (McNamara & Houston 1996). This thesis explores potential trade-offs between state and behaviour and determines if energy reserves can be employed as a useful state variable. In contrast to other empirical studies, parental states (energy reserve at dawn) were experimentally manipulated using a direct method, namely changes in overnight temperature (Warming, Chilling and Control). The effects of these temperature changes were quantified using indirect calorimetry. Responses to these experimental manipulations were measured by behavioural observations, a common method in behavioural ecology, but were also quantified in terms of energy expenditure, with the aid of the doubly labelled water technique. Thus, this thesis provides a unique quantitative approach, in that it measures both manipulations and responses in the currency of energy. Individual energy reserves at dawn significantly affected resource allocation decisions the subsequent day. Birds with surplus energy upon release increased the number of feeding visits to their nestlings whilst in parallel increasing energy expenditure. Those with an energy deficit at dawn, conversely decreased nest visitation rates along with energy expenditure. There were no effects of temperature manipulations upon mass or fatscore changes over the trial period, suggesting a regulation of somatic investment at a threshold level, whilst reproductive effort was varied depending on parental state. The responses to positive manipulations (warming) were congruent across two species with differing foraging ecologies: the swallow, an aerial feeder foraging in a variable environment; and the great tit, foraging in relatively stable woodland. Thus, the behavioural and energetic responses seen here were not the result of species-specific strategies. This points the way towards a general rule within state-dependent behaviour. The trade-off identified here implies that reproduction carries a cost, and that reproductive effort will be reduced if an animal's survival is jeopardized and vice versa: a life history response, mediated by an individual's body-state. Furthermore, the response of birds to positive and negative manipulations was large enough to be readily detected, even amongst the considerable variation in energy expenditure related to individual differences. This suggests that body-state not only plays a key role in allocation decisions, but that it is comparable in the scale of its effects to other major influences on energy expenditure of free-living animals.