1. The functional response, i.e. the feeding rate as a function of prey density, links resource availability to population level demographic rates. The functional response is often central to behavioural-based population models that predict the effect of environmental changes on populations based on the assumption that each individual tries to maximise its fitness at all times. 2. The functional response is a well studied phenomenon for some groups of foragers and many mechanistic models of the functional response have been derived to account for variations in foraging behaviour. However, only a few functional response models have been applied to granivorous farmland birds and these models often have simplifying assumptions that are likely to be inappropriate for these foragers. Consequently, the functional response models that often lie at the core of behavioural-based population models are untested and quite possibly provide a poor prediction of the functional response. 3. The objectives of this thesis are to: (i) test the ability of functional response models to predict the functional response of granivorous birds; (ii) identify incorrect assumptions in these models; and (iii) derive new functional response models that better describe the observed behaviour of these foragers. 4. Here I have found that many of the underlying assumptions of simple functional response models are inappropriate for granivorous birds and several new mechanistic models have been derived that attempt to describe the behaviour more accurately. Vigilance was shown to frequently interact with incompatible behavioural processes (i.e. searching) and affect the feeding rate at low prey densities. Handling time was divided into two components (i.e. attack and process) and a simple mechanistic model was derived to predict the attack component of prey handling. The searching mode of granivorous birds was shown to be akin to a pause-travel forager at low prey densities and, lastly, habitat structure was shown to reduce the feeding rate by restricting the searching area. 5. These finding will help in the development of more biologically 'realistic' functional response models and provide a more accurate prediction of the functional response when used to predict the effect of environmental change on the intake rate of foragers. z