Diet choice in pre-migratory pink-footed geese (Anser brachyrhynchus) and greylag geese (Anser anser)
Pre-migratory Icelandic pink-footed and greylag geese staging in Aberdeenshire, Scotland, feed on winter barley (Hordeum vulgare), winter wheat (Triticum spp) and sown pastures of Italian ryegrass (Lolium multiflorum). Previous studies, elsewhere, have reported that arctic-nesting geese need to build their reserves of energy and nutrients at their staging site since birds feeding during the breeding period are more vulnerable to nest predation and since, in some instances, there is virtually no food in the breeding area. Hence, geese are expected, when feeding at their spring staging sites, to select their food plants in such a way as to maximise their intake of both energy and nutrients. This thesis investigated the diet choice of pre-migratory Icelandic pink-footed and greylag geese. The chemical composition of the food plants eaten by geese and the digestibility to geese of these food plants as well as the intake of food by geese, were measured. Food intake on the fields of the various food plants was also related to the energy expenditure on such fields by measuring the intake of organic matter, energy and protein per peck and per pace on each field. Furthermore, diet choice of captive geese was also measured with the aim of investigating what cues geese use to select their food plants as well as investigating the role of field-related factors in the choice of diet by geese. Neither the chemical composition nor the digetibility to geese of the food plants were significantly different between barley, wheat and ryegrass. Similarly, food intake was not significantly different between geese feeding on the different food plants. When food intake was related to the indices of energy expenditure (pacing and pecking), however, ryegrass was the most profitable food plant with barley being more profitable than wheat in this respect. Nonetheless, barley was the least preferred food plant by both wild geese and captive geese suggesting that its relative rejection in the wild is not entirely due to field-related factors. Moreover, unlike wild geese, captive geese did not show a preference for ryegrass over wheat suggesting that the relative rejection of wheat in the wild may be a consequence of possible vartiations in field-related factors such as disturbance and the great cost of walking on wheat in relation to ryegrass. In conclusion, geese would maximise their net rate of energy gain by feeding exclusively on ryegrass (since they needed to walk less on ryegrass fields for the same amount of energy intake on cereal fields). Nevertheless, although geese spent more time feeding on ryegrass fields, they sometimes fed on cereal fields. This may be an attempt to diversify their diet in order to ensure the intake of all their required nutrients.