10.2.7 Direct observations of crop
For some granivores species the seeds and some invertebrates within the gullet
can be identified. The feathers on the dorsal surface are blown aside and the gul-
let contents identified through the translucent skin of the neck (Newton 1967).
This works for both nestlings and adults.
10.2.8 Regurgitates
Some species regurgitate food if disturbed. For example, swifts and swiftlets can
be caught when they return to the nest whereupon they often regurgitate boluses,
which can be preserved in alcohol and identified (Lourie and Tompkins 2000).
Young herons regurgitate whole fish and other items. The nestlings are then
deprived of a meal and repeated collections from the same nest could affect
nestling survival.
10.2.9 Cafeteria experiments
Cafeteria experiments consist of providing a range of prey items and seeing
which are taken by which species. Cafeteria experiments may be carried out in
the field or with captive animals. A typical experiment would be to place out
identical patches of seed of different species on the ground (or on platforms to
reduce loss to rodents) in such a way as to minimize the differences between
patches, such as disturbance or distance to cover. The patches are then watched
to measure the number of items taken by each species or the time spent in each
patch. Other options would include providing bunches of fruit or containers
with invertebrates. For captive birds, it is sufficient to count or weigh the food
and recount/reweigh it later to measure relative consumption. However, in the
field, observations are usually necessary to ascertain the bird species involved.
Cafeteria experiments are useful for understanding some components of the
choice, but need not reflect the actual choices in the real world. Seeds or inverteb-
rates preferred in experiments may be inaccessible in natural conditions.
10.2.10Morphology
The structure of the bird can give insights into the likely foraging behavior. In
general, insectivores which pick items from foliage have fine bills, sallying insect-
ivores have wide bills, which are not deep and they often have long and stiff rictal
bristles, seed eaters have broad and deep conical bills, specialized fruit eaters have
wide gapes and mouths, birds feeding in dark conditions have large eyes, birds
having to break tough prey have thick bills and the length of the bills
of wading birds indicates the maximum depth to which they can probe. The
240 |Diet and foraging behavior