ten different arctic breeding wader species the eggs were produced from food
originating from tundra habitats rather than from coastal habitats in temperate
regions.
10.3 Determining prey size
10.3.1 Direct observation of prey size
Direct observations can be sometimes a reasonable method of estimating prey
size, for example, in relation to bill length. The relative measure (i.e. 2.5 times bill
length) can be converted to actual length by multiplying by mean bill length
(there may be a sex difference in bill length). The accuracy of this method can be
assessed by holding prey items next to a stuffed bird and having observers estim-
ate prey length in similar conditions to those used in the field. If the feeding birds
have been videoed or photographed then these measurements can be made from
the images. Sitters (2000) placed canes marked at 10 cm intervals on mudflats so
he was able to assess the bill length of individually marked Oystercatchers
Hematopus ostralegusby comparing on a video screen. He then used these measures
to estimate the size of prey taken.
10.3.2 Determining size from prey remains
Prey remains can sometimes be found, and used to determine the size and species
of items taken. Thus parts of carcasses left by predators can be measured, as can
shells of molluscs and hard parts of other prey. For herbivores the size of the leaf
at the base may give a measure of the size of piece bitten off. Thus Summers and
Atkins (1991) measured the petiole (leaf stem) widths of Sea Aster Aster tripolium
and showed the petiole width correlated with length. They used this measure to
estimate the size of leaves eaten by Brent Geese Branta bernicla.
10.3.3 Determining prey size from regurgitates
Regurgitated prey can be measured directly. Quinney and Ankney (1985) col-
lected the boluses from parent Tree Swallows Tachycineta bicolorreturning to the
nest. The collected insects were placed in 70% alcohol and at a later date the
lengths of the insects could be measured.
10.3.4 Measuring fragments in pellets, droppings, or stomach
Large items within droppings can be measured with calipers. Smaller items can be
measured using a graticule eyepiece in a microscope, which has been calibrated
using a slide with a known scale. It is necessary to decide which items are suitable
for measurement. These should be those that can be measured consistently and
242 |Diet and foraging behavior