spacing of the bill lamellae of dabbling ducks indicates the likely size range of
prey. Similarly sex differences in morphology are often related to sex differences
in foraging behavior (Durell 2000).
10.2.11Neck ligatures
Neck collars have been applied to chicks so they cannot swallow food. This food
is then collected and the collar removed. It has the advantage that the prey is
undigested. Collars may be made of pipe cleaners, thread, or copper wire. If feeds
are infrequent, then it may be necessary to compensate by feeding the chick.
There can be increased mortality of chicks, adult birds may remove prey from
collared nestlings, and smaller items may be swallowed (Rosenberg and Cooper
1990). Less food was delivered to nestling Grey Catbirds Dumatella caralensis
with ligatures thus underestimating prey intake and larger items were often dis-
gorged thus biasing diet ( Johnston et al. 1980). This method is now rarely used
due to the welfare and conservation considerations from both the loss of food
and the risk of damaging the chicks.
10.2.12Emetics and flushing
Emetics and stomach flushing do not usually kill the bird, but occasionally they
do and these methods are usually considered too invasive. As an emetic 0.8 cm^3
of 1–1.5% antimony potassium tartrate per 110 g of body mass is administered
via a syringe usually through a vaseline-coated narrow flexible plastic tube
pushed gently down the esophagus. The bird is then placed in a dark box with a
carpet of absorbent paper and released 15–20 min later. Of 3419 birds of
82 species studied in Venezuela, 3033 diet samples were obtained, of which 2712
had recognizable food, but 70 birds died (Poulin et al. 1994). Nectar is difficult
to detect but pollen grains are obvious.
Stomach flushing apparently has lower risks. A vaseline coated narrow plastic
tube is inserted into the stomach and lukewarm (often weak saline) water
pumped in through a syringe until the contents of the esophagus and stomach
are voided. In many countries use of both emetics and flushing would be illegal
without permits.
10.2.13Isotope differences between habitats
Marine, freshwater, and terrestrial foods typically differ in isotope “signatures”
(see Chapter 9 for description of methods). These can then be used to identify
likely feeding habitat. Sampling feathers grown during particular periods can
then indicate whether birds concerned were feeding on marine, freshwater,
or terrestrial foods. Thus Klaassen et al. (2001) showed that for each of
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