Food Sampling Biases Associated with the "Ligature Method"

Abstract

-Biases associated with the ligature technique of sampling food from nestlings were quantified for the Gray Catbird and Brown Thrasher. Food samples collected after a one-hour interval did not accurately represent the diet because nestlings with ligatures gaped less intensely, gasped when trying to gape, and disgorged food from their throats. Food samples collected from ligatured nestlings after one-hour intervals were compared with samples obtained after each feeding trip and with observations of nestlings without ligatures. Biases in the former method were demonstrated by three different analyses: average volume of food delivered/ nestling/h, size of individual food items, and number of nestlings fed/h. Ligatures did not seem to affect appreciably diet composition. Several methods have been employed to collect food samples from nestling passerines. Analysis of stomach contents (e.g., Evans 1964, Crase and DeHaven 1977) has the disadvantages that birds must be sacrificed, food items obtained are greatly fragmented, and samples are biased in favor of items less readily digested (Hartley 1948, Dillery 1965). Also, sizes of individual food items and rate of feeding are difficult to determine. Analysis of feces (Evans 1964) does not require sacrificing birds, but identifying and quantifying diet composition are even more difficult and incomplete than stomach content analysis (Hartley 1948). Nest boxes with windows have been used for cavitynesting species (Betts 1955, Royama 1966); food items delivered are directly seen or photographed with a perch-triggered camera. When many items are brought to the nest at once, however, only a few can be seen and identified (Betts 1956). Use of artificial nestlings to collect food was moderately successful for Betts (1956), but if many food items were brought to the nest in one trip, not all could be collected. In other studies, this method failed (Evans 1964, Orians 1966). In many recent studies of nestling foodhabits, constrictive ligatures have been placed around the nestlings' necks to prevent them from swallowing food delivered by the parents. Food that accumulates in the nestlings' throats is collected and later analyzed. This method has the advantage that food items are usually intact, which facilitates their identification and size determination. Also, individual nestlings can be used repeatedly. Various materials have been used for ligatures, including plasticcoated wire (Owen 1956), metal bands (Kluyver 1961), pipe-cleaners (Orians 1966), light string (Westerterp 1973), heavy thread (Pinkowski 1978), and enameled copper wire (Best 1974, this study). Several problems with the ligature technique have been reported. Orians (1966) and Willson (1966), using pipe-cleaners, stated that ligatures could not be used on nestling Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) less than three days old because either the nestlings were strangled or food items slipped by the ligatures. Food slipped past ligatures in older nestlings of other species (Owen 1956, Walsh 1978). Ligatures have been reported to affect feeding behavior. Young Black-capped Chickadees (Parus atricapillus) would not gape for food when wearing metal collars (Kluyver 1961). Orians and Horn (1969) collected more food from nestling Red-winged (Agelaius phoeniceus) and Yellow-headed blackbirds than from Brewer's Blackbirds (Euphagus cyanocephalus), possibly because the latter would not beg as vigorously when wearing ligatures. Another problem is that adult birds may remove food from nestlings' mouths and eat it (Van Balen 1973, Robertson 1973). Nestling Yellowheaded Blackbirds disgorged food if the sampling period was too long or food delivery too rapid (Orians 1966). During a study of Gray Catbird (Dume-