Ligature use in food studies of precocial birds-methodological results from Antarctic skua chicks

Abstract

This paper is the first methodological investigation of the ligature application for food studies in semi-precocial birds. In a population of Brown Skuas, South Polar Skuas and mixed pairs, ligatures were attached to chicks for a period of 28 hours. Of 408 theoretical sampling visits, 8% could not be carried out. The number of times the ligature was applied did not cause differences in mortality of chicks nor did I find a growth difference for treated and untreated chicks . The method did not work equally well for three skua forms with different diets. Parent birds disgorged food equally often to ligatured and control chicks . This method is probably less accurate when used for diets with a high proportion of fish. While I was applying the ligature, body weight loss showed a non-linear decline, with a drop appearing in the first hours after application. The chicks may compensate for weight loss . In chicks whose stomachs were flushed after ligature use, no food was found, leading to the assumption that the ligature method is highly efficient. A projection of the total food consumption by field data revealed values of only 45% (Brown Skua) and 25% (South Polar Skua) of that of the model by Drent and others (1992) . The advantages and disadvantages of ligaturing are discussed in regard to its further use in precocial birds.