An evaluation of the use of a spooned spatula to asses the diet of cardueline nestlings
Published: Volume 55(1), June 2008. Pages 49-57.
Aims: Sampling diet of passerine nestlings can be associated with bias and danger to the birds, according the methods adopted, but an understanding of diet is essential to development of conservation measures. This paper describes a new technique for examining the diet of cardueline finches that store food in gullet pouches prior to digestion. We applied this method to nestlings of goldfinches Carduelis carduelis of 31 nests. Goldfinch nestlings were selected because the diet of their chicks is poorly known.
Location: Our study was conducted near Sagunto in eastern Spain (39º 42' N, 0º 15' W). The area was located within extensive orange monocultures that are widely spraying in the east of Spain.
Methods: A small spoon was used to obtain the food directly from the gullets. Samples were obtained from 3 - 10 day old nestlings and samples from nestlings in the same nest were combined. Animal prey seeds were separated. Seeds of the same species were put together. We dried each sample at 103 ºC for two hours to assess dry weight. After drying, each food type from a nest was weighed. In order to test the method effect on the nestlings we compared the breeding success between the sampled nests and another 31 nests that were not manipulated and hatching close to the sampled nests.
Results: Thistles of two species, Sonchus oleraceus and Carduus pycnocephalus were the main food and the contributions to the diet varied across the breeding season. The two species of thistles showed opposite trends. The contribution of animal foods in the diet decreases as the season progresses. Daily survival rates and the average number of fledglings per nest were similar in both groups of nests suggesting that the use of a spoon to analyse the diet have no impact on the nestlings.
Conclusions: The main value of this method is that food samples are obtained before swallowed and thus the food is easy to recognize. This is an inexpensive method because the spoon tool can be used repeatedly.