RADIO-PTILOCHRONOLOGY - TRACING RADIOACTIVELY LABELED FOOD IN FEATHERS

Ingested radioactive sulphur, S-35, in amino acids. is readily incorporated in growing feathers. In a growing rectrix, or tail feather, the radioactivity will be deposited in the part of the feather that develops during the day(s) after ingestion. In an autoradiographic picture it will appear as a dark band across the feather. In a combined laboratory and field study of Willow Parus montanus and Marsh Tits P. palustris, I used this (1) to assess the accuracy of ptilochronology and in an effort to improve that method and (2) to test the technique's usefulness for studies of food retrieval in food-hoarding bird species. The study confirmed that one growth bar (consisting of one light and one dark portion) represent one day's growth of the feather. It also demonstrated that radioactive labelling can be used to pinpoint the exact date at which a particular growth bar was formed. It can also be used to identify simultaneously grown bars in successively plucked rectrices thus extending the period during which ptilochronology can be used, e.g. to study birds' nutritional status over the whole winter. During selective provisioning of radioactive seeds to either partner of a Willow Tit pair, the bird first ate a couple of seeds and then stored the rest. However, some radioactivity showed up in the rectrix of the partner (who was only fed unlabelled seeds on this occasion) indicating that it had been able to pilfer the partner's caches.