BREEDING BIOLOGY OF THE THORN-TAILED RAYADITO (FURNARIIDAE) IN SOUTH-TEMPERATE RAINFORESTS OF CHILE

We conducted a study of the breeding biology of the Thorn-tailed Rayadito (Aphrastura spinicauda) in secondary forests on the continental island of Chiloe ´ (42 8 S), southern Chile. Rayaditos are small insectivorous furnariids inhabiting the south-temperate forests of Chile and Argentina. We followed the reproduction of rayadito pairs breeding in nest-boxes. Rayaditos build their nests mainly of rhizomes and stems of epiphytic vines, grasses, and hairs during periods of at least a week, and show a marked population asyn- chrony in laying dates of more than two months (October-December). Rayaditos lay clutch- es of 3-6 eggs with a mode of 4 and laying occurs on alternate days. Eggs are 50% larger and hatchlings are 30% larger than expected from allometric equations. Most broods hatch synchronously. Nestling growth curves adjust well to logistic functions and at 2 weeks nestlings attain masses similar to asymptotic values. Nestling growth, which occurs over 3 weeks, is 27% slower than expected from allometry. Fledglings attain adult size with respect to tarsus length, but have less developed plumage and higher body mass than adults. Ray- aditos exhibit clutch and brood reduction, suggesting possible food limitation. The protracted breeding periods may preclude second breeding attempts for most pairs in Chiloe ´. There is evidence for declines in parental quality with season. The low seasonal fecundity, large eggs, and prolonged dependence periods of a truly south-temperate species like the Thorn- tailed Rayadito reflect a 'slow' life history similar to that of tropical passerines.