Reproductive Performance and Nestling Growth of the Brown Shrike Subspecies Lanius cristatus superciliosus in Hokkaido.

Abstract

A subspecies of the Brown Shrike, Lanius cristatus superciliosus, has undergone a severe decline in Hokkaido for the past three decades. I studied the reproductive performance and nestling growth during the breeding seasons in Hokkaido. I examined 41 active nests during 1992-1996. The mean nesting success was 53.7%. The mean clutch size was 5.3, and the mean number of eggs hatched per nest was 5.1. The mean number of fledglings per successful nest was 4.4, and the mean fledging success per successful nest was 90.3%. The main cause of nesting failure was depredation by unknown predators. The frequency of parasitization by Cuckoos, Cuculus canorus, was very low, and Brown Shrikes either removed the cuckoo egg or deserted their nest containing one Cuckoo egg. The gains of body mass and tarsus length for 75 nestlings from 15 nests (a total of 273 nestlings) were fitted to Ricklefs's logistic regression curves from the hatching day to 11 days after hatching. The asymptotic mass (g), growth rate, and age (days) at the inflection point were 27.1, 0.4, and 5.7, respectively. The asymptotic tarsus length (mm), growth rate, and age (days) at the inflection point were 24.9, 0.3, and 4.4, respectively.