Survival of adult, territorial Longtailed Wagtails Motacilla clara : The effects of environmental factors and individual covariates

Abstract

The Longtailed Wagtail is a non-migratory African passerine that is confined exclusively to small, fast-flowing rivers in a largely arboreal environment. The breeding adults hold permanent, life-long, linear territories in their riverine habitat and this makes it easy to locate colour-marked birds. They are confiding by nature and permit close approach, often to less than 10 m, and this allows their unique permutations of colourrings to be read. Using data from the 21 year period, 1 August 1978 to 31 July 1999, of a dozen territories it has been shown that the breeding territories have not changed at all, even though there has been a continual, but slow turnover of territory holders. A total of 109 territorial adult birds were monitored for a total of 1121 bird-quarters and survival was estimated for each of four quarters in a year. The average survival rate is estimated at 68.8% yr -1 (95% confidence limits: 63.3% to 69.3%) and this is high for such a small bird (approximately 20 g) and there have been some remarkably long-lived individuals, e.g. 10 to 12 years. In this paper, a generalized linear model is built of the survival of territorial adults. It is shown that bigger birds have a higher survival rate and that there are seasonal differences in survival that are ascribable to the cost of breeding and possibly cost of moult. There is an underlying long-term quadratic trend in survival that is related to increasing environmental degradation and decreasing chemical pollution.