BRINK B VAN DEN, BIJLSMA R & HAVE T VAN DER (2004) The effect of rainfall on condition, moult and survival of Barn Swallows Hirundo rustica in southern Africa. LIMOSA 77 (2): 109-120.

During the winters of 1992/93, 1993/94 and 1994/95, the authors studied Barn Swallows in their wintering quarters in Botswana. At a roost containing up to three million swallows along the Boteti River, c. 30 km south of Maun, and during a few days at the Shashe Dam, south of Francistown, and at the Gaborone Dam, a total of 18,439 swallows were trapped and ringed and of 1200 birds biometrical data were recorded. Four birds were recaptured, indicating fidelity to specific wintering grounds. In this paper, data on moult and body weight were related to variations in rainfall.
      The water level of the Boteti River fluctuates with variations in the influx from the nearby Okavango Delta and with local rainfall. In 1992/93 the water level was high and rainfall was moderate (rainy at first, dry later during the study period). Extensive reedbeds were used by 100,000-500,000 swallows. In 1993/94 the water level was lower, and there were moderate showers during the entire study period. The reedbeds were dry but were still used by two to three million swallows. In 1994/1995 the Boteti River had fallen dry and there was hardly any rainfall. The reedbeds were burnt down by local people and only a few hundred swallows were using the remnants. Rainfall had a strongly positive effect on availability of insects as food for the swallows.
      Changes in the amount of rainfall and food availability both within and between seasons were linked to changes in body weight. The proportion of juveniles was high in 1992/93 (72.9%) and 1993/94 (80.9%), but was considerably lower in 1994/95 45.4%), suggesting lower survival rates compared to adults in periods of drought with low food availability. Moult rate of primaries and rectrices was fastest during the relatively wet season of 1992/93 and slowest during the dry season of 1994/95 (Fig. 3). Juveniles lagged behind one to two primaries and started moulting rectrices in late December, when adult primary moult scores were 15-25 already. They also showed lower moult rates but with similar differences between seasons.
      It is suggested that droughts are connected to habitat destruction by humans and large roosts in reedbeds are threatened by transformation of marshes into cornfields with growing human population. A shift towards smaller roosts in other vegetation types like elephant grass may result in higher vulnerability to predation by non-human predators, but may also decrease the losses to hunting and aerial sprays against Red-billed Quelea's Quelea quelea, which also use reedbeds in large numbers. Barn Swallow Hirundo rustica

[free pdf] [dutch summary]