The influence of weather on the body mass of migrating swallowsHirundo rusticsin South Wales

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Abstract
Swallows were tape lured into mist netting when coming to roost in a Phragmites swamp at a coastal site in south west Wales between July and October 1988. In total, 1214 Swallows were caught on 40 nights, and 716 were weighed and measured. A low retrap rate indicated that the day‐to‐day turnover of birds at the site was probably large. Daily changes in total catch, estimated flock size and Swallow body mass were assessed in relation to daylight weather conditions (wind speed and direction, rainfall amount and duration, temperature, sunshine) measured at a nearby meteorological station. Covariance between different weather features was small, so that their correlations with Swallow biology could be assessed separately. Estimated flock size and catch declined significantly with increasing rainfall, but more so with increasing wind speed. Wind direction had no discernible effect. Mean body mass varied by up to 14% of the overall mean (19.5 g, range of means 18.1 to 22.3 g). Although age and body size had significant effects on individual mass, they were not responsible for day‐to‐day variability. Instead, the most significant correlate with mass change was date, with mean mass increasing on average by 0.03g per day between July and September. However, weather patterns caused marked fluctuations around this trend, mean mass decreasing significantly with increasing windspeed and rainfall, but increasing with maximum daily temperature. Together, date and these meteorological factors explained 84% of the variance in daily mean mass in a multiple regression which fitted the data closely. Mass still showed highly significant correlations with either date or weather conditions even when variability due to other significant factors had been removed. More information is required to assess the reasons for these changes, but fluctuations in weather conditions are likely to affect the supply of aerial insects, while the progressive increase in mass could reflect either pre‐migratory conditioning, improved foraging skills in juvenile birds, or the development of a lipid reserve as insurance against adverse conditions.