Population ecology of barn swallows

The barn swallow is an excellent model species to investigate large-scale population dynamics of bird populations. In this study, we aim to identify the breeding habitats factors affecting productivity in different regions and exchange of individuals among barn swallow populations in Switzerland.

Aims

This study aims to evaluate the current state of barn swallow populations in Switzerland and to enhance our understanding of their population dynamics. To reach this goal it is crucial to identify the factors affecting the population dynamics and to investigate their relative importance. We focus on the following population parameters:

a) Breeding success (number of breeding attempts, clutch size, number of fledglings, nestling body mass).
b) Annual survival of adults and juveniles in terms of return rates to the local study populations.
c) Dispersal of juveniles within and between study populations.

The findings provide information on conservation measures applicable to barn swallow populations in Swiss farmland.

Approach

The study was carried out from 1997–2004. In 12 study areas in different regions of Switzerland, data on barn swallow populations breeding in farms were collected for at least four years. We visited all barn swallow nests weekly to record the relevant breeding parameters. Moreover, all nestlings and as many adults as possible were caught, ringed and re-trapped. In 1999 and 2001, the agricultural land use was mapped for all the farms in the study areas. Data on behaviour and body condition of adults and juveniles after the breeding season were recorded for several years by regular counts and trapping for barn swallows at roosts in reed beds.

Significance

Populations of barn swallows declined throughout central and western Europe over the last decades. Therefore, the European Union for Bird Ringing (EURING) launched a barn swallow project to investigate the mechanisms underlying this population decline. International studies on the breeding biology, population dynamics, on the behavioural ecology and body conditions during migration and at stopover sites as well as on the situation in the African wintering quarters were established to pinpoint the critical periods in the annual cycle and the factors affecting population dynamics.

Results

Years with the strongest general decline in numbers of breeding pairs coincided with a low reproductive output the year before and/or a low annual survival of barn swallows. Periods of adverse weather conditions crucially impaired breeding success. Accordingly, a high amount of rainfall negatively affected population size the following year. However, such losses can be balanced within a few favourable years.

Number of breeding attempts and an early start of egg laying positively affected reproductive output. Breeding colonies in farms with cattle declined less than those in buildings without cattle, and nests in the presence of cattle yielded higher annual reproductive output than nests in buildings without cattle.

Local adult survival rates were higher in males than females. Reproductive success was an important factor affecting breeding dispersal and population turnover. While unsuccessful males usually remained in the local population, many unsuccessful females emigrated to other sites.