Behaviour changes could threaten swallows’ survival.

A new study has revealed the impact of parasitic infection on the flight patterns of swallows and the consequences for their survival.

Although the swallows appeared visually unharmed, tracking of birds with avian blood parasites revealed significant consequences to the swallows’ flight movements.

Sub-clinical infections, such as parasites, can have a number of effects on infected birds, including lethargy, weakness and a restricted radius of action. These symptoms can be subtle, but they have noticeable impacts on the birds’ abilities to forage, reproduce and survive.

Researchers from Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) and the University of Potsdam sought to better understand the effects of these infections on swallows and how it impacts their movements.

Over the course of several years, the team captured house martins and barn swallows for the project. The birds were analysed for their parasite infection load and fitted with mini-transmitters that send high-resolution data to an ATLAS tracking system.

The swallows were also analysed for a capture-recapture study, which would track their long-term developments and the effects of infections on their survival.

Tracking data revealed that infected birds of both avian species had a reduced range of flight compared to birds which were not infected. Birds which were infected rested more and spent less time foraging.

By reducing the range and duration of their flight, these birds would often have to forage in agricultural areas which were closer to their colony. This left them with habitats where insects were less abundant and less food could be found.

In contrast, birds which were not infected were able to avoid landscapes where food was more scarce.

The study also revealed that between 11 and 76 per cent of swallows become infected with parasites each year. With the parasitic infections now linked to behavioural changes, researchers have raised concerns about how this might impact the survival probabilities of the birds.

Swallows depend on an insect diet for their energy supplies, especially during breeding season. Not being able to source a reliable insect diet and the impact of parasites on their host’s immune system mean that low energy levels could impact the demographics of swallow populations

Stephanie Kramer-Schadt, head of the department of ecological dynamics at the Leibniz-IZW, said: “It is important for the field of ecology to focus more on diseases and pathogens as possible causes of variation in movement behaviour, as this can provide information about the state of populations. Under favourable environmental conditions, many animals can be infected without the infection having strong negative effects.

“However, if these conditions deteriorate – for example because of climate change or changes in land use – this can increase the negative effects of an infection. In extreme cases, this could even lead to an accelerated decline or extinction of the affected species without the cause being immediately apparent.”

The full study can be found in the journal Communications Biology.

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