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Swallows infected with parasites move less and in smaller ranges than healthy ones – with detrimental effects on their foraging success and their survival. As a result, infected individuals foraged in less productive areas, such as cultivated farmland, clearly avoided by their healthy conspecifics. Although infected swallows show no externally recognisable signs of infection, scientists from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) and the University of Potsdam now demonstrate the negative effects of these infections using the high-resolution tracking system ATLAS, which records precise position data of swallows at one-second-intervals using ultra-light transmitters.
The results were published in the journal “Communications Biology”.
Infected animals often show no recognisable sings of infection. Particularly in the case of parasitic infections, individuals are often apparently visually unharmed, although these infections do have negative effects on the host. Typical effects of such sub-clinical infections are lethargy, weakness and a restricted radius of action. These effects may be subtle, but may have important consequences for the foraging success of individuals and thus for their reproduction or survival. Scientists from the BioMove graduate school, funded by the German Research Foundation (DFG), are investigating the effects of sub-clinical infections with avian blood parasites in swallows and their consequences for animal movements. Over several years, they caught house martins (Delichon urbicum) and barn swallows (Hirundo rustica), analysed them in terms of their parasite infection load and equipped them with mini-transmitters that fed high-resolution data into the ATLAS tracking system. In addition, swallows were analysed in a capture-recapture study in order to track the long-term developments and the effects of infections on survival.
“Using the ATLAS tracking system, we could reconstruct movement profiles of many swallows during the breeding season – of both healthy and infected animals”, says Marius Grabow, PhD student in the BioMove project at the Leibniz-IZW and first author of the scientific paper. “Infected individuals of both species had smaller home ranges than their uninfected conspecifics. Infected birds also rested more and spent less time foraging.” These combined effects led to infected swallows repeatedly having to forage in habitats where insects were less abundant. In particular, the infected birds reduced the range and duration of their foraging flights, probably owing to physical limitations caused by the infection, and often foraged in agricultural areas close to their colony, where there is considerably less food to be found. In contrast, non-infected swallows clearly avoided these landscapes when foraging. These comparative findings were possible because the study area in the German district of Uckermark is a mosaic of intensively utilised cultivated, agricultural land with few patchily distributed high-quality habitats for insects and insectivores.
The scientists could show that sub-clinical diseases were associated with behavioural changes. In the study, between 11 and 76 percent of swallows were infected per year, and infected individuals had lower survival probabilities between years. Especially during the breeding season, swallows depended on high energy supplies such as insects. Reduced food intake of many birds could therefore influence the demography of the populations. “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”, says Prof Stephanie Kramer-Schadt, Head of the Department of Ecological Dynamics at the Leibniz-IZW and Professor at the Technische Universität Berlin. “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. Conversely, changes in movement behaviour are also relevant for the transmission and spread of pathogens, making these scientific findings important for predicting the spatial spread of wildlife diseases.”
Parasites use their host's resources. As a result, the host has to invest additional energy for immune functions. Infected animals are therefore often lethargic and suppress activities that require a lot of energy. The BioMove scientists hypothesised that movement behaviour can serve as a reliable indicator of host performance, regardless of the mechanisms involved. “To test this, we focused on a widespread group of blood parasites that cause so-called avian malaria and are naturally common in many passerine species”, says Prof Dr Ralph Tiedemann from the University of Potsdam and head of the Laboratory for Molecular Evolutionary Biology. “Avian blood parasites such as plasmodia or leucocytozoa have complex life cycles with a two-host life cycle in which they infect the host's red blood cells. In the acute phase, the red blood cells are eventually destroyed, resulting in reduced oxygen transport to muscles and organs. This is an ideal study system to investigate the physiological effects of the infection on the host.”
This scientific investigation is a collaboration between BioMove, led by Prof Florian Jeltsch from the University of Potsdam, Tel Aviv University and Hebrew University of Jerusalem, where the ATLAS system was developed. ATLAS is short for “Advanced Tracking and Localisation of Animals in real-life Systems” and is a so-called reverse GPS tracking system. This means that the animal transmitters only send an encoded signal and the position is recorded and calculated by local antennas on site. This allows the animal transmitters to be smaller and lighter as they do not have to calculate the position. On the other hand, this system requires an antenna infrastructure at a local level. In the current BioMove investigation, this is being provided by a large number of local supporters in the Uckermark district, who greatly supported the set-up of antenna sites for the system and the catching of the swallows.
Prof Dr Stephanie Kramer-Schadt
Head of the Department of Ecological Dynamics
Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW)
phone: +49(0)30 5168714
email: kramer@izw-berlin.de
Marius Grabow
PhD student in the Department of Ecological Dynamics
Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW)
phone: +49(0)30 5168725
email: grabow@izw-berlin.de
Prof Dr Ralph Tiedemann
Head of the Laboratory for Molecular Evolutionary Biology
University of Potsdam
phone: +49(0)331 977-5249
email: tiedeman@uni-potsdam.de
Grabow M, Ullmann W, Landgraf C, Sollmann R, Scholz C, Nathan R, Toledo S, Lühken R, Fickel J, Jeltsch F, Blaum N, Radchuk V, Tiedemann R, Kramer-Schadt S (2024): Sick without signs. Subclinical infections reduce local movements, alter habitat selection, and cause demographic shifts. Communications Biology 7:1426 DOI: 10.1038/s42003-024-07114-4
House martin tagged with an ATLAS transmitter for the recording of high-resolution movement data
Marie Klett
Leibniz-IZW/Marie Klett
Blood sampling from a barn swallow for the identification of blood parasites
Eva Sánchez Arribas
Leibniz-IZW/Eva Sánchez Arribas
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