Infections with parasites often entail no recognisable signs in many wildlife species, but can have still negative effects across generations. Infected animals often have a slightly reduced body size and their offspring have a harder start in life. A team of scientists from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW), the Technische Universität Berlin and the University of Potsdam now have shown for the first time, that the impaired reproductive success is connected to altered movement behaviour: Infected starlings have a smaller action radius, which limits their access to high-quality foraging habitats. The paper is published in the scientific journal “Proceedings of the Royal Society B: Biological Sciences”.

The scientific team led by Marius Grabow and Stephanie Kramer-Schadt from the Leibniz-IZW Department of Ecological Dynamics and the Technische Universität Berlin fitted 29 starlings (Sturnus vulgaris) in two colonies in the Uckermark district in northeastern Germany with lightweight high-resolution transmitters from the ATLAS telemetry system, measured their bodies and took a small amount of blood from each bird. At the same time, they systematically monitored the nests in two colonies during the breeding season in spring, recording the time and number of eggs laid, chicks hatched and fledglings fledged and taking the chicks’ body measurements at 15 days of age. Thanks to the high spatial and temporal resolution data from the ATLAS system, the scientists were able to precisely track the movements of the parent birds and monitor where the parents flew to look for food during the first 15 days of their offspring's lives. This enabled them to link movement behaviour with feeding habitat selection and reproductive success.
Blood analyses conducted by Prof. Ralph Tiedemann, Chair of Evolutionary Biology / Systematic Zoology at the University of Potsdam revealed that 11 of the 29 adult starlings had infections with blood parasites (Haemosporidia), which cause so-called avian malaria. Typically, these infections lead to reduced performance, which may explain the restricted movement patterns of the infected animals. Grabow, Kramer-Schadt and their colleagues have now been able to show for the first time in wildlife what effects this reduced performance has on foraging during breeding time and how this affects offspring.

Offspring of infected birds weigh 12 percent less
Statistical analyses showed a clear correlation between the body condition of the young starlings and the infection of the parents. Infected birds spent less time foraging and stayed closer to the nest. This forced them to be much less picky in their choice of foraging habitats than completely healthy conspecifics. They fed more frequently in arable land, which is less abundant for starlings, than in meadows, where starlings are better able to find food, such as white grubs. Presumably, they were therefore able to feed less frequently and on lower quality prey, which was reflected in the poorer general physical condition of the infected adult starlings. Although this had no demonstrable effect on the number of eggs laid or chicks hatched, it did have a pronounced negative effect on the physical condition and thus the probability of survival of the young starlings: The offspring of infected parents were on average 12 percent lighter than the offspring of non-infected parents.

Plausible explanation for lower reproductive success and chances of survival
Animals infected with blood parasites often show no recognisable symptoms and animal populations appear healthy, although the infections do have negative effects on the host. Typical symptoms of these so-called sub-clinical infections are lethargy or a restricted action radius. These effects are often subtle, but have important consequences for the foraging success of individuals and thus for their reproduction or survival. “For the first time, we were able to establish a logically coherent chain of determining factors and to show statistically clear cause-effect relationships”, says first author Grabow, PhD student at the Leibniz-IZW in the “BioMove” graduate school funded by the German Research Foundation (DFG) and led by Prof Florian Jeltsch at the University of Potsdam. “The robust correlations between infection, action radius, selection of foraging habitat and poorer physical fitness of parents and young provide a plausible explanation for the disadvantages of young starlings at the start of their lives and thus for their lower chances of survival. This allows us to show that even infections that are not immediately visible can have serious consequences across generations.”

ATLAS tracking system enables precise insight into movement and behaviour
The high-resolution telemetry system ATLAS was developed at the Hebrew University of Jerusalem and Tel Aviv University and records precise position data of animals every second with ultra-light transmitters. 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 their position is recorded and calculated by local antennas on site. This allows the transmitters to be smaller and lighter. On the other hand, this system requires an antenna infrastructure at the local level. In the current investigations within the BioMove graduate school, 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 starlings.