A study led by the University of Liverpool and the Centre for Ecology & Hydrology (CEH) has found that parasites affect flight ability of wild seabirds, which may make it harder for them to raise chicks.
The researchers studied a population of European shags on the Isle of May National Nature Reserve, Scotland, and measured how parasites affect energy levels and behaviour of individual birds, something which hasn't been done before in a wild population.
They used an endoscope to count individual worms in the birds' stomachs and miniaturised electronic tags recorded the movement and energy of the birds.
They then calculated the total energy used on each day, and the energy used for flying, diving and resting.
Researchers found that the total energy used per day did not depend on the amount of parasites, but females with higher levels of parasites had more costly flight and spent less time flying each day, presumably to avoid using too much energy.
A paper, titled The energetic cost of parasitism in a wild population, has now been published in the journal Proceedings of the Royal Society B following the four-year study.
Co-authors included Dr Sarah Burthe, Dr Francis Daunt and Isle of May field manager Mark Newell, from CEH, as well as researchers from the University of Liverpool, Biomathematics & Statistics Scotland and Tokyo University.
Dr Burthe, an animal population ecologist at CEH, said: "It is extremely unusual and challenging to be able to measure both parasites and energetic expenditure in wild animals, so this is a very exciting development.
"We already know from previous work that parasites can negatively affect the ability of parents to successfully raise chicks.
"The really interesting thing about this current work is that it shows the mechanism by which parasites affect the host."
The research was carried out from 2014-2017 on the Isle of May as part of the long-term study conducted at the nature reserve by CEH.