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A study of the microbiome (i.e. the microorganisms in the intestines) of great tits (Parus major) in Europe, including Tartu, has revealed that the composition of gut microbes is more diverse in winter than in summer. No significant differences were observed between populations.
The gut microbiome has been linked to a number of health indicators, such as obesity, allergies, metabolism and susceptibility to pathogens. However, there has been little knowledge to date of the variability of the microbiome in a single species: while more is known about the microbiome of mammals and humans, very little is known about that of birds. Until now, studies of bird microbiomes have mostly focused on captive birds, mammals and invertebrates.
To study bird microbiome, faecal samples were collected from eight populations of great tits: the northernmost in Oulu, Finland, and the southernmost in La Hiruela, Spain. Populations from Estonia, Sweden, Hungary and the Netherlands were also included.
Researchers, including Vallo Tilgar, a bird ecologist at the Institute of Ecology and Earth Sciences of the University of Tartu, assumed that the microbiome of the northern populations of great tits would vary more throughout the year, as the differences between summer and winter are greater than among the southern populations. In addition, it is known that the microbiome of e.g. mice changes depending on whether they eats insects or seeds.
In birds, the microbiome of the thick-billed murre (Uria lomvia) and the barn swallow (Hirundo rustica) is known to depend on their diet. However, habitats also play a role, as observed in the Eurasian blue tit (Cyanistes caeruleus), whose microbiome is more diverse in dense woodland compared to those of the same species living on more open landscapes.
The study found that the microbiome of great tits did not differ significantly between populations. However, it was found that the microbiome was more diverse in winter than in summer, with great tits in mixed forests having a richer microbiome compared to those in deciduous forests, and that different bacterial groups were present in each season. Among environmental factors, temperature was linked to the microbiome – the colder the climate, the more diverse the microbiome.
Because of the geographical distance between the populations and the fact that they differed in respect of a number of traits (e.g. physiology, phenotype and certain genetic differences), the similarity in their microbiomes was surprising. This could be due to the food they eat, which is more varied for forest-dwelling birds and may offset the high degree of variability between birds.
It was assumed that in winter the nutritional conditions, and therefore the microbiome, would be poorer, but in reality it was the complete opposite. Since great tits feed on overwintering insects (butterflies, beetles and bivalves) and plant food (buds and seeds) in winter, the supplemental food provided by humans could explain why the composition of the microbiome varied seasonally and was more diverse in winter.
The birds did not receive any supplemental food in summer, whereas in winter they were often found near bird feeders. The greater diversity of the winter microbiome and the higher abundance of certain bacterial groups may also be due to the function of bacteria: cold weather can diversify the microbiome, because birds need to boost their digestion and energy absorption in order to maintain body heat. The more diverse winter microbiome of great tits may also be due to a more diverse diet.
The diversity of the microbiome benefits the birds, as bacteria with similar functions can replace or complement one another when needed, making the birds more resilient to environmental changes. In summer, when the birds are nesting, their bodies come under constant stress due to the demands of reproduction. This strain can mask the differences in the microbiome between habitats.
As such, the gut microbiome is dependent on environmental conditions and changes therein. How a changing environment affects the microbiome of birds is a topic for future research.
Scientific paper can be found here: Seasonal and environmental factors contribute to the variation in the gut microbiome: A large-scale study of a small bird
This article is written by Marko Mägi. It was originally published on the “Birdwatcher blog”.
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