Great tits thrive in old-growth forests

After a forest is logged, that land is often re-planted with new saplings. Within a few decades, those trees will have grown pretty big, and the forest will look much like a forest once again, with birds singing among the shade of the boughs.

But a new study finds that, despite this apparent rebirth, younger forests may not offer those birds the same quality of habitat as an old-growth forest—with differences between the two forest types stretching all the way down to a cellular level.

Researchers in Latvia compared wild forests more than 100 years old with managed pine forests just 40-50 years old. They studied how many insects were living in each forest type by measuring the amount of frass (insect poop and other droppings) that fell from trees. They also took blood samples from 15 day-old great tits—a common European songbird—to measure the birds’ stress levels.

In addition, the team measured the length of the birds’ telomeres, the DNA that sits on the ends of chromosomes and protects those chromosomes from damage. While scientists are still trying to understand exactly what different telomere lengths might mean for health and fitness, some previous studies in birds have found evidence that shortened telomeres were associated with lower survival.

[ Related: Immigrating great tits learn from their new neighbors ]

The researchers on this new paper found that younger forests had less frass, indicating fewer insects, than the older forests. The great tit nestlings in the younger forests tended to have higher stress levels than nestlings in the older growth forests—and nestlings from those younger forests had, on average, shorter telomeres than nestlings from the old-growth forests.

These results, collectively, may tell a story. “Since insect larvae are the preferred food for great tit nestlings, the shortage of food may divert energy resources away from growth, which can show up as physiological stress,” the researchers write. And stress, they note, has previously been associated with telomere shortening.

“It’s a very complete paper,” Simon Verhulst, an evolutionary biologist at the University of Groningen in the Netherlands, who was not involved in the new research, told Popular Science.

Telomere shortening is a natural part of cell biology. Telomeres shrink a little bit every time a cell divides, until those telomeres get too short, and the cell dies. And in many species, telomeres generally get shorter with age—but scientists are still trying to understand how, exactly, shortened telomeres might impact an organism’s overall health. In humans, for example, while researchers have linked shortened telomeres with a variety of diseases, they’ve also found evidence of health risks in people with extra-long telomeres. And while some scientists have previously suggested telomere length could be used as a sort of “biological clock,” others have suggested that this outlook doesn’t really capture what happens to our bodies as we age.

Previous research has also found some nuances in the relationship between telomeres and longevity in birds. A study on Seychelles warblers, for instance, found that survival was linked to both shorter telomeres and faster telomere shortening rates. But a study on jackdaws (co-authored by Verhulst), on the other hand, found that while more telomere loss was associated with lower survival, shorter telomeres themselves did not seem to be related to survival. It’s also possible that some of the negative health and fitness effects linked to shorter telomeres aren’t directly caused by those shorter telomeres, but caused by some other factor that also impacts telomere length. 

The researchers on this new study in Latvia did not find any survival differences between the nestlings from each habitat, although increased mortality could always show up later in life. They also note that other factors could have led to higher stress levels among the nestlings in the younger forests, such as an increased risk of predation on adult birds that somehow impacts the nestlings. In addition, variations in telomere length can be passed from one generation to another, so it’s possible the great tit parents living in each forest type had different telomere lengths initially and simply passed those differences onto the chicks. One way to test for that would be a “cross-foster” experiment—switching eggs from one forest type to another.

“An important extra step would be to do a cross-foster experiment to show that it’s really something that happens after hatching,” Verhulst said. 

[ Related: Great tits are killing birds and eating their brains. Climate change may be to blame. ]

Another possible complication in this new study was the researchers’ method of measuring insect abundance. Larger trees may simply have more insects to drop frass, which could impact the results if tree sizes weren’t standardized between forest types. But that doesn’t necessarily mean that the study’s conclusions are incorrect. “I am fully convinced that there is a difference in food availability between these old and young forests—I don’t think, actually, their measurements show that,” Verhulst said. (The researchers of the new paper could not be reached for comment.)

But this isn’t the first study to show that habitat modification can impact telomeres in great tits—a 2016 paper (with cross-fostering) found that tits from urban environments had shorter telomeres than those from rural areas. And overall, the differences between the birds in this new study indicates that something seems to be different among the great tits in young and old forests, despite the fact that the species is found in both forest types. “This study, through their telomere measurements, are showing that there can be sort of cryptic effects on viability of individuals and thereby, possibly, populations,” Verhulst said.

That’s notable, as old-growth forests are rare in Europe. According to a 2021 report by the European Commission, primary and old-growth represent just three percent of all forests in the EU. If these venerable trees truly offer better habitat to wildlife like great tits, protecting that three percent could be crucial.

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