"As you can probably remember from your childhood, it used to be rather common for snow to disappear only in April, just as it was the case this year: the winter was snowy and long, especially in the eastern part of Estonia. However shorter winters, with snow disappearing well before March became the new normal," said therologist Uudo Timm.

Climatologist Jaak Jaagus said that based on his new calculation, the seasons have shifted. The average air temperatures, measured at about twenty weather stations in Estonia between 1951 and 2020, confirmed this. "The general tendency for the beginning of the first semiannual (spring) season has been to shift earlier and for the beginning of the second semiannual (autumn) to shift later. That is not the case for the beginning of late autumn that has stayed the same," Jaagus said.

The biggest shift occurred in times of the beginning of early spring, which now starts on average 20-30 days earlier than 70 years ago. Spring begins about 10-15 days earlier, except in northeastern Estonia, where the change has been even smaller. "Summer begins now statistically significantly earlier only in southeastern Estonia. The beginning of autumn, however, has been also delayed by 10-15 days, and the beginning of winter even more," said Jaagus, adding that the late autumn will be delayed only for the western coast of Estonia.

"In result, the summer has become 20-30 days longer, and the winter one to two months shorter," said the climatologist.

Estonian birds nest a week or two earlier

"If spring starts weeks earlier, it affects both the flora and the fauna that is dependent on the former," said Uudo Timm. Thus, data from a 75-year-old observatory in Oxfordshire, England, shows that tits are nesting there weeks earlier than usual. Marko Mägi, a bird ecologist at the Institute of Ecology and Earth Sciences at the University of Tartu, has been using this example of the case in England to discuss the effects of global warming with his students. "I could tell them, 'But no one else in Europe has paid much attention to it,'" he recalls.

According to Mägi, more and more research has accumulated in recent years, which shows that also elsewhere in continental Europe, birds have started nesting earlier. "This is partly reflected in our Estonian data as well," he said. For example, Mägi's team has been collecting data from southwestern Estonia since the 1970s on the onset of the breeding times of the two species of local birds: the great tit and the European pied flycatcher. "Our study confirms that the birds have started nesting there earlier," Mägi said.

Mägi also examined the data collected since 1956 by Endel Edula, an amateur ornithologist from Viljandi County, on the late nesting birds. "When we analyzed this data with one of our master's students, it turned out that common starlings, for example, have moved their breeding time to about 11 days earlier in the last 50–60 years, while the European pied flycatcher and the great tit start nesting in Viljandi County about six to seven days later," he said.

Mägi said that when looking at these changes, it is worth keeping in mind the lifespan of smaller birds, which may be only one or two years. "If you look at the whole population and think about, for example, the shift that occurred in the case of the common starling, which is 11 days in half a century, it should be called a revolutionary change, not an evolutionary one," said the bird ecologist.

Although Estonian birds noticeably react to climate change, they do so a little slower than other European birds, because the climate here is colder, Mägi said. This is especially true for long-distance travelers from sub-Saharan Africa, such as the European pied flycatcher. "The signals it catches in Africa, that now is the time to move, are not quite as accurate as they used to be, so they don't align well with the situation here in Estonia. But it would arrive a little too late, if they were indeed correct," he said.

However, there are known cases in the Netherlands where pied flycatchers have arrived earlier than usual. "They start nesting there at about the same time as the great tits, and the competition between them has become much more intense there than it used to be a decade ago," says Mägi. This means that as time moves ahead, we will see an increasingly intense struggle for nesting sites between those species.

"In Estonia, for example, pied flycatchers start laying eggs about seven days earlier, but their migration pattern has not changed," said Mägi. The common starling, after wintering in Europe, arrives in Estonia relatively early; the great tit might not even have bothered flying away from Estonia for the winter. "So it's relatively easy for those species to respond to climate change," the bird ecologist said.

Possible effects of the disturbances in hibernation on population of mammals and reptiles

"Talking about the weather change, Estonia is a big country," said Uudo Timm, specifying that the air temperature in different parts of Estonia can vary by 20 degrees. The layer of snow, which is important for mammals' hibernation, may cover the islands and in western Estonia for only a couple of days, while in eastern and southeastern Estonia the snow-cover is almost half a meter deep for the duration of several months. It is a little riskier to generalize on the effects of the shifting seasons in other respects, for instance, in the case of fauna," he said.

Global warming has mostly changed the duration of snow-cover and sea ice in Estonia. For winter dormant animals this is an important factor, as the snowpack should preserve a uniform temperature in their wintering place. "Especially for smaller animals a uniform temperature during their hibernation or the dormant state of torpor is crucial," he said.

In a taxonomic sense, cold-blooded reptiles also belong to the animal kingdom's winter-dormant class of animals. If the snow disappears by the end of February or March, the heat of the sunny day may start already warming up the ground. "Then it's no wonder that around this time, in some warmer places, after the snow is gone, vipers, grass snakes or lizards start moving around already in March," said Timm. If there was snow on the ground, they would still be inactive.

However, some warm-blooded mammals, e.g., male bears, can survive relatively well also in snow-poor winters. "If they are accidentally woken up, either at the end of the year or in the early spring, they might no longer be able to fall asleep," said the theriologist. The sows, on the other hand, even if woken up, will wait until April to leave the nest as their cubs would not able to accompany them otherwise.

Overwintering in caves or basements bats also depend on the stable winter temperature. Timm said, the change of seasons in the spring does not affect them as much, because the heat reaches deep underground rather slowly, and added, that "Falling asleep in the autumn may be delayed for them, as this timing is dependent on whether there is still food available for them." When the first frosts arrive, northern bats can crawl into barns in warmer winters and move elsewhere only if it gets below -15 to 20 degrees Celsius," Timm said.

Will Estonia become a mecca for biodiversity?

As for the birds, for instance the case of the great tits of Estonia and England show that the birds have adapted fairly well, Mägi said. "The question rather is whether they can also adapt to changing conditions in the fast pace of global warming today, and here I am more skeptical about whether all the species will be able to do so," Mägi said. Especially the availability of food for migratory birds is altered due to the shifts in changing seasons. For example, in the Arctic the nesting birds arrive to their breeding grounds only by the end of the most favorable period. "They should have reached there two weeks earlier, then they could have had enough food and could have raised their offspring properly," Mägi said.

Uudo Timm said, among the species most detrimentally affected by the climate change in Estonia are those living  in the north of the country Estonia, i.e., in harsher winter conditions. The biggest loser would be the ringed seal, which, unlike the gray seal, needs a cover of ice for breeding. "For example, none of the last year offspring of the ringed seal have survived, as there was no permanent ice cover at the time of calving," he said. The winners, on the other hand, are among the southern species for which Estonia was the most northern limit of their range so far. Examples of such species are roe deer and red deer.

"A roe deer must be able to clear where they rest their underbelly of snow to prevent pneumonia and other diseases," Timm said, noting that this year's thick snowpack could have been detrimental to the health of deer in eastern and southeastern Estonia. However, the bare land of the islands and of western Estonia must have been for the roe deer this year rather comfortable. The deer's habitat in Estonia is constantly expanding. "Since for deer a deep layer of snow is a discouraging factor, these animals benefit best from the shortest possible period of deep snow," said Timm.

Mägi brought up an example of a study of the Acrocephalus warblers in the Czech Republic, where it is shown that a shorter winter prolongs their breeding season. "It gives the bird an attempt to retry at nesting again and again if the first nesting attempt did not succeed," he explained. However, as the reed nests of warblers are often looted at the beginning of the breeding season, or they may fall prey in some other way, the older birds are more likely to succeed in bringing up their offspring.

Both Mägi and Timm admit that the warmer the climate gets in Estonia, the more alien species arriving from the south can be expected here. "However, there are no mammals among first migrators expected as their speed of movement is slower than, for instance, of flying birds or insects," Timm said. If global warming continues at its current pace, Estonia's bird diversity may reach its peak in the second half the century." People interested in birds could be increasingly tempted to visit Estonia, as the climate here is mild and the number of bird species expands.

Although, for example, new species of birds and insects arrive to Estonia every year, those are often the species that can retreat to their original habitat in any unfavorable year, said Timm. However, the earlier birds and insects begin breeding the riskier it becomes for them. "It's in nature's general interest not to allow definitive changes at a breakneck speed," Timm said, and added: "In the past, when species have survived all kinds of disasters, it was often their conservatism that speared their lives." However, if unfavorable conditions persist long enough, the winners could be among those that dared to take risks."

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