The common crane  (Grus grus), in Estonian "Sookurg," is a summer visitor to Estonia. The species is noted for its large size, distinctive call, long lifespan (the species can live up to 40 years) and the fact they are practically always seen in pairs or larger groups.

Their populations have been growing in recent decades, while climate change has made their migration considerably shorter — since they are able to spend winter in locations closer to Estonia, in more southerly parts of Europe.

However, with this success has come issues, notably for juveniles, who often are not in good shape.

Estonian University of Life Sciences (Eesti Maaülikool) avian ecologist Ivar Ojaste spoke to ERR Radio show "Labor" to tell more.

Migration requires cranes to be in good health and get access to abundant food. However, agricultural practices in much of Europe have evolved over recent decades, and this — as well as more effective habitat protection — has made it easier for common cranes to find food. "The common crane is one of those success stories, where it was at a low point 60–70 years ago, but over time its numbers have increased," Estonian University of Life Sciences Ojaste told "Labor."

At the same time, climate change and more favorable feeding opportunities are significantly shortening the cranes' migration routes. The birds are departing the northerly climes later and later. "If we start with fall migration, then the autumn departure has generally shifted for us, and it actually differs within Estonia. In western Estonia they migrate later; in eastern Estonia earlier — the difference can average up to two weeks," the ecologist noted.

Ojaste said scientists have observed a broader pattern in which the share of long-distance migration is decreasing and wintering areas are shifting in a northerly direction, sometimes considerably more northerly. "When we talk about the 1960s–70s, the main wintering zones for cranes were in Morocco. From there they shifted in the 1980s–90s to Spain, then over the past 15–20 years there has been a shift toward France and Germany," Ojaste added.

The researcher noted that individual variation between the birds themselves also plays a role. "The larger the population, the more 'personalities' there are, with their own habits and behaviors influenced by environmental changes. When conditions allow, they tend to stay further north," the scientist confirmed.

Despite the overall success story, the large population size conceals a worrying trend, particularly among young cranes. A research group analyzed 37 years of data on birds which had been ringed with colored markers, in order to grasp the life history of this long-lived species. The data showed that since the mid-1980s, the survival probability of juveniles has been steadily declining.

"Where it used to be around 96–97 percent, today it is somewhere around 60 percent — so something quite significant has happened," Ojaste noted. Scientists link the mortality of young birds to increased nesting density. When the best nesting sites get occupied, birds must settle for poorer habitats. However, birds raised in these areas find it harder to prepare for a fuller life. Ojaste outlined more. "It is known that juveniles raised in poorer conditions are generally in weaker condition. They are more sensitive to negative environmental changes, unexpected cold periods, or increased predation—this is where additional pressures begin to accumulate."

Nature has its limits, however, and the number of common cranes can no longer grow at the same rate it had been doing. A lack of space is forcing young birds to seek new nesting sites, expanding even into their historical areas in Hungary. "This is a normal fluctuation in population size. The species has reached its ceiling, as the carrying capacity of the environment has been filled. This is supported by the fact that the breeding range of cranes has shifted westward and also southward in Germany," Ojaste continued.

Climate change and the adaptability of the birds will continue to shape the future of common cranes. Commenting on the outlook, the scientist recalled an earlier study too. "About 10 years ago, together with Aivar Leito, I published an article showing that in about 50 years, some cranes will likely begin over-wintering in Estonia in greater numbers. This change is inevitable."

This year, the first Estonian satellite-tagged cranes arrived back here in the middle of last week. "However, the majority of cranes are still somewhere in Europe — in Germany, Poland, or Lithuania," Ojaste concluded.

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