Birds are not only a matter of research for the graduate biologist and PhD student, they are also a passion, he says. And you can believe that immediately when Stiels, who lives in Bad Honnef, enthuses about a wagtail that has successfully bred in the neighbourhood. "The other day, one of the young birds was sitting in our garden."

These observations make Stiels the envy of every bird lover. As a rule, nowadays we' re happy about every robin, jay or wren we see. Even the common sparrow, aka the house sparrow, is much less common than it was in the days of our parents and grandparents. Or is that just a subjective perception?

Stiels brings the scientific concept of the shifting baseline syndrome into play: people notice changes differently because of different standards of comparison. Older people notice the decline in species diversity in birds much more keenly because of their experience.

The bird world around us is changing dramatically. The fact that this is not just "perception", but scientifically proven, is shown in counts - and a unique study of the urban bird community in Bonn-Dottendorf. For this small region, of all places, ornithologists can draw on comprehensive data material spanning 50 years. This is thanks to the work of the well-known biologist Michael Wink, who was later to become Professor of Pharmaceutical Biology and Founding Dean of Molecular Biotechnology at the University of Heidelberg.

Born in Bonn, when he was a schoolboy he had a paper round in Dottendorf and did some true pioneering work: from 1969 onwards he mapped the breeding bird population in an area of about 103 hectares in the district. He published the meticulously compiled results, including methodology and a map of his study area. "For us ornithologists, this is a real stroke of luck," says Stiels, who is writing his dissertation on bird distribution.

The result of the scientific comparison of bird populations 50 years ago and today: there have been massive changes in species and population patterns in bird communities. The everyday picture in the streets and gardens of Bonn has changed drastically since 1969. The blackbird no longer occupies first place in the bird world of the village, but the great tit. And the house sparrow is considered extinct in Dottendorf. Woodpigeons, blackcaps, dunnocks and blue tits have replaced the former flocks of greenfinches, tree sparrows and swifts. Fifty years ago, the house martin was the sixth most common bird. Today it is the robin. The European Serin can be heard just as rarely as the redstart or the willow warbler. Instead, wrens, song thrushes, kinglet and chaffinches hop from branch to branch.

The OAG reports a "dramatic change in the bird world". In an essay in the journal for birdwatchers "Der Falke", the authors' collective describes how sharply the richness of species has declined. In 1969, 57 bird species were breeding in Dottendorf, in 2019 there were just 39. In half a century, 22 species died out in this area alone, while four new species settled there. "What is particularly alarming, however, is that even once very common species disappeared," they write.

What are the causes? Is it the changing landscape? The ever-expanding residential development? The changed attitude towards garden design? In the past, fruit trees, berry bushes and hedges were common. Today, gardens are characterised by other, mainly visual and aesthetic aspects.

The ornithologists took a close look at whether habitats have changed. To do so, they compared historical and present-day aerial views, looked at changes in settlement development, industrial areas, gardens and forests. With more than 50 per cent of the urban area covered by housing and roads, Bonn is now very densely populated. Dottendorf, however, borders on the Kottenforst. Over the years, many village structures have been lost.

"In the study area, near-natural, open habitats have decreased by about 40 percent," say the ornithologists. There used to be 15 typical open land species in Dottendorf. These are areas that are not developed but are not covered by trees, in other words, agriculturally used areas such as fields, gardens and grassland. Twelve have become lost. At the same time, birds that typically breed in settlements, such as swifts, house martins and house sparrows, have also become extinct.

If you look back even further, to our grandparents' childhood and youth or even to the 19th century, the scale is even sadder. According to the Bonn Nature Conservation Union (Nabu), 130 species were breeding in our region around 1900. In the first decade of the 20th century, capercaillie, hen harrier and the European roller (Coracias garrulus) became extinct. The hoopoe was probably once seen as a "traveller through" the Sieg meadows, but it disappeared there 100 years ago. The last black grouse were seen in the 1930s, the last rooks in the 1940s.

More and more farmland birds are also disappearing from Bonn and the region. Hazel grouse and red-headed shrike, the nightjar, the wryneck and the wheatear - they all belong to long-forgotten species. According to Nabu, 18 bird species became extinct in Bonn and the Rhine-Sieg district on the left bank of the Rhine in the 20th century.

In the Rhine-Sieg district on the right bank of the Rhine, ornithologists say that bird diversity has "completely disappeared". Even if information boards along the "Green C" point out partridges, yellowhammers, lapwings, skylarks or redshanks - it is not usual to observe them there during a walk. And anyone who wants to see the red kite gliding through the sky should go to the Bergisches Land, where populations have stabilised thanks to a comprehensive protection programme. There they like to fly over freshly mown meadows in search of dead animals that fall victim to mowing machines. Their prey consists mainly of voles, earthworms, amphibians, fish and birds, but also carrion or other edible waste.

And the lapwing was once a common sight in open landscapes such as the Hangelar Heath. It used to be one of the "common species". Today it is an endangered species. The Biological Stations, for example the group in the district of Euskirchen, started targeted protection measures a few years ago. Lapwings often breed in ground clutches on fields that run the risk of being ploughed under in spring. So conservationists talk to farmers so that they either adapt the cultivation of the fields or drive around existing nests when working in the fields. They mark them with sticks, and when manure is applied, the conservationists cover the nests with buckets.

The population of hoopoes is also "extremely low", says Stiels. Even though they have recovered in some regions through classical nature conservation, the hoopoe is an endangered species. The striking black-and-white-striped bird with the rusty-brown-red head and feathered cap could perhaps even benefit from climate change, says the ornithologist, because it loves warm temperatures.

The experts do not see any displacement by non-native bird species. "That doesn't seem to be a decisive factor," says Stiels. The best-known species of newcomers are the green-collared parakeets. According to the current census, 2800 collared parakeets live in Bonn. "There are some individual indications of a competitive situation - by other non-native species," says the spokesperson of the Ornithological Society. The much larger Alexander's parakeets are now also on the rise in Bonn. "They are competitors for breeding sites. This could have an effect on the reproduction of the collared parakeets. We'll have to keep an eye on that."