In North America, the number of birds has dropped significantly over the past 50 years. More than a quarter of the animals in Canada and the US have since been lost, researchers report in the journal "Science". They assess the loss as part of the worldwide extinction of species.
The scientists around Ken Rosenberg of Cornell University in New York State have evaluated data on the occurrence of 529 bird species living in the United States and Canada that volunteers had recorded since the early 1970s. In addition, the team analyzed recordings of 143 weather radar stations over a period of more than ten years.
Thus, only the record is a rarity. Only the occurrence of very few species is systematically documented over such long periods of time. This makes it hard to understand today how the populations of various species have evolved over the past decades. Among other things, this causes great uncertainties in studies that try to quantify the extent of insect deaths, including in Germany.
Birds in forests and meadows particularly affected
Rosenberg and colleagues were able to draw on a large database compared to this. According to her analysis, today 2.9 billion fewer birds live in the US and Canada than 50 years ago. That corresponds to a decrease of 29 percent.
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"We expected the populations of endangered species to continue dwindling," explains Rosenberg. "However, the study now also shows a significant decline in species that have been widespread so far." For example, 90 percent of the decline is due to twelve bird families - including sparrows, finches and swallows.
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The population loss in three areas, the researchers consider particularly important:
- For birds in field and meadow landscapes. Since 1970, the population has fallen by more than half (53 percent). In the habitat, there are now 720 million fewer birds.
- Many waders were already endangered in the seventies. Since then, the population has fallen again by a third.
- The number of birds migrating north in the spring has decreased by 14 percent over the past ten years, according to radar reports.
However, there are also species whose population has grown. This affects some predatory and waterfowl, including the bald eagle, the heraldic animal of the United States. Among other things, the researchers attribute this to the ban on the insecticide DDT in the early 1970s and the fact that the species are often under special protection.
Why the number of birds in the US and Canada so much decreases, the scientists have not investigated. But they assume that humans are responsible for many different ways. The biggest influence is therefore the loss of habitat through growing cities and expanding agriculture.
In 2013, researchers also published a study, according to which domestic cats in the US kill up to four billion birds each year. Even glass panes can become a deadly trap. In addition, many species of birds are increasingly losing their food supply because the number of insects is also declining. The global temperature increase could further aggravate the situation in the coming years, the researchers warn.
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Scientists around the world are currently studying how humans influence biodiversity in detail. Birds are indicators of the state of the environment, warn Rosenberg and colleagues. Their study indicates that humans are now affecting ecosystems in the US and Canada so badly that their livestock population is sinking. It should also be taken into account that many birds also perform important tasks for humans, for example distributing seeds and keeping pests at bay.