They've been back since the beginning of May: it's not uncommon for swifts to draw attention to themselves with shrill screams as they whirr over the roofs of cities or circumnavigate blocks of houses.
Originally dependent on high rocks, these acrobats in this country usually look for nesting sites on buildings.
With preference in gaps that open up in old buildings between the roof and masonry, or in special nesting boxes.
In their breeding areas, they use the times when the food supply in the form of flying insects is particularly plentiful.
After three months they fly away again towards the south.
But what if the weather is so wet or cold that the swifts can hardly find any nutritious prey?
They can often simply avoid unfortunate low-pressure areas by flying at high speed.
With offspring in the nest, however, this is not an option.
Instead, saving energy should be the order of the day.
Studies in the laboratory have actually shown that swifts, which have to fast for a while, significantly reduce their energy expenditure and thus their body temperature during rest phases.
Scientists working with Arndt Wellbrock, Luca Eckhardt and Natalie Kelsey from the University of Siegen have now proven that free-living birds can also save heating costs in this way if necessary.
Common swifts, which had nested on the underside of a motorway bridge near Olpe, served as research objects.
Numerous nests were equipped with small devices that recorded the temperature at regular intervals.
Oxygen consumption and carbon dioxide production could also be measured in some of the swifts that had lodged themselves in specially prepared nesting boxes.
As Wellbrock and his colleagues report in the "Biology Letters" (doi: 10.1098/rsbl.2021.0675), they were able to observe several times that oxygen consumption - i.e. the production of metabolic energy - fell significantly during the night hours.
When the resting birds switched their metabolism to the back burner in this way, the temperature in the nest fell by an average of eight degrees Celsius.
It then dropped to 21 degrees,
while it is usually around 30 degrees even at night.
After all, swifts, far from being the hottest of birds, usually keep their body temperatures around 39 degrees.
Nestlings also rely on stockpiling
During an observation period of eight years, the biologists found that most swift families spent one or two nights in energy-saving mode during the breeding season.
How often this was necessary depended on the weather.
While it was mostly sunny and warm from May to July, only a minority of the swift families were occasionally forced to save energy.
If more than one bird spent the night in the nest - a maximum of four nestlings plus parents - the proportion of each individual remained an open question.
According to the researchers working with Wellbrock, lack of food is the most important reason why swifts sometimes drastically reduce their energy expenditure at night.
However, nestlings rely not only on thrift, but also on stockpiling.
Fed up fat reserves help them to get through lean times unscathed.
After all, a well-fed young swift can weigh up to sixty grams, which is 50 percent more than an adult bird.
Before the young fledging, they usually have to fast for a few days in order to lose enough weight.
They have been training their flight muscles for a long time.
At first only flapping their wings, as far as their accommodation allows.
Later, with their wings stretched downwards, they lift their bodies so that their feet lift off the ground.
Problems in Central and Southern Europe
With this type of push-up, the young swifts can probably also estimate whether they have reached their ideal weight.
In any case, they can only leave their nursery, which is usually three, four or more stories high, by flying.
Once flown, they remain in the air day and night.
So they then move south to their winter quarters, always in search of fruitful hunting grounds.
Before they return to Europe the following year, they never have solid ground under their feet, or at least only very sporadically.
Returning from southern Africa, the one-year-old swifts sometimes inspect possible nesting sites.
In order to be able to start a family, however, they have to wait another year.
The European breeding range of the common swift extends to Norway, Sweden and Finland.
Thus, these extremely persistent fliers can also use the mosquito swarms in summery Scandinavia as food for their offspring.
A prerequisite for breeding successfully there was probably the ability to drop the body temperature considerably on cool nights in order to save on heating costs.
In central and southern Europe, however, common swifts are increasingly having another problem: Summer heat waves sometimes overheat the nesting sites there.
If the nestlings then seek cooling by stretching far out of the nest cavity, there is a risk of a fatal fall.