A couple days ago the first barn swallows were seen back in our country. Barn swallows stay in Belgium roughly from April until October and each autumn migrate to the to West and Central Africa. They stay there during our winter and return to Europe in Spring to breed. Many consider spring not complete without barn swallows and the sighting of the first swallows is guaranteed to get the news. Like for example (translated from Dutch):
The swallows are two days earlier than in 2013 and 2014, the big arrival is expected by the end of March. “This date is moving forward under the influence of climate change,” says Dominique Verbelen of Natuurpunt. “In 1985, for example, the arrival only started on April 5.”
There were some things that were not really clear. For example how accurate that arrival date could be determined? What is the significance of a two day earlier date? How it generally related to earlier dates? And especially what “arrival” on April 5 he was referring to? Indeed, those swallows don’t come back to our country in one giant flock. The largest part of the animals will come more or less two weeks after the arrival of the first individual birds, called “scouts”. The birds that were sighted a couple days ao are the first scouts of this year. Therefor his estimation that the largest part of the flock are to be expected later this month. That was what he called the “big arrival”.
So is this April 5 the date that the first swallows arrived in 1985? In that case, the current swallows are one month earlier than the birds of 1985. Or is this April 5 the date that the scouts arrived in our region? In that case, no comparison could be made yet.
In other articles I learned that the registration of barn swallows started in 1985 and after a search I found the database of Natuurpunt.
This was interesting information. The earliest scout in 1985 was spotted on March 25. A lot of measurements were registered on the 6th of April of 1985, so this was probably the start of the big arrival. This means that it is not possible to compare the two values that were given in the article: the date of the big arrival is not known yet, it was just estimated. The early bird being spotted on March 6 of this year was the first spotted scout. The birds spotted at 5 (probably 6) April 1985 was the spotting of the big arrival. This is not what one would think when reading that news article.
Also, from the two sentences in the news article it looks as if the current date of March 6 is the earliest one until now (the date is said moving forward and the current birds are earlier than those of 1985). This is not true either. 9 of the 31 years (29%) have a first sighting that is earlier than March 6. If you count the outliers (birds sighted in January or February of a year), one get 5 years extra.
Another thing that caught my attention was that although most of the later dates were in the earlier years and most of the earlier dates came from the later years, there was a abrupt drop in arrival date since 2007. Before 2007 there was a gradual slowdown from March 25 until around March 15 (without taking the outliers into account). From 2007 on the scouts consistently came back in the beginning of March and this stayed that way until now. What is the reason of this abrupt step change? When wading through the dataset, this step change seems to come around the time that the measurements were increasing considerably.
In the beginning of the observations there were only limited observations of barn swallows throughout the year (hundred or a couple hundreds), but around 2007 the observations skyrocketed (in 2014 almost 13,000). It might be a coincidence, but the number of measurements could influence the results. The more eyes that look out for barn swallows, the more the chance that one will be officially spotted. This remembers me of the “increase” in thunderstorms or hurricanes, which had its cause in better observations systems. Storms/hurricanes that could not be measured in the past are now counted. How significant is an increase then?
To conclude: do these birds come back earlier over the years? I have no reason to doubt that they do. I also have no problem believing that the climate is changing and that those migratory bird react on that, but the differences are not as large as the media want to let us believe.
Trust, yet verify 