Every spring and autumn, millions of birds migrate over our country. They do this mainly at high altitudes and at night, which makes the phenomenon largely invisible to us. But not for weather radars! On 9 March 2021, the Royal Meteorological Institute of Belgium (RMI) and the Research Institute for Nature and Forest (INBO) of the Flemish government, in cooperation with the Royal Belgian Institute of Natural Sciences (RBINS), launched a web application on which anyone can view this migration in real time across the Benelux. It shows that bird migration started exceptionally early this year, and that has everything to do with the unusually warm weather in the second half of February.
What does meteorology have to do with bird migration? A lot, it turns out, and in both directions. Birds wait for the ideal weather conditions to start their long journey to their wintering grounds in the south and back to their breeding grounds in the other direction. But equally intriguing is the fact that one of the most important measuring instruments in meteorology today, the weather radar, can measure this migration in detail. Indeed: weather radars do not only detect water droplets in the atmosphere, but also birds moving through the air.
The new website offers a unique overview of bird migration across the Benelux, based on ten weather radars located in Belgium, the Netherlands, France and Germany. The graph that is shown first when loading the website is the so-called MTR (Migration Traffic Rate). This parameter expresses the number of birds that pass through the vicinity of the radar at a given moment, through an imaginary screen that is 1 km wide and infinitely high (in practice, 5 km is taken as maximum height). It can be compared with the stream or the flux of birds that flows through the air.
The colourful figure underneath shows how these birds are distributed across the vertical layers of the atmosphere. The parameter shown in this figure is the bird density, expressed as the number of birds per cubic kilometer. However, certain assumptions have to be made to arrive at this number (e.g. about the typical size of a migratory bird). It also excludes bird movements close to the ground, as radars are not able to detect those. More explanation about the correct interpretation is provided on the website itself.
Spring Migration in Full Swing
The website (and the accompanying figure) shows how bird migration started during the exceptionally warm days in the second part of February, especially - because of the wind direction - in the eastern part of the Benelux. This is unusually early, as mass spring migration usually takes place during the month of March to mid-April, the exact period depending on meteorological conditions such as prevailing wind direction, average temperature and presence of precipitation zones. The reverse movement, towards the south, is usually made between mid-September and the end of October. Whereas previously this was a phenomenon that took place largely invisibly high up in the sky (and especially at night), it is now being made a little more visible for us ground-based earthlings!
Building on Existing Expertise
In the meteorological and hydrological applications of weather radar, the bird signals are neatly eliminated from the radar data. But for ornithologists and ecologists, this information is actually very valuable. Even though radars cannot distinguish between bird species, the bird signals provide invaluable insights into the dynamics and evolution of bird migration on a large scale that cannot be gained in any other way. In addition, this detection is also important for aviation: collisions between aircraft and groups of birds can sometimes have fatal consequences, and the detection of birds by weather radars can help to avoid these collisions.
RMI, INBO and RBINS have all built up considerable expertise in weather radar-based bird detection over the past ten years. A few years ago, the RMI installed special software that separates the bird signal in the radar images from the precipitation and then quantifies it. The end product of this bird detection has been shared with the Bird Control Section of the Air Component of Defence for several years, and with various European research projects. These have given a tremendous boost to the study of bird migration and the behaviour of insects and bats on the basis of radar. The research field even got its own name: radar-aeroecology.
The research institute INBO has been playing an important role in these European studies for years. In the past, its Open Science Lab for biodiversity developed innovative visualisations to better map bird migration. In recent years, RBINS has also contributed to the study of bird migration using radar. In particular, RBINS focused on the possible influence of offshore wind farms off the Belgian coast on bird migration. This included comparing bird signals from weather radar with measurements from a special bird radar installed on a platform in a North Sea wind farm.
Transparent and Accessible
In the past, occasional news items about this form of bird detection have often met with great interest from the general public. With the launch of the new website, the project partners want to make bird detection by weather radars accessible to everyone interested in the fascinating phenomenon of bird migration. Moreover, the development of the web application was done in a fully transparent way and it is available as open source software, making it portable and reusable for similar initiatives.
The web application is a result of the BELSPO valorisation project CROW, building on the expertise previously acquired by the same partners in the BELSPO project RAVEN, for which RBINS acted as coordinator. The objectives of RAVEN were to validate registrations of birds by weather and bird radars, to link small-scale observations by bird radar to large-scale observations by weather radar, and to process radar data from two complete migration seasons in order to estimate the total number of birds flown through the weather radar measurement volume during a complete migration season.
Short video on radar aeroecologie: https://youtu.be/uwvXKmOViws
