There is a lot of information on the behaviour of aquatic warblers in neighbouring countries, where they are still found in large areas, but there is almost no information on how the birds behave in the Nemunas delta region, Lithuania, where their habitats are fragmented and distant from each other. It was therefore decided to carry out a more detailed study of the movements of aquatic warblers in this region.

COLOURED PLASTIC RINGS USED IN THE STUDY

The ringing method was used for the study. In addition to the usual metal rings, the birds were marked with combinations of coloured rings, which allowed each individual to be identified without having to catch it again. The combinations used were those in which a green ring was placed on the bird’s right leg. The green ring represents the country code. It is only accompanied by a metal ring on the right leg. On the left leg, there are combinations of 2 or 3 rings. The colours used are white, yellow, orange, red, green, blue, purple, and black.

The Nemunas delta region of the study area was divided into six distinct parts, separated from each other by habitats that are unsuitable for breeding of aquatic warblers.

The surveys started in early May, when the first birds flew back from their wintering grounds, and ended in early August, when most of the birds leave for their autumn migration.

In 2014 – 2021, 369 birds were caught and ringed with Lithuanian metal rings. Most of them are also marked with a combination of coloured rings, by which each individual can be identified at a distance. The table below shows the number of birds caught and ringed in different years of the study, in different territories.

Number of ringed aquatic warbler individuals in different areas by year. 

A total of 18 first-year birds and 27 adult females were ringed. The remaining 324 birds were all adult males. The locations where the aquatic warblers were caught and ringed in different areas are marked on the maps. They are not published in this article due to the aquatic warblers‘ globally protected status.

The survey recorded 439 repeated detections of ringed warblers in their breeding territories in the Nemunas delta region. 172 ringed individuals (about 40% of all ringed birds) were detected and identified again at least once.

BIRDS ARE NOT ATTACHED TO THEIR BREEDING SITES

The ringing and detection sites were mapped according to their coordinates, which clearly shows that not all breeding birds in the Nemunas delta region are strongly attached to the breeding areas. Even within the same breeding season, some birds were moving from one area to another (red lines on the map). Some birds have been found changing breeding territories during different seasons, being ringed in one area and then a year or two later being found in another area (yellow line map).

MIXING WITH BIRDS IN REMOTE AREAS

We also recorded five birds that were ringed in the areas inhabited by the aquatic warblers during the breeding season outside the Nemunas delta region. Two of them were ringed in Lithuania, in the vicinity of Lake Žuvintas (Alytus county), and three were ringed in Poland, in the Biebrza National Park (NP) in the territory of the voivodeship of Podalskie.

The distance from the breeding area in the Nemunas delta to the breeding area in Poland is 250 kilometres, and 180 kilometres to Žuvintas. The maps show that birds from aquatic warbler populations (as far apart as several hundred kilometres) mix with each other.

DIFFERENT AUTUMN AND SPRING MIGRATION ROUTES

During the survey, 24 birds were recorded as ringed abroad (2 in Belgium, 1 in the Channel Islands, 3 in Spain, 17 in France, 1 in Slovakia) on their migration routes. As these birds were only ringed with standard metal rings, coloured combinations of the rings were added on when they were caught in Lithuania, so that the birds could be identified later on without having to catch them.

Of the 24 birds recorded, 23 individuals were ringed abroad during the autumn migration (21 in August and 2 in September). One individual was ringed during spring migration (in April in Slovakia). Seven birds were also found ringed in Lithuania during the autumn migration (all in August).

The data collected confirmed previous information from other countries that most of the aquatic warblers fly to their wintering grounds in Africa during autumn migration via the Western European flyway via Belgium, France, Spain and Portugal. The birds reach these countries as early as August and September.

Meanwhile, in spring, the birds return along a more easterly migration route through central Europe (as evidenced by an individual caught in Lithuania that was ringed in Slovakia during spring migration).

The furthest-ringed bird found in the Nemunas delta region was ringed in Spain. The distance from the ringing to the detection site was 2420 kilometres. The closest bird ringed on the autumn migration route was in Belgium, 1,265 kilometres from the detection site in Lithuania.

The longest interval between ringing abroad and detection in Lithuania was 1786 days (4 years 10 months 26 days). The bird was already at least one year old at the time of ringing, so it was found in Lithuania at approximately 6 years of age.

Aquatic warblers ringed in the Nemunas delta region and found abroad in Belgium (1), Spain (1), Portugal (1) and France (4) on their autumn migration route. The furthest distance from the ringing site along the migration route was in Spain, 2700 kilometres from the ringing site, and the closest was in Belgium, just over 1200 kilometres away. Six birds were detected in the same year as being ringed (10 to 72 days between ringing and detection), with only one individual detected (photographed) in Portugal 388 days later (just over a year after ringing).

MORE FREQUENT RINGING OF YOUNG BIRDS

The vast majority of the aquatic warblers ringed abroad during the autumn migration and later found in Lithuania were ringed as first-year juveniles (20 out of 23). The remaining 3 birds were caught as adults (older than one calendar year) during the autumn migration.

Of the 251 adults (older than one calendar year) ringed in Lithuania, 5 birds (about 2%) were found abroad during the autumn migration, while 2 individuals (about 11%) of the 18 juveniles (birds in their first calendar year) ringed in Lithuania were found abroad.

Although the sample size is not large, the data suggest that young birds on the autumn migration route are more likely to rest and feed more often and/or for longer periods of time, in contrast to mature individuals, and are therefore more likely to fall into ringers’ traps.