I'll share this call for informations and recordings:
Very rarely garden warblers (Sylvia borin) sing so atypical songs that you need to see the bird in order to find out, which species you have heard. For years we have tried to collect informations and recordings of such mysteriously singing garden warblers. By now we have got recordings of 31 birds: 16 from Germany, 2 from Holland, 1 from Switzerland, 6 from Denmark, 5 from Sweden, and 3 from Finland. Examples of the songs of the 31 birds can be heard at the site: http://www.ginster-verlag.de/Raetselsaenger.html
Ten of the 31 birds (category b on the above mentioned site) sang the same type of song, which consisted of very short elements with a rate of 10-15 per second. The overall pattern of each song was a wawing up and down in frequency - a character also found in normal garden warbler songs. Some individuals were recorded in more years.
The mysterious garden warblers have been found in a rather narrow belt, reaching from Finland to southern Germany and Switzerland. Why do some garden warblers sing such peculiar songs - and never normal garden warbler song? Three hypotheses have been forwarded.
One suggests that the birds have been hatched late in the season, when adult garden warblers have stopped singing. If they have to learn their songs from adults in the first calendar year, they are then forced to develop their singing based on only an „innate" template. According to this hypothesis, the birds sing a crude model of garden warbler song. This might apply to the 10 birds in group b on the above mentioned site. The remaining birds sing a bit more elaborate songs, which may be because they have had some experience with garden warbler singing. If the hypothesis is true, we might expect the mysterious garden warblers to be found especially in colder climates with shorter breeding seasons. However, one problem with the hypothesis is that we do not know when garden warblers acquire their song models.
Another hypothesis is based on the observation that one of the individuals did not respond to play back of garden warbler song. The observers concluded that the bird was deaf. The reason for the abnormal songs should thus be, that the birds cannot hear and acquire the necessary song models. However, a number of studies of singing in birds, which have been experimentally deafened at an early age, show, that the birds develop very diffuse song elements (probably due to the lack of auditory feedback). The abnormal garden warblers sing well-defined song elements. That elements appear fuzzy on some recordings is mostly due to the recording quality and/or the acoustic conditions during the recording.
The third hypothesis relate to the geographical distribution of the recorded birds, which as mentioned are found in a rather narrow zone from Finland to Central Europe. It is suggested that two genetically different populations meet in this zone, and that the mysterious singers are „hybrids" between parents of each genotype. Why these „hybrids" should develop abnormal songs is unknown, but one suggestion is that they are deaf. However, at present no data are available on the suggested divide between genetically different garden warblers in the zone with abnormally singing garden warblers.
The conclusion must be, that we simply do not know the cause of the abnormal songs.
Therefore we ask for more observations and recordings.
1) Have you heard birds with this type of song? Please let us know, and if possible, send us available informations and recordings.
2) If you happen to meet birds with this type of song in the future, we should like to know all about when and where. Preferably, we should also ask you to record to singing of the bird(s) with whatever available equipment. The most import thing is to get rather close to the bird, so that echoes from the surroundings are reduced.
3) Is the bird deaf? You may help to solve this question by playing garden warbler alarm (mobbing) calls to the bird. Does it react to the calls? If you need a recording of garden warbler alarm calls, please, contact one of us.
4) Does the bird attract a mate and produce offspring? All observations on the behaviour of the bird in relation to conspecifics is interesting.
5) If possible, it would be optimal to ring the bird, so that it could be recognised, if it returns in future years.
Contact one of us, if you need any information on the project, and if you can help us with informations and recordings.
Goetz Rheinwald & Poul Hansen
Email: goetz.rheinwald @ t-online.de
Email: poulh @ nathist.dk