Finding mates under limited conditions
Every year Gull-billed Terns are returning from their West-African wintering grounds to the same breeding sites in Europe. They particularly go back to places where they have been breeding successfully before. Moreover, during migration they seem to use familiar routes and may even pass this knowledge through to their chicks by taking them to these places en route. Their geographical memory seems to be amazingly precise. This conservative strategy leads Spanish terns return to Spain while only terns from Neufeld return to Neufeld. In many life-history respects this strategy may be advantegous on a general or species level, but in this particular case of the small and isolated Neufelderkoog-population there is also a downside.
Since the Spanish, French or Italian terns do not know that there is suitable breeding habitat in Neufelderkoog (only the local ones do) they have no reason to fly that far away north from their home turf (at least 1200 km straight). Thus all members of the Neufelderkoog-colony are probably descendants of the former Danish-German population which has been condensed and isolated a long time before, maybe 200 years. This bears the risk of inbreeding and ongoing decline in genetic diversity. Until today almost 400 chicks have been ringed in Neufelderkoog, but we have no indication of genetic interchange between colonies or populations yet.
The depicted couple is spending some time in the pre-breeding season at Neufeld harbour. Both partners are colour ringed indicating that both have been born in the Neufelderkoog colony (the bird on the right in 2017).
Peaceful nights and electric fences
Currently 74 Gull-billed Terns are having a quality tryout of the breeding site by using it as overnight roost. Hence the electric fences are now in place keeping those areas free from mammalian predators. The terns seem to notice that at night (when predator activity is most dangerous for them) those protected places are quiet and peaceful. As we have learned in previous years this makes the terns like the area even more for breeding. Over time this may have lead to the remarkable site-fidelity we now see at the Neufelderkoog-colony.
As many studies have shown mammalian predators have become the most important factor compromising breeding success in ground-nesting birds, at our colony it is foxes and raccoon dogs.
Seasonal record at the evening count
In the prebreeding season Gull-billed Terns perform an interesting behaviour. Over the day they are foraging far away from the later breeding site, but return at sunset, in order to stay the night there sleeping and resting. Hence the evening count is a good way to estimate how many terns are already around at the moment. Tonight there was a surprise - 73 individuals have been counted which in 10 years was the largest flock this early in May. Usually the maximum (ie all breeding birds) were counted between May 10 and 20th, still some time to go. Thus expectations are high that a good number of Gull-billed Terns will show up this year for breeding.
Colony site close to the River Elbe at dawn with hundreds of Black-headed Gulls, Common and Gull-billed Terns. Photo: Moritz Häring