Slamming the brakes for a meal
For Gull-billed Terns flying is the easiest thing. When in a hurry they effortlessly speed up to 80 km/h (50 mph), but they also like to slowly soar against the wind when in search of food. This body is optimized for all kinds of flying tasks on mimimum fuel consumption.
When hunting (which is more like gathering than hunting) they rely on their big eyes, not missing out a single thing on the ground - a moth, a tip of an earthworm, a frog in a ditch, the smallest movements of mitten crabs in the mud. It all starts with a seemingly relaxed search flight when suddenly the target ist detected, followed by an immediate stop (like slamming the aerial breaks) and a downward swoop. The beak's final grasp almost never fails. It is a special feature of Gull-billed terns that they never dive while feeding. Only the head dips in, not the body. Gull-billed Terns have a large proportion of terrestrial food among their diet and do not care about fish very much. Compared to other tern species of that size these preferences are quite unique, indicating tight evolutionary adaptions to a special habitat between land and water.
This beautiful shot of a Gull-billed Tern (one second away from catching a mitten crab) was taken by the Dutch photographer Fred Visscher in the Harbour of Neufeld on July 16th, 2021. The perfection lies in the dynamics of that very moment, frozen in time. While it is obviously a still photo it tells a lot about motion and the events a few seconds before an after the shot.
Just by chance this photo tells yet another story - that bird is individually tagged with colour rings. Actually, that is why we know it quite well. Here is the story so far: This bird was ringed as a chick on June 26th, 2015, in the Neufelderkoog-colony. Since then it has been reported by birdwatchers in Germany and the Netherlands more than 30 times. When a few weeks old, right from its early flying days, it showed up at several Dutch sandplants joining its parents in its first turn of migration. Since then and over the years it has been reported a number of times from those very spots. The first year of its breeding was 2019.
Saudações de Portugal
Recently a dead adult Gull-billed Tern was found behind the colony - carrying a ring from Portugal. Of course, a dead tern is always a sad loss to us, but beyond its death this ringed one could perhaps give answers to the still open question whether or not genetic exchange is going on between our colony and other European colonies (which in our case are all more than 1000 km away) and, if so, to what extent. Two options are possible: In case this bird was ringed as a chick or breeding adult in a Portuguese colony, we would indeed expect some degree of exchange. In case this bird was ringed out of the breeding season, particularly during migration, there would rather be more support for the no-exchange scenario. Moreover, this bird could then well be from our colony at Neufelderkoog where it was found dead at its likely birthplace.
The matter of genetic exchange and the risk potential coming with inbreeding has drawn our attention for quite some time (see blog of May 16th) with good reason. The Neufelderkoog colony is located more than 1000 km away from the next "neighbour" colony. In terms of population biology this is like a far away island in the ocean.
Since processing of international ring records do take a lot of time (and we just could not stand to wait) we asked Dr. Sandra Bouwhuis, scientific director at the Institut für Vogelforschung in Wilhelmshaven, to lend a hand on this in order to shortcut the process. And the answer was quick!
The Gull-billed tern with the ring I 15390 was tagged as adult (2 years or older) on September 8th, 2018, at Salinas do Samouco (Setúbal, Portugal) at night around 1:30 o'clock. Body mass was 194g which is a normal weight for a healthy adult tern. This place is a salt production site with lots of ponds in the River Tejo estuary where migrating birds gather to roost, a view to the Portuguese capital Lisboa included. From other ring records we know that Gull-billed Terns are using the western European/Atlantic coastlines as orientation during migration and particularly like to stay in estuaries. The date of ringing lies well outside the breeding season when most individuals have left Central Europe already, particularly when they don't have chicks. Hence the birthplace of the dead specimen still remains unclear, but with high probability it is Dithmarschen, the Neufelderkoog-colony. Perhaps there will be some more specific answers coming soon with the help of molecular genetics.
Last weekend 42 chicks have been ringed in the main colony. This is exactly this years number of breeding pairs. We are happy to see that more chicks are about to come up because there are still nests with breeding adults for which hatching is expected in the next 10 days. There was no indication of deseases or predation in the colony. The chicks appeared to be healthy and vital so that we have reason to hope for a good survival. It could well turn out to be an above-average year in terms of reproduction. Fingers crossed! We will know in August.