When rare bird species are discovered, many birdwatchers and ornithologists are thrilled. If the sighting is widely communicated through the established networks, a group of passionate birdwatchers will quickly gather to see the bird and want to add it to their “life list” or “year list”. For this, trips of several hundred kilometers are regularly undertaken, and even an entire country is sometimes traversed. However, today it is often about completing one’s own photo collection, which is an even greater challenge because good photos rarely come from a great distance and require time and effort. Rare birds often bring together many interested parties from near and far, and the lively exchange of experiences and social contact is always an important part of the event.
One such example was the surprising discovery of a Yellow-billed Diver on the 13th. January 2024 on the Thalinger Reservoir, 13 km southeast of Linz in Upper Austria, directly on the border with Lower Austria. Aside from the bird’s usual wintering on the Atlantic along the Norwegian, Scottish, and Irish coast, its stay in central European inland was exceptionally long, lasting at least 45 days, as the bird remained until the 26th. March in a very limited area. The newly published book by Ernst Albegger, “Rare Bird Species in Austria,” mentions only nine previous sightings between 1840 and 2014, with a stay ranging from a few days to two weeks. In only one case did a juvenile Yellow-billed Diver linger for 4 weeks. While Yellow-billed Divers usually only stay in inland areas for relatively short periods, razorbills often stay here for months, often an entire winter, and sometimes even remain until the following summer, as has been the case in recent years at large lakes, such as Lake Constance.
In any case, the Yellow-billed Diver discovered at Thalinger Reservoir last winter is one of the best documented specimens, as it was observed and photographed from a relatively short distance, raising the question of what may have prompted the bird to stay in an area that is quite small for its species for so long. The bird regularly displayed noticeable behaviors, such as seemingly “choking” movements that lasted for varying lengths of time, with its head held straight or vertically upward, as seen in figures 4 and 5.
With high-resolution optics, a thin string protruding from the beak was sometimes visible, which protruded from the beak like this, but was usually not visible as it lay within the beak.
The string, which in rare cases extends straight down, shows a typical tension for fishing lines. Such lines are often used for catching smaller baitfish, therefore the hooks are relatively small and therefore do not lead to the quick death of larger birds.
External injuries were not visible at first glance with a standard spotting scope (Fig. 1). Therefore, in the following days, a Zeiss Harpia high-performance spotting scope with a Morpheus 4.5 mm eyepiece was used, which provides 116x magnification with excellent sharpness to allow for a closer look at the beak and throat region. It was eventually noticed that a fine thread protruded from the beak slit on the right side, which was attached to the right half of the lower beak (Fig. 2), but occasionally it also protruded downward and was visible below the beak at very high magnification. The straightness of the line on the outside of the lower beak, but especially the line that occasionally protrudes downward (Fig. 3), shows the tension typical of fishing lines, which supports the assumption that the bird had probably swallowed a smaller fishing hook or lure, which would have caused the bird considerable pain and would have put its survival in serious question in the long to medium term.
The Diver regularly showed unusually long and intense “choking movements,” which indicated a problem in the upper digestive tract.
Accordingly, the Yellow-billed Diver spent a long time intensively removing limbs from the signal crayfish often caught in the reservoir, which is a completely normal behavior for both species of great crested grebe, but appeared unusually long in terms of processing time. Even medium-sized specimens often had their dorsal shield removed before being swallowed (Fig. 6). Larger signal crayfish were obviously unable to be swallowed and were released again. Even with larger fish, the bird had trouble swallowing them. Thus, it was observed several times that fish of more than double the beak length, which are normally no problem for the common Diver and the red-throated Diver, could only be swallowed after almost half an hour of struggling. All this suggests that the bird suffered from a complication in the upper digestive tract, which is likely due to an irremovably stuck fishing hook. Unfortunately, fishing hooks are so durable that they do not even begin to rot within a bird’s lifetime, which means the rare Diver has no chance of long-term survival.
Medium-sized signal crabs were usually swallowed only after removal of their extremities and, as shown here, their large dorsal shield.
It has been observed multiple times that Common Divers are interested in natural and artificial bait connected to fishing lures. A few years ago, such a phenomenon proved fatal to a Common Diver near Plobsheim on the Rhine, who died miserably.
In this context, it is also worth mentioning the stay of a Yellow-billed Diver in Germany at Diemelsee from the 13th to the 28th. December 2016, which showed a distinct injury on the neck, which could have been caused by a fishing net or a rope associated with it. Thus, the stay of the world´s largest Diver at the Thalinger Reservoir, which was celebrated by many birdwatchers, had a rather sad background. In any case, there is much evidence that this bird too has fallen victim to human civilization.
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