Knotted willows characterise many Dutch landscapes, where they are often seen in a row along creeks that have existed for decades. These willows are pruned every few years, which causes the characteristic wide “knot” at the treetop to form over time. Complex shapes can form due to this pruning, and it is quite common to see older knotted willows with a slightly, or entirely, hollow centre. In the latter case, the tree continues to grow broader and taller towards the sides, which could eventually result in a gnarled bowl-shaped treetop.
Last Spring, when I was on my way to a scenic lake located just outside the city of Leiden, I noticed something quite surprising. Along a long bicycle path, which is flanked by hundreds of old knotted willows on the left and right, one knotted willow caught my attention. For, right in the middle of its bowl-shaped treetop, a white goose (presumably Anser anser domesticus) was sitting on a nest. The shape of the large treetop was perfectly suited for a nest, as the centre had deepened but was still rigid, and the raised pruned edges enclosed the middle of the tree trunk. Unfortunately, I forgot my camera at home, but a few bystanders made sure to take a photo. It is rare for this species of goose to build or occupy a nest in a tree, as opposed to the Egyptian goose for example (Alopochen aegyptiaca), especially along a frequently used bicycle path with cyclists passing by at eye level. After doing some desktop research, I found an article in an ornithological journal by Kleefstra and Bles (2000), in which similar observations were described. Although, in their case the nest was located higher, was pre-used by birds of prey and the geese in question were Anser anser (the undomesticated, but highly similar, greylag goose). Kleefstra and Bles (2000) predicted that this observation, which had not been documented in the Netherlands before at the time, would become less rare in the future given the availability of nesting sites in trees and the increase in population density and predation and/or disturbance at ground level.
In the two preceding decades, similar observations had been documented in Germany, suggesting that there was a trend, while this behaviour was still seen as very rare in Sweden (Kleefstra and Bles, 2000). This made me wonder: has this behaviour been recorded (more often) since? A further search online yielded several recent posts and news articles about Anser anser nesting in various trees: multiple in the Netherlands (several cases in Nijmegen and one in Drenthe), one in the UK (Scotland) and one at an unknown location in Europe. Thus, although this phenomenon in Anser anser (domesticus) still seems to be uncommon at the moment it might be less rare than in the past, at least in the Netherlands. However, more observations are needed to establish whether there is a trend or not.
During the online search, I noticed that this behaviour has also been recorded in and around the Netherlands in the canada goose (Branta canadensis), even though this species, just as Anser anser and subspecies, usually nests close to the ground (see for example: Biesbosch, Amersfoort, Leerdam & Galmaarden). In Leerdam, the Netherlands, the couple of canada goose was observed on a similar knotted willow along a bicycle path, where they attacked cyclists time after time to protect their nest. In Galmaarden, Belgium, a knotted willow was also chosen by this species as a nesting site. The Netherlands is one of the most densely populated countries in the EU, and the observed uncommon nesting behaviour makes me wonder if the lack of space and increased disturbance could have something to do with the tree-nesting phenomenon. The case that was observed by Kleefstra and Bles in any case suggests that these pressures can result in this uncommon nesting behaviour in Anser anser in a non-urban environment, so this might also be the case in (sub) urban environments. Perhaps periodic high water tables also makes it more likely that these birds seek a higher nest.
This Spring, I wondered if the willow nest would be occupied again by the goose I observed last year. I ventured out and eventually found the exact same willow tree. It was not an empty nest or a goose I found on its top, but a peacefully nesting light-coloured duck. Nesting in trees is not uncommon in many duck species, so this phenomenon was less surprising. What was peculiar, however, was that the duck in question seemed to be a mallard duck female that did not have the typical colouration of this species. The phenotype may in fact be leucistic, which is not common among wild ducks. However, just as with the domestic goose, the wild ancestor (Anas platyrhynchos) can easily hybridize with various feral and domesticated ducks. Domesticated ducks were often bred to select certain phenotypes (such as white), making it more likely that uncommon phenotypes will be expressed.
I will continue to monitor the nest to find out if it will still be occupied by Anser anser (domesticus) in the next two months after the duck will have left the nest. Or, perhaps, another unexpected occupant will again be found. One thing is for sure, this old knotted willow seems to be a very attractive nesting site!
References
Kleefstra, R. & Bles, A. (2000). Boomnesten van Grauwe Ganzen Anser anser: een nieuwe trend? Limosa, 73, 148-150.
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