From Threat to Shelter: How Birds Repurpose Anti-Bird Spikes

Cities are becoming more and more grey, with less and less living biomass. The consequence is that there is not enough natural nesting material available for the urban birds. These birds adapt to use alternative anthropogenic nesting materials. Examples include synthetic twine, string, and rope or artificial plants made of plastic (Hiemstra et al. 2021). Besides that, more and more birds nest in and on buildings as a substitute for trees. This way, they adapt to urban environments where natural nesting sites have become scarce.

Anti-bird Spikes

However, these birds in urban areas cause nuisance with noise and excrement. People therefore want to get rid of these birds and thought they had found the perfect solution: anti-bird spikes. Rows of upward pointing spikes made of metal or plastic are placed on building roofs and statues to deter birds. However, these spikes don’t seem to deter all birds, some birds even use them for their advantage. Peregrine falcons (Falco peregrinus) for example use these pins as their personal pantry, impaling their prey on the spikes for later consumption (Verkerk 2018). Other birds found ways to remove these anti-bird spikes from buildings. By doing so, they once again have free access to rooftops and ledges. Here, they can rest, observe their surroundings, and nest safely without fear of harm (O’ Connor 2019).

Exclusive Nesting Material

Some birds take it even further. They use these anti-bird spikes as nesting material, with the nest existing of these spikes almost exclusively. This has been found for multiple corvid species: European magpies (Pica pica) and carrion crows (Corvus corone). Evidence shows that both species actively remove and steal these pins from the buildings, skillfully dislodging them.

As magpies are known to put in a lot of effort to obtain nesting materials, such as ripping off twigs from trees, it is not surprising that they show this behaviour. Crows build their nest with the spikes pointing inwards, using them as a supporting structure to secure twigs into place, without hurting the breeding individual. Magpies however seem to use the spikes for a different reason, the same reason we as humans use them: to ward off birds. The spikes in the magpie nests are facing outwards. This forms a prickly dome protecting the eggs from unwanted predators such as other corvid species. It makes it more difficult for intruders to reach inside without risking injury (Hiemstra et al. 2023).

Natural versus Man-made

In natural environments magpies use thorny twigs as a defence mechanism against these nest predators. It might be that the thorny bushes are less available in the urban areas. The anti-bird spikes are then an urban replacement of the natural material. However, it is a possibility that the spikes function as a better bird deterring mechanism than the natural thorny twigs. Therefore, the magpies might prefer the artificial material over the natural (Hiemstra et al. 2023). Either way, the use of these anti-bird spikes is a beautiful example of the flexibility of birds to adapt to changing environments and life in the city. These birds used human made material that was made to harm them and turned it into something that protects them instead.

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References

Hiemstra A.F., Gravendeel B. & Schilthuizen M., 2021 – Birds using artificial plants as nesting material – Behaviour 159: 193-205. http://dx.doi.org/10.1163/1568539X-bja10115 

Hiemstra A.F., Moeliker C.W., Gravendeel B. & Schilthuizen M. 2023 – Bird nests made from anti-bird spikes – Deinsea 21: 17 – 25 https://natuurtijdschriften.nl/pub/1026185/DEIN2023021001002.pdf

O’Connor C., 2019 – Calls for bird birth control to save Geraldton’s historic Saint Francis Xavier Cathedral from corellas – ABC Mid West & Wheatbelt. https://www.abc.net.au/news/2019-07-05/corella-contraceptive-needed-to-address-plague/11274434

Verkerk M., 2018 – De wilde stad – Amstelfilm, Amsterdam

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Featured photo: Carrion crows (Corvus corone) on their nest, partly made of anti-bird spikes, in a poplar tree. Rotterdam, The Netherlands; 7 April 2009. Garry Bakker.
Bakker G., 2009 – Natuurlijk Rotterdam: Spitsvondige kraaien maken nest van duivenpinnen – Straatgras 21: 52. https://natuurtijdschriften.nl/pub/538267/STGR2009021003004.pdf