drunk birds?

Suicide season

The Rufous-bellied thrush (Turdus rufiventris) is a songbird that occurs in most of eastern and southeast Brazil (where it is the national bird), Bolivia, Paraguay, Uruguay and central and northern Argentina. While in Uruguay they are known as “zorzales”, here in Salta they are called “Zorzales chalchaleros” or just “Chalchaleros”. While the English name for the species describes its distinctive reddish-orange underparts, the vernacular name in Salta reflects its preference for feeding on the fruits of a tree known as Chal-chal.

The Rufous-bellied thrush -very similar to the African Olive thrush (Turdus olivaceus)- occurs in different habitats (forests and urban treed areas) and it is common around rural houses where its song is one of the pleasures of rural living. An omnivorous bird that prefers arthropods and fruit it can eat broken maize during our dry winter season when food becomes scarce. As I mentioned earlier, one of its favourite fruits are produced by the Chal-chal (Allophylus edulis)[1].

We do have Chal-chal trees in the farm but not close to the house. However, about ten years back we planted a row of Hawthorn bushes (Crataegus spp.) to act as a wind-breaker against the predominant eastern winds that often blow in this latitude. This resulted in a rather unexpected high wall of trees that not only help to stop the wind but also yield what I estimate to be several hundred kilogrammes of red berries, towards the end of the summer months.

This plethora of fruits attracts a number of birds that include the large Dusky-legged guans (Penelope obscura), Chachalacas (Ortalis canicollis), Toco toucans (Ramphastos toco), Blue-and-yellow tanager (Rauenia bonariensis) and Sayaca tanager (Thraupis sayaca). Although the latter are rather spectacular, the most common birds that come to feed on the hawthorn are the Rufous-bellied thrushes. We probably have a few dozens of them constantly moving to and from the red berries.

Year after year, the resulting thrush heavy and uncontrolled air traffic causes casualties announced by loud bangs coming from our only large east-facing window in the house (that also faces the row of hawthorn trees about twenty metres away). Usually, about two or three thrushes (no other species do this) either die or get stunned after heating the glass. So far we have accepted this as an unavoidable consequence of the increased number of birds brought about by the abundance of food.

The offending window showing the reflection of the surrounding garden.
A stunned thrush after crashing.
Sometimes the thrushes came at such speed that they broke through the mosquito screen as shown here by the Bolivian guiro (a musical instrument).

This year, however, the suicides (birdicides?) reached alarming levels and yesterday we had four hits (three dead and one recovered), a rather alarming number! Although the first bird that hit during early morning recovered, a second one crashed about an hour later so we decided to do something about it.

I remembered having read somewhere that if you drew lines on the glass with a highlight pen, somehow the birds eyesight would see them from far and avoid the window. I drew the lines and, satisfied with my job I called Mabel to see it. The moment we were close to the window a bird nearly hit Mabel’s head and the loud thud indicated another fatal outcome! The fluorescent lines did not work so, do not try this at home!

So, “encouraged” by Mabel I placed a rather obstrusive zig-zag of flourescent yellow tape that occupied the top of the glass, at the area the sky was reflected. We decided that it was better to interfere with our view rather than nhaving more casualties!

The yellow tapes in place.

So, proud with my work but now tired, I went for my obligatory siesta (a pleasure of these regions!) to recharge my batteries.

When one hour later I woke up, Mabel was very upset as a fourth bird had killed itself!

In desperation and after some more thinking, we remembered that we had bought some bird netting to protect our fruits. We placed the netting in front of the window in a way that resisted me throwing the Bolivian guiro that was the closest to a thrush I could find for a test!

The Bolivian guiro itself.

Below I show you the netting and a video showing how we expect it to work.

A lateral view of the netting.

We believe that the deaths will stop now but our discussion has turned now to resove the reason that compels the birds to do this.

A couple of years back we thought that the birds could see a mirror that we have in front of the window and tried to fly through. As the birds continued hitting the glass when we covered the mirror, this idea was abandoned.

However, we are convinced that the birds see the reflection of the sky in the window and try to fly through.

The presence of predators, in particular the Sharp-shinned hawk (Accipiter striatus) make us also believe that it could chase the thrushes and the latter, trying to escape, bump themselves against the window. In favour of this hypothesis is that we have seen the hawk catching thrushes and other smaller birds around us. However, it is unlikely that the hawk would try to kill four birds the same day when one would be sufficient for a few days.

That leaves us with the last hypothesis that had been put forward by Mabel: the ripe fruits of the hawthorn ferment in their crops and their small livers are not able to process the resulting alcohol with the result that they get drunk! The fact that the berries are ripe now and likely to ferment faster, supports this hypothesis. In addition, after “googling” the idea, I found that at least one similar event involving the hawthorn and Cedar waxwings (Bombycilla cedrorumhas) happened in the USA [2] and other instances of drunk birds also exist [3][4].

[1] I did not find an English name for this species.

[2] S. D. Fitzgerald, J. M. Sullivan and R. J. Everson (1990). Suspected Ethanol Toxicosis in Two Wild Cedar Waxwings. Avian Diseases 34, pp. 488-490.

[3 See: https://www.washingtonpost.com/news/morning-mix/wp/2018/10/04/drunk-birds-are-causing-havoc-in-a-minnesota-town-police-say-theyll-sober-up-soon/?noredirect=on or https://time.com/5415378/drunk-birds-minnesota/

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7071081/