The moths are drinking tears from the eyes of sleeping birds

In a strangely poetic act, butterflies will sometimes drink the tears of the eyes of a sleeping bird. It sounds harmless, but this rare interaction is unquestionably a one-way affair.

Scientists actually have a word to describe their unique tear behavior, which they call "lachryphagia". To feed in this way, the mites gently insert their straw proboscis into the eyes of their reckless hosts. Scientists have seen mites drinking tears from various mammals and even reptiles like turtles and crocodiles. But on birds, not so much.

A new study published in Ecology is more than the third to document lachryphagia involving larval moths and their feathered hosts, the other two cases being in Madagascar in 2007 and in Colombia in 2015. This third documented case of Leandro João Carneiro de Lima Moraes, a biologist with Brazil's National Institute of Amazonian Research, did field work near the banks of the Solim Soles River, in the center of the Amazon.

Just before midnight, November 7, 2017, Moraes saw an adult female standing on a branch.

"On approaching, I noticed that this individual has an active erebid butterfly (G. macarea) located above his neck," he writes in his report. "This butterfly was constantly moving its proboscis towards the eye area of ​​the birds, sometimes resting inside the eye and probably feeding on secretions (tears)."

Later that night, Moraes witnessed a similar scene involving a different bird and moth, both belonging to the same species as before. In both cases, the diet did not disturb the birds during their sleep, remaining completely still while Moraes was watching. While feeding, the moths rested on the necks of the birds, doing their best to avoid waking their guests.

That said, it is surprising that something so intrusive does not bother birds. In his article, Moraes says that their immobility is probably the result of a torpor of birds while they sleep at night. In this inactive state, their metabolism decreases considerably, as does their body temperature.

This feeding behavior may seem harmless, but there is a risk for birds, which do not benefit at all from this relationship. Lachryphagy "can diminish the physical form of [a] target the animal by increasing the risk of eye diseases, "writes Moraes, adding that tear-shaped butterflies prefer placid and slow animals, but" immobile sleeping birds may be good targets for this behavior ".

For moths, the benefit is obvious. The tears are filled with nutrients, such as sodium and protein, namely albumin. The tears serve as a supplement to the diet of the mites, improving their flight performance and their chances of reproduction. Moraes says that sodium is a particularly valuable nutrient for moths because it is difficult for them to find it elsewhere in the region, which is likely to be flooded.

The rarity of this event suggests that butterflies do not look for birds very often, but Moraes says that more observations are needed to be sure.