After I lay down my handful of un-shelled peanuts it’s only a matter of minutes before I can hear the “thwap!” as the steller’s jay hits the cable wire that runs above my balcony’s railing. It balances there for a moment before descending onto the pile I’ve offered it. Quickly, and with obvious purpose, it springs down the railing and picks up a nut. Its mohawk feathers bounce as it snaps and cocks its head around in various direction. After only a few seconds, I hear the sound of rejection; the distinct hollow tap as the nut is returned to the railing. The jay repeats the same process with two more nuts before abruptly flying off with one that, to my eyes, appeared identical to the first two.
Certainly most manner of corvids engage in some kind of choosy behavior though I don’t think any of them go about it with as much frenetic spunk as jays do. So the questions arise: Why are they being so picky? What do they know about the rejected nuts that my eyes can’t see?
According to a new study1, Mexican jays are actually ‘weighing’ nuts during this process. By using specialized slow motion cameras, researchers showed that those snappy head movements are actually a way for the birds to get a tactile feel for the the nut’s weight and listen to the sound the peanuts makes as it rattles in its shell. By providing nuts that were visually similar but different in mass, the researchers were able to show that jays could consistently select nuts with the most nutmeat density. A further test showed that large shells that were altered to contain only one nut were typically selected first, only be be rejected, while single nut shells were accepted. This suggests that jays either have a sense for how much nut should weigh (and thus reject nuts that contain less than they should) or that the correlation between hollow sounds and nut density lead to the ability to choose denser nuts.
So the next time your visiting jay delights you with its sassy head snaps remember; it may simply be amusing to you but for jays, it’s an impressive product of evolution that helps keep them alive.
Literature cited:
1) Piotr G. Jablonski, Sang-im Lee, Elzbieta Fuszara, Maciej Fuszara, Choongwon Jeong, Won Young Lee. Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina). Journal of Ornithology, 2015
corvidresearch.blog 
I like how crows and ravens just smash and grab, no weighing involved. 🙂
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I see this behavior every day I visit the carrion crows.
Taking one nut, shaking their heads very quickly. You can hear this rattling sound – then they either take the nut or dismiss it and try another one.
The explanation of the study seems to be the most probable cause for this procedure. The small peanuts (just one kernel) are the ones they take the least.
Hm. Have to think about and observe this in more detail, live and by my videos.
Carrion crows certainly are very “choosy” too, if they have choices.
The rattling sound I hear has an obvious reason: they try to get a peanut,
alongside, deeper into their throat by shaking their head vertically just to gain space in the beak for the *next* peanut. They rarely take only one peanut at one time. When the throat is filled with, say, 2 peanuts, they will try to somehow get some more (up to three) nuts into the beak – this time crosswise.
Then… it depends on the shape of the nuts and how to arrange them to be able to get them all at once – thus maximizing the usable mass of kernels. (It’s always hilarious to watch these trials!).
This may be a difference between the jays’ and “my” crows’ behaviour.
Ah. Corvids are the most thought-inspiring animals I know – aside from humans.
This is very common behaviour for the crows here in Vienna (Austria), mainly corone x cornix hybrids. Many of them also check the weight of whalenuts if they are offered a choice an pick the heaviest ones, regardless of the size.
We have repeated observed that the California scrub jay, Aphelocoma californica, will ‘weigh’ up to three peanuts before selecting the heaviest one of the three. It will pay no attention to a fourth peanut.
Choosy Corvids choose Jif.