How do crows break open hard shells of molluscs?

Updated: Feb 28, 2021


Northwestern Crows on Mandarte Island in Canada are known to feed on ‘whelks’. Whelks are molluscs, particularly sea snails which have a shell around them. Crows need to break open these tough shells in order to feast on the juicy snails inside. The crows pick up these whelks, take a vertical flight up into the sky and release them from the top. After doing this a few times, the shells break open and the crows are able to enjoy the snail inside.


However, this seemingly easy process involves making complicated and important decisions.


First, the sea bed has whelks of different sizes. Which whelks should the crows choose? Second, the flight upward is energetically costly for the crows. How high should the crow fly so that the whelks break but do not disintegrate into smaller pieces? And third, where and how should the crows drop the whelk? Carefully designed experimental, observational and theoretical studies performed way back in the 1900s have answers to these questions.


1. Which whelks do the crows choose?

Crows choose the largest of all whelks, because bigger whelks have higher caloric content and break open more easily. The benefit of opening the shell should always be more than the cost. If large shells are not available, crows switch to eating something else, but refuse to put effort into opening the smaller whelks. Crows assess size of whelks first by sight and then by weight. Some whelks were even picked up with the bill and laid down again before making a final selection!


2. How high should the crow fly so that the whelks break but do not disintegrate into smaller pieces?

The crows consistently flew on an average 5.2 metres up while dropping the whelk. From this height, the crows had to drop a whelk on an average 4 times for it to break open, some needed as many as 20 drops! The number of drops required to break the shell reduce if the crows fly higher. Why then, do they still drop shells from ~5m? That is because first, crows are more likely to lose track of and lose shells dropped from further up. There is a higher possibility of whelks disintegrating into smaller pieces and being stolen. And second, going higher up is more energetically costly than doing multiple rounds of 5m height and this has been proven theoretically by scientists! Crows know math, do they? Or do they just ‘learn’ the most optimum way out!?

3. Where and how should the crows drop the whelk so that the chances of them breaking increase? All whelks were dropped on rocks and not over water or on grass. In addition, crows had specific dropping sites. Surface of such dropping sites was hard but whelks were unlikely to bounce into water from these carefully selected sites.

To add to this, most crows dropped the shell when they were facing down, as shown in the figure. This enabled them to see and track exactly where the shells are landing!”

So much ‘calculus’ and 'economics' behind such a superficially simple process of eating a snail! But animals have nailed the business and hardly ever work in loss. Nature has really taken care of it all! :)


If you have observed or read about how animals can take 'optimum decisions', do let me know in the comments! :)


References:

- Zach, Reto. "Selection and dropping of whelks by northwestern crows." Behaviour 67.1-2 (1978): 134-147. Link: https://brill.com/view/journals/beh/67/1-2/article-p134_5.xml

- Zach, Reto. "Shell dropping: decision-making and optimal foraging in northwestern crows." Behaviour 68.1-2 (1979): 106-117. Link: https://brill.com/view/journals/beh/68/1-2/article-p106_6.xml


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