Blue Jays Harvesting Acorns: Coming Soon

It is almost September and the acorns are nearing maturity. A fascinating Eliza Howell bird-watching occasion will begin soon.

Blue Jays in southern Michigan in September are both migrants and year-round residents. Blue Jay migration is different from that of most other birds. They are found throughout their general range in both summer and winter, but large numbers move from one location to another in spring and fall. Mid-September to early October is the time of the largest numbers in the park, many more than in the rest of the year.

Eliza Howell has a number of large oak trees within the road loop and the acorn crop is the main attraction for the jays.

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Photo by Margaret Weber

Last year (2017), I did more careful observation than previously. It began with noticing that many of the acorns that I was crunching as I walked under one of the pin oak trees were really just the caps; there were far fewer nuts on the ground. I thought that the jays were probably responsible for removing the caps but I had never really given careful attention to their harvesting behavior, beyond observing their preference for oak and beech trees.

I decided to watch. In the late morning of September 22, I sat in the shade of another oak tree and watched the pin oak the Blue Jays were frequenting.

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My observations:

  • The number of jays coming and going was considerably greater than I had expected. Those flying out almost always often had an acorn in the beak.
  • Individuals in the tree moved quickly from one small branch to another, efficiently securing acorns (with only a few being dropped or knocked to the ground).
  • After picking an acorn, the jays flitted to a larger limb, where they held the acorn with the feet and pecked or hammered at it.
  • While in some cases they might have been breaking the acorn open and eating it, the hammering was most often to remove the cap, which fell to the ground.
  • The jays then appeared to swallow the nut whole and move on to get another.
  • I knew that jays do not eat acorns whole, so I wondered exactly what was going on. More watching revealed a bulging throat pouch in some of the jays.

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Photo by Margaret Weber

I knew, of course, that Blue Jays carried off acorns to store for future use, but I assumed they carried only one at a time, in the beak. Reviewing some published reports on the acorn storing behavior of jays, I found that they typically carry several acorns at a time to cache. They hold acorns in their “gular pouch” (usually 2 – 3, sometimes more), may carry another in the mouth, and have in the beak as well. They take them to the caching location (which may be a half mile away or more), regurgitate them, and hide each individually on the ground.

The next day I went back for further observation of the jays in the same pin oak tree. From a stationary position where I could see the entire tree from top to bottom, but where I could not observe the “back side,” I watched the Blue Jays coming and going to the tree for a 30 minute period on a sunny morning, from 9:56 a.m. to 10: 26 a.m.

  • I recorded jays flying into the tree 94 times and flying out 98 times.
  • I estimate that there were 7 or more birds in the tree at any given time.
  • I have no way of knowing how many individual jays returned how many times, so cannot estimate of the total number of individuals harvesting pin oak acorns from that one tree.
  • If I assume that the average trip out included at least 2 – 3 acorns, and recognize that this was just a 30-minute segment of one day (the jays have been “working” the tree for a number of days and for hours each day), it is likely that the number of acorns harvested by Blue Jays from this one tree this one year may be 8,000 or more.

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Photo by Margaret Weber

I now have additional questions. Based on what I observed, I now wonder whether I can continue to assume that most of the Blue Jays present in the park every September are migrating through. As I watch the jays in the tree, there was clearly much more “pouching” of the acorns than eating on the spot. Do migrating Blue Jays that are not going to be in the area for more than a short period of time harvest acorns and cache them? Maybe a number of year-round jays from neighborhoods outside the park come to the park in September for the acorn harvest?

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The acorns in August remind me of what is coming and I am looking forward to September watching. Perhaps I will learn a little more about these common birds this year. Perhaps I will have additional questions.