The Strange Case of Blue Jay Migration

Blue Jays have been a major presence in Eliza Howell Park during the last month, the time of their fall migration. Each year I anticipate their acorn harvesting frenzy (see “Blue Jays Harvesting Acorns,” August 27, 2018) and this year they again worked the oak trees in great numbers.

Note: All bird photos by Margaret Weber

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Their month-long period of intense activity in Detroit is exciting to observe, but I am increasingly aware of their unusual – and not fully understood – migration behavior.

Most birds that migrate through our area in the fall leave all or much of their breeding ground for the winter. Blue Jays, by contrast, are found in the very same areas all year round. It is interesting to compare the following two range maps. The first is White-throated Sparrow, also moving through here in September, and the second is Blue Jay. (These maps are taken from the Cornell Laboratory of Ornithology website.)

White-throated Sparrow range

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White-throated Sparrows leave almost their entire breeding area in the fall.

Blue Jay range

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Blue Jays are found throughout winter in the very same areas in which they breed. A few scatter to the west, but the entire breeding area remains occupied.

Such a range map usually suggests that a species is non-migratory. Blue Jays, however, migrate and do so in great numbers. Every fall the raptor counters at the two locations by Lake Erie (Detroit River Hawk Watch at Lake Erie MetroPark and Holiday Beach Conservation Area in Amherstburg, Ontario) count Blue Jays as well as raptors. As of October 10 this year, Holiday Beach watchers had reported over 350,000 Blue Jays moving through since early September. And the migration wasn’t fully over yet.

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This is what is known (and/or thought likely) about Blue Jay migration, based on my review of some of the published research:

  • The species is partially migratory; only some migrate
  • Most that migrate go only a few hundred miles south in the fall
  • It is estimated that fewer than half migrate
  • Those that migrate (and those that don’t) include both young birds and older birds
  • Individual birds might migrate one year and not the next and then migrate again in the following year
  • The migration number in a particular year is suspected to be related to food sources

The jay numbers will soon decline in Eliza Howell as the migrants move on. But I do not know whether individual birds like this one, seen in an oak tree in September, will stay for the winter.

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I do not know whether this one, nesting here in June, will be around in the coming winter

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I had assumed, if I thought about it much at all in the past, that the same birds (the non-migrants) were present in summer and winter and the migrants all passed through. Now I doubt that.

One of my other questions, not addressed in the limited research I have reviewed, is whether some of the individual jays harvesting acorns in the park leave/migrate before the winter and do not ever go back to eat the acorns they hid. Is acorn harvesting possibly community and not just individual food-gathering?

The Blue Jay is a common bird, so it is tempting not to give it a second thought. But I rarely see jays these days without thinking about what I do not know about them.

“Acornucopia”— and Wildlife Benefits

“Cornucopia” being abundance or profusion, vast numbers of acorns might be called “acornucopia.” Eliza Howell Park is home to many productive oak trees and the acorns seem especially plentiful this year.

There is a great variety of oak trees in the park. On an afternoon walk on September 19, I was noting the varieties of acorns and collected these examples in a very short period of time.

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Acorn production varies from year to year and from species to species. Overall, 2018 is a bountiful year. Some trees have as many acorns as I ever remember seeing.

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One of my favorite oaks, a type of red oak, is a large tree with several low hanging branches filled with acorns that fit my mental image of the archetypal acorn.

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For many wildlife, an abundance of acorns means a valuable food source as winter approaches. Dozens of mammals and birds eat acorns. Acorns make up about 25% of the diet of White-tailed Deer in the fall. An abundant acorn crop often means healthier deer heading into the breeding season.

This stag, which crossed the EHP footbridge right before me one day last fall, may well have been foraging for acorns.

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Rabbits, raccoons, opossums, even foxes, eat acorns. And squirrel consumption of acorns is so well known that I need not comment.

Many of the mammals feed at night, but it is easy to watch the different birds getting their acorns. The greatest in number are Blue Jays (see “Blue Jays Harvesting Acorns,” August 27, 2018). They, along with Red-bellied Woodpeckers and American Crows, pick the acorns in the tree. Grackles, on the other hand, feed on the ground under trees for the fallen ones. Jays crack the acorns open with repeated pecks with their beaks; grackles have a sharp ridge in their beak (keel) which they use to open the shell.

Some birds eat acorns whole and let their gizzards do the work of grinding the food. Wild Turkeys, birds that eat many acorns, have been found in EHP in recent years.

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Wood Ducks also eat lots of acorns. They will be leaving the park soon, heading south for the winter, but they are still around now and are often on the banks of the river, where their nutrient sources include acorns.

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Photo by Margaret Weber

Every year, in late summer and fall, the presence of acorns calls my attention to the number and variety of oak trees in the park and I spend some time trying to get to know them better. It is not easy to identify some of them by species name, since there are so many different species in the eastern U.S. and oaks do hybridize.

I recently collected a few acorns, each with a leaf from its tree, for more leisurely study at a later time.

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We have all heard that “mighty oaks from little acorns grow.”

A few acorns do grow into large trees, but most of the many, many thousands of acorns present in Eliza Howell right now serve other valuable purposes: food for wildlife and an opportunity for me and others to admire, enjoy, and learn.

 

 

 

 

 

 

 

 

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.