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
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
White-throated Sparrows leave almost their entire breeding area in the fall.
Blue Jay range
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.
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.
I do not know whether this one, nesting here in June, will be around in the coming winter
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.