Here’s to you, American Robin(son)…

Ok, that title is pushing it a little bit.  It’s been a long day and the Simon & Garfunkel reference made me smile.  But the robins have been on my mind of late.  Last week, a bunch of people asked me about the groups of American Robins that were flying in and out of the big holly bushes on campus.  When I walked outside, I saw huge numbers of these birds perched in the oaks, on the ground, and flying back and forth into the bushes.  Even from a distance, it was possible to see the brightly colored berries disappearing from the bushes.  In just a very short little video (filmed with my phone, so don’t expect much), you can see just how active these birds were (and if you look closely, you can see the fruits disappearing!

Now, since I had so many people marvel at these robins and all their activity, let’s hit this observation from a few different angles:

1.  What were they doing?

2.  Why so many?

3.  Was Simon & Garfunkel really necessary?

Ok, number 1.  What were they doing?  Well, the birds were clearly eating the fruits!  Throughout the fall and winter, American Robins rely heavily on fruits for the largest portions of their diet.  Winter fruit resources do tend to be fairly spottily distributed, so having lots of individuals foraging on a single ripe tree isn’t so surprising.  Additionally, from the holly’s perspective, it’s a great way to get your seeds dispersed.  The sidewalks (and driveway, and peoples’ cars) around that area were covered in robin poop that was full of seeds.

2.  Why so many?  Well, around here, some of our robins are winter residents.  They breed here and stay here year-round.  A lot of the robins that are here now, however, are migrants who have come down for our balmy winter to escape the frigid north.  Cold weather is a stressor on birds and they may gather in larger numbers in trees at night to insulate themselves against the cold.

Walsberg, G.E. and J.R. King.  1980.  The thermoregulatory significance of winter roost-sites selected by robins in eastern Washington.  Wilson Bulletin 92:  33-39.

In smaller groups, a robin might actually try to defend these fruit resources.  That defensive behavior has been observed with a single American Robin successfully driving off flocks of up to 15 Cedar Waxwings.  The authors do point out, though, that in groups larger than 15, the robin was unable to protect the fruit.

Pietz, M.A.J. and P.J. Pietz.  1987.  American Robin defends fruit resource against Cedar Waxwings.  Journal of Field Ornithology 58:  442-444.

So, onto #2.  Clearly with such a large group of robins, no single individual could defend the resource.  As a result, we end up getting a free-for-all of feeding.  Birds just rushing in and out of the bushes (and pooping all over) in order to stock up on the fruit.   The Walsberg and King paper talks about flocks in Washington of usually 20-40 birds.  But they do mention another paper that describes an entirely different sort of flock in Arkansas.

Black, J.D. 1932.  A winter robin roost in Arkansas.  Wilson Bulletin. 44:  13-19.  

He describes a roosting site spread over quite a large area and containing upwards of 250,000 birds!!!!  Uhm…wow!  Now that is a flock!  It makes the 100 or so birds on campus kind of boring.  250,000?  Here IS to you, American Robin(son)!!!

#3.  See?  Absolutely necessary.

 

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About thomasbiology

I'm an Associate Professor of Biology at Queens University of Charlotte with a background in animal behavior with an emphasis in bird song. I've got two secret goals with this blog (well, since I'm sharing them, they're not so secret): 1. To encourage people to look at the natural world around them- not just as a hiking destination, but to notice all the little things moving around them all the time; and 2. To show some of the science that relates to these little things moving around. There's some really fascinating research out there that so few people get to see.
This entry was posted in birds, ecology, foraging, migrations, thrushes. Bookmark the permalink.

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