Sibling rivalry (again)

Clearly, I’m in a title rut, now.  I’m willing to keep running with this theme, though, because there’s so many fun things going on right now.  Last week I was over at a friend’s house.  Sitting in his backyard, I noticed a pair of American Robins that were clearly flying to a nest on one of the supports over his deck.  When the parents flew off, I walked over to take a quick peek at the nest and saw five young robins sitting in the nest, heads pointed up, squeaking and jostling around in the nest.

Now all that movement is kind of fascinating.  First of all, it meant that they weren’t hiding from me (not surprising, since they were on someone’s deck, already.  They should have been used to seeing people- and dogs – moving around below them).  Secondly, the parents had been making multiple feeding trips back and forth.  Since they were both off the nest at that moment, they were likely to return again, soon.  So that meant that these kids could have been jockeying for better positions in the nest.

It turns out that American Robin chicks are quite adept at this kind of positioning behavior.  One group of researchers in Canada put video recorders near American Robin nests and tracked the movement of the kids in the nest with respect to where the parents landed to feed them.  Two things stick out from what they found.  First, parents tended to return to the same spot on the nest to begin feeding.  Secondly, the chicks were sensitive to that pattern and would fight to get into the best position. It’s sort of like watching people waiting outside Target on the day after Thanksgiving.  Everyone just sorts of tries to work towards the front to be the first one in.

McRae, S.B.; P.J. Weatherhead; and R. Montgomerie.  1993.  American robin nestlings compete by jockeying for position.  Behavioral Ecology and Sociobiology 33:  101-106.

As a result, there isn’t really a true hierarchy.  No one individual is the most dominant, they all just sort of take turns being the ‘pusher’.  At least that’s true when there’s plenty of food to go around.  When food is scarce, well, some of those chicks may not survive because they’re not good enough at doing that pushing- and that would usually be the last ones who are born.  Like the Peregrine Falcons, American Robins don’t lay all their eggs at once.  One article from 1898  (and here’s my little aside for today:  if you’re never read old science, do it!  It’s so different and fun!)  puts the interval at 24 hours, but there is likely some variability with that.

Howe, R.H.H.  1898.  Breeding habits of the American Robin (Merula migratorius).  Auk 15:  162-167.

So in a good year, all that sibling rivalry is just good-natured fun.  In a bad year, that rivalry affects which of those birds will survive (and the youngest ones are likely to be the losers in that situation). Being the youngest child in my family, I’m very sensitive to that kind of situation!!

If you’re keeping an eye on the Peregrine Falcons nesting in Charlotte (FalCam), then keep this sort of competition in mind and keep answering these questions:

1.  What kind of food are the chicks getting?

2.  Which of the chicks eats first?

3.  Which of the chicks eats more?

Now get to work!



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 ecology, falcons, raptors, reproduction, thrushes. Bookmark the permalink.

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