The Battle of the Sexes

It’s been a long, blank couple of weeks here.  I haven’t just been slacking off, I swear!  I actually just spent about three weeks in Spain.  As a result, I’m going to mix it up a bit on here.  Too much excitement for you to handle?  Sorry…hold on to your pants, then.

First of all, I have to begin by saying that this trip was not a birding trip.  Most of the time was spent in cities.  We checked out Picasso’s Guernica (stunning) and cathedrals in several cities, including Toledo and the Sagrada Familia in Barcelona (breathtaking).  While I was without binoculars, I was still able to see some amazing birds on this trip, just by keeping my eyes open.  For instance, while standing in front of the cathedral in Seville, I looked up to see several White Storks circling overhead.  These birds are impressively large.  I’m guessing that they were following a migration route at that time.  In the air, they moved like Turkey Vultures, riding thermals gracefully with no flapping at all.  I watched them until they moved past the towers of the cathedral and out of sight.

One of the other birds that I saw a lot was the House Sparrow.  “House Sparrows? Ugh!”  Normally I would agree with that kind of assessment.  Locally, they rank with Pigeons and Starlings on my trash bird list.  In Spain, though, I had to check myself a bit.  After all, that is their native range; I was the invader this time.  I had to cut them some slack.

Behaviorally, these birds seemed to be pretty similar to the ones we get locally.  I spotted them around tapas bars and cafes, picking up crumbs in much the same way that they do around here.  Maybe their groups were a little smaller or less aggressive, but that was hard for me to evaluate in any systematic kind of way on this trip.  Well, this morning, my curiosity got the better of me, so I started to see what I could find out about differences in House Sparrows in the Europe and North America.  The first thing that really caught my eye actually just speaks to the behavioral flexibility of these birds- a trait that has certainly facilitated their global success.

Breitwisch, R. and M. Breitwisch.  1991.  House Sparrows open an automatic door.  Wilson Bulletin 103: 725-726.

Yes, you read that title correctly.  House Sparrows in New Zealand figured out how to open a sliding glass door in order to enter into a convenience store where they foraged.  Several males either flew in front of or perched on the sensor that opened the door to the store.  Regardless of your opinions on these little birds, you’ve got to admit that that is an impressive feat.

As I dug around for intercontinental comparisons, I came across one set of interesting patterns related to body size and sexual dimorphism.  First of all, to be sexually dimorphic just means that males and females have distinct physical differences.  In the case of House Sparrows, you can see the differences in color patterns quite clearly.  Males have the large black badges on their chests, while females have a paler brown pattern over the entire body.  Male and female House Sparrows also have different sizes, with males being slightly larger than females.   It is in these size differences that things become interesting because birds in Europe show slightly different patterns of size variation than those in North America.  Now, we’re not talking about overall body size.  What I mean by size variation is how similar different birds within the same population are to each other.  If a population shows low size variation, then most of the birds in that area are very close in size to each other.  On the other hand, a population with high size variation would have individuals with a range of body sizes.

House Sparrows in Europe tend to have more size variation in southern populations than in northern ones.  House Sparrows in North America show the opposite trend.  These overall patterns of variation may be a function of competition for access to food.

Hamilton, S.  and R.F. Johnston.  1978.  Evolution in the House Sparrow-VI.  Variability and niche width.  Auk 95:  313-323.

Now, when we add in the question of sexual dimorphism to this overall pattern of size variation, we find something very interesting in these birds.  In both Europe and North America we find a very similar pattern of sexual size dimorphism.  Northern populations always show larger degrees of sexual size dimorphism than southern populations.  This statement means that the size difference between males and females in northern populations is, on average greater than the size difference between males and females in southern populations.  Since this pattern holds true in spite of the larger scale differences in overall body size, something else has to be happening more universally in House Sparrows.

Johnston, R.F. and R.K. Selander. 1973.  Evolution in the House Sparrow. III.  Variation in size and sexual dimorphism in Europe and North and South America.  American Naturalist 107:  373-390.

These researchers think that the variation in sexual dimorphism is under different pressures than the overall patterns of size in these birds.  While overall body size variation is a result of environmental factors and interspecific (between species) competition, the patterns of sexual size dimorphism seem to result from intraspecific (within species) competition.   Populations of House Sparrows in areas that are farther north have more intense competition for access to resources in the winter and that plays out in how males and females interact with each other.  Even in flocks around here, you can see these interactions take place.  House Sparrows have a definite dominance hierarchy in their foraging groups.  Larger males (with bigger badges) have the best foraging locations and get the best food.  Males, on average, tend to rank more highly in the hierarchy than females.  You can sometimes see males chase females away from larger pieces of food.  Well, as you move north, these hierarchies may become stricter as competition in winter gets more intense.

It’s an interesting pattern.  In spite of the large-scale environmental changes between populations- both within North America and between North America and Europe, the interactions between the sexes still seems to play out in the same way.


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, morphology, sparrows. Bookmark the permalink.

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