A Couple Of Newbies And A Question

Reminder:  You can enlarge any of the photos in this blog by clicking on it.  Click again for a full-screen image.

I’ll start off by saying that the photos that I’m using for tonight’s blog are not the greatest.  I detest cluttered backgrounds and the first photo is just filled with clutter.  And, even more than that, I really detest man-made intrusions like wires and insulators, and my second photo, unavoidably, has both.  But, these photos do illustrate a point and help me pose a question, so I’m using them for that reason.

When Ned Harris and I were driving around in southeastern Arizona last week we had several encounters with very young Swanson’s Hawks.  Swanson’s Hawks are cousins of Red-tailed Hawks and there are some superficial similarities between the species.  However, these species have very different lifestyles and there are some significant differences in their morphologies that reflect their lifestyles.  Swanson’s Hawks, for example, have longer and more tapered wings than Red Tails, which undoubtedly aid them in making very long distance migrations.

This first bird is still a nestling.  I photographed it perched on a branch just a foot or two away from its nest.  Its two siblings (not shown) were still on the nest.

_N4B9685 copy

This bird is definitely about ready to fly, judging from the state of its plumage.

The second bird is slightly more advanced.  We found it a few miles from the first one.  It was perched on a utility wire, probably very near its nest tree.  It is still highly dependent on its parents and was calling loudly for them when we photographed it.

_N4B9733 copy

My question relates to the youth of these birds.  I am familiar with several species of hawks in our area and they seem to time the arrival of their offspring differently.  Red-tailed Hawks are early breeders.  They hatch their chicks in early April and by May, the youngsters are airborne.  Cooper’s Hawks hatch their chicks in late May or early June and their youngsters take wing around the beginning of July.  Swanson’s Hawks evidently don’t hatch their chicks until late June and their offspring are fledged in late July.  What explains these timing differences?

Well, I think it may have something to do with these birds’ feeding preferences.  Red Tails primarily hunt rodents.  Rodents in these parts tend to give birth in early spring when there is plenty of new vegetation to eat.  For the Red Tails, timing their breeding so that there’s an ample supply of young and naive rodents to hunt and to feed to their hungry offspring makes sense.  Cooper’s Hawks hunt birds, primarily, if not exclusively.  Baby quail, particularly, are abundant in late May and June and these birds are an excellent food source for the hawks’ offspring.

Swanson’s Hawks will feed on small rodents.  But, they really love to go after grasshoppers.  In late summer it’s possible to see big flocks of Swanson’s Hawks gathering in fields under cultivation, catching grasshoppers and gobbling them down as fast as they can eat them.  They need the calories in order to prepare for their fall migration, which takes them all the way down to Argentina.  Young Swainson’s Hawks get an opportunity to fatten up quickly on a food source that is plentiful and easy to catch.

So, the timing of breeding with these birds may be an evolved trait that has a lot to do with what’s available to feed to the kids, or for the youngsters to catch on their own once they’ve fledged.  Speculation on my part, but I think it makes sense.

Images made with a Canon 5Diii, 400 DO, M setting, ISO 400.  The first photo was shot at f6.3 @ 1/1000, the second at f7.1 @ 1/1250.

One response to “A Couple Of Newbies And A Question”

  1. Liesl Kii says :

    As always you commentary is very interesting.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s