Eagles young and old

Here's an iconic American critter, from Seward, Alaska, 2012:

Bald Eagle (Haliaeetus leucocephalus), Seward, AK, 8/7/2012

 

Everyone recognizes these, of course. But a bird that confuses a lot of people is this one:

Bald Eagle, Red Rock Dam, Marion Co., IA, 12/7/13

As the caption states, this is also a Bald Eagle! Obviously, it looks rather different, and it illustrates an important (and often frustrating) point about birds -- young ones typically look different than adults. We're not talking about size here; with a few notable exceptions, birds are full-sized (or very close to it) by the time they leave the nest. Rather, the plumage is different, in color and often in shape. (Many hawks, for example, have longer wings and tails as juveniles than they do as adults.) In the case of Bald Eagles, they may take 5 years to reach fully adult plumage, although they do change every year in the meantime. (Here's a slightly older youngster to demonstrate, caught in a classic hunting moment.)

As always, the question here is what gives? Why bother taking several years to acquire adult plumage? After all, you're already full-sized, and you're still growing just as many feathers. So what's the point?

One possible answer is diet and energy -- some plumage colors require certain pigments in a bird's diet, (carotenes -- this is why Scarlet Ibis and Flamingos in zoos often appear bleached out), and others take more energy to produce (melanins). But many birds actually produce more melanin as juveniles than as adults. (Swans and Snow Geese are mostly gray rather than white as juveniles, and many sparrows show streaks as juveniles that they lose during their first molt.) So energy probably isn't the explanation.

On the other hand, the patterns that we examine so closely on birds are there for a reason. Generally speaking, brown streaky, spotted, or mottled plumage means camouflage, whereas bright colors and bold patterns are for display -- either attracting mates or discouraging rivals (or both). If we take that word immature at face value, this makes sense -- sexually immature birds can't breed, and since most of the signaling that we're talking about involves breeding, there isn't much point. On the other hand, predators don't mind eating youngsters -- they're often easier to catch, in fact! So, as a youngster, it makes sense to be dull and streaky. For some species, most notably sparrows and warblers, this probably explains a lot of the age-related variation we see.

For other species, though, there's a bit more of a puzzle. Many species become sexually mature while still in their "immature" plumage -- Bald Eagles are capable of breeding at three years old, for instance. (1) So. what's going on here?

The usual explanation is that birds that show this delayed plumage maturation are capable of breeding, but not likely to have the opportunity (no open territories, too much competition from older males) or to succeed (due perhaps to insufficient experience in finding food for the offspring). In that case, putting yourself on display increases both your chances of being eaten and the amount of harassment you'll have to put up with from older birds protecting their territory. (2)

Unsurprisingly, this idea has been tested on a number of occasions. For example, Conover, et al. looked at the issue in Mute Swans (Cygnus olor). They found that males that delayed their maturation by one year had a better chance of survival, but males that didn't do so started breeding earlier. (3) They didn't state what the lifetime reproductive output for each strategy was (unsurprising, really -- Mute Swans live quite a long time), and without that info, it's hard to say whether this mix of strategies is evolutionarily stable, or if it's a recent response to a relatively recent colonization of the area. (In this case, the Chesapeake Bay -- Mute Swans are native to Eurasia.)

Learning all of the myriad plumage variations in birds can be frustrating, even maddening at times. (I'm looking at you, confusing fall warblers!) But when you start digging into the whys and wherefores, there's a lot of neat stuff going on -- something to think about next time you see one of the magnificent Bald Eagles I started this post with.

(1) Wheeler, Brian K. (2003) Raptors of Western North America. Princeton University Press, Princeton.

 

(2) Hawkins, Gerard L (05/01/2012). Delayed plumage maturation and delayed reproductive investment in birds. Biological reviews of the Cambridge Philosophical Society (1464-7931), 87 (2), p. 257.

 

(3) Conover, M. R., Reese, J. G., & Brown, A. D. (2000). Costs and benefits of subadult plumage in mute swans: testing hypotheses for the evolution of delayed plumage maturation. American Naturalist, 156(2), 193-200.