Tuesday, January 26, 2016

A Little Blue Heron in the Salt Marsh

As I mentioned in my previous post on the Cottonmouth, I recently took a 2000 mile solo road trip during my Winter Break. I spent most of my time birding throughout coastal South Carolina, racking up a total of 90 species (including 8 lifers) from December 28, 2015 to January 2, 2016. This wasn't my first time birding in South Carolina; just a few months prior I visited with my ornithology class for a 4 day field trip (which you can read about here: Part 1, and Part 2). It's an absolutely wonderful state to go birding in!

Pinckney Island National Wildlife Refuge
One of the locations I visited was Pinckney Island National Wildlife Refuge in the very, very southern tip of South Carolina. It's a very interesting preserve, and most of it is open saltwater or salt marsh. I went to visit a productive freshwater pond named the Ibis Pond, but first had to walk by an extremely shallow salt marsh. And in that marsh was a small white bird.

Juvenile Little Blue Heron

And that small white bird was a Little Blue Heron. You might be wondering why a "Little Blue Heron" is white, and this is because it is a juvenile Little Blue Heron. This species begins life in all-white plumage and individuals transition to their deep blue plumage upon maturity.

I would like to take a moment to talk a bit about the name "heron." As you might know, there are herons and egrets; both look essentially the same, aside from species-level color differences. You might be thinking there is some kind of difference between the two groups, like there's something that makes a heron a heron, and an egret an egret. I definitely thought that until I started my research for this post. Turns out, there is no significance to the names. Although the Little Blue Heron is called a heron, that name doesn't mean much in itself; in fact, the Little Blue Heron is most closely related to the Snowy Egret. So why the two groups of names? It's basically due to linguistic and societal differences. Birds in the family Ardeidae are called the herons. The name "heron," when talking about etymology (the study of a word's origin and history), comes from certain Germanic languages including Middle English. Egret, on the other hand, is still Germanic, but is based on the Old French word "aigrette," which is pronounced "egret." Aigrette can translate to "little white heron," but also refers to long, decorative plumes that certain heron species grow during breeding season. So basically, the French term "aigrette" was borrowed by English and morphed into "egret." This term was then applied to white herons, many of which also have those decorative plumes during their breeding season. However, when you look at the evolutionary history for these long-legged waders, you see that there's no biological basis for the differences in names; the birds we call egrets and the ones we call herons are all intermixed in various genera, and they don't break down into two completely separate groups. This is one of the many reasons why common names can be deceiving!

Immature Little Blue Heron
Back to the Little Blue Heron! This is a medium-sized heron species that stands about 2 feet tall. Here in the United States, Little Blue Herons are most commonly a resident of sub-tropical swamps and marshes in the southeast. However, like their cousin the Snowy Egret, the Little Blue Heron will also very sporadically breed in the northern half of the Eastern United States. Little Blues will also sometimes irrupt northward in the late summer after breeding is finished. When it comes to Ohio, the Little Blue Heron has an interesting history. A few pairs have actually nested in Ohio, and these pairs have been confined to the wading-bird nesting colony on West Sister Island in Lake Erie. These nesting individuals show up to forage in marshes throughout Ottawa and Lucas counties, including Ottawa National Wildlife Refuge and Magee Marsh. Even then, this is a very rare species to see in Ohio. To read more about the Little Blue Heron in Ohio, check out this link: OBBA Account

Little Blue Heron foraging
Although they are rare in Ohio, the Little Blue Heron is pretty common within their core range. They are typically a bit shyer than other herons, preferring to forage along the edges of wetlands either singly or in very small groups. The Little Blue is a stand-and-wait predator, meaning it prefers to quietly stand in one location and wait for prey to come to it instead of actively hunting them out. They typically feed on fish, amphibians, and crustaceans, but will also take insects and small mammals. This individual was in extremely shallow water (about 1 or so inches deep), and fish were nowhere to be found. I watched him as he slowly and carefully stalked about, turning his head to inspect the water for life. Eventually he struck the substrate and pulled up a bristle worm, as pictured above. Bristle worms, class Polychaeta, are a diverse group of mostly-marine segmented worms, and easy pickings for many wading birds.

Little Blue Heron South Carolina
While researching information for this post, I came across two very interesting studies that I would like to summarize. Both studies were done by Gloria S. Caldwell, and the papers can be found at this link: Caldwell 1981, Caldwell 1986. To give some background, most herons/egrets have the same plumage coloration throughout their life. The Little Blue Heron is a bit unusual in this as it is dimorphic in regard to age. The juveniles are white, while the adults are dark blue. Is there a significance to this color dimorphism? Turns out there is, and it’s incredibly interesting. Caldwell (1986) found that herons with white plumage were attacked by hawks more often than herons with "dark" plumage. At first glance, it seems that the juvenile plumage of the Little Blue would be a hindrance to survival, but it’s actually a bit more complicated than that. Caldwell also found that flocks of herons have a significantly lower chance to be preyed upon than an individual foraging alone, and this makes sense; the more eyes you have watching for predators, the less of a chance a predator will be able to commit a successful attack. How does this flocking feature factor in? Caldwell (1981) found that Snowy Egret flocks will more readily accept juvenile Little Blues than adult Little Blues (in fact, adults were attacked by the Snowy Egrets, while juveniles were left alone). Caldwell suggests that the white plumage of the juvenile Little Blues makes the Snowy Egrets, a white bird themselves, more apt to accept the juveniles than the dark-colored adults. So, although white juvenile Little Blues are more likely to be attacked by predators than the adults, they can mitigate this negative effect by foraging in a mixed flock with Snowy Egrets. In addition to all of this, Caldwell found that juvenile Little Blues actually had a higher success rate catching fish while in a flock than when they foraged alone.

Immature Little Blue Heron
Let me take a moment to sum everything up so far. Juvenile Little Blue Herons, which are white, are better accepted into mixed foraging flocks than the dark-colored adults. By being part of a flock, the juvenile Little Blues have a better chance to avoid predation. In addition, by being part of a flock, juvenile Little Blues have better success at catching fish. Therefore, by having a lower chance of being killed by a predator, and by being better-fed as they mature, a juvenile Little Blue Heron has a much better chance of making it to adulthood and passing on its genes. Having white plumage is, therefore, very beneficial, and natural selection ensures this white juvenile stage is maintained.

Little Blue Heron adult and juvenile
Now you might be wondering why there isn’t a selective pressure for the adults to be white, instead of dark blue, as well. There are some young white subadults that attempt to mate every year, and you would think those individuals would be heavily selected for, but those subadults almost always fail to mate. This implies there’s something else now at play, but what is it? It all has to do with breeding! As I mentioned previously, white herons are preyed upon at a higher rate than dark-colored herons. This is simply because white really stands out among the greens of swamps and marshes. To see this in action, look at the photo above; the white juvenile on the left stands out like a sore thumb, while the dark adult on the right blends in relatively well. Being inconspicuous is very important when it comes to nesting; nestling birds are incredibly easy pickings for predators as they can’t fly away and escape. Many predators know this, and many will actually watch for adults bringing food back to a nest in order to locate the nest. Once a predator knows where a nest is, and if they can get to it, then it’s all over for the nestlings. Because of this, adult birds try to hide the location of nests at all costs. If you are a conspicuous, attention-grabbing white, you might as well let predators waltz right up to your nest. By transitioning to a dark coloration upon maturity, Little Blue Herons reduce the chances that predators will attack them or their nest, which not only helps ensure their survival, but the survival of their nestlings as well.

In the end, natural selection selects for white-plumaged juveniles and dark-colored adults. By having this dimorphic coloration system, the Little Blue Heron optimizes survival across its entire life cycle from nestling to breeding adult.

That's it for this post! Thanks for reading!


  1. ...I enjoyed the post! Pinckney Island, SC is one of my favorite places. In late spring and early summer when nesting is in full swing, the place is amazing! I miss it!

    1. Thank you! I would love to get down there during the summer sometime. Hopefully it happens!