Signs of Spring 5: Great blue herons

A colleague here at Penn State asked me a great question last week. She wanted to know if there was any reason why she has been seeing so many great blue herons lately. She said that she not only saw one flying over campus last week but also has seen them quite frequently over her home in North Huntington. (Photo by Kozarlua (Wikimedia))

The great blue heron is one of my favorite birds. When Deborah and I lived on our farm in Northern Allegheny County we had regular visits from a great blue heron every three or four days all through the summer months. It would circle down and land on the edge of our pond and spend an hour or so feasting on bluegills and frogs. Our little pond was just one stop on the heron’s regular hunting circuit. 

Great blue herons are the largest of the North American herons reaching heights of four and a half feet and wingspans of over six and a half feet. Their size and distinctive flying posture (they fly with their long necks curled up in an “S” and their legs stretched out behind them) make them readily recognizable as they soar past. They primarily consume fish and frogs from ponds, lakes, streams, and estuaries but very opportunistically will also eat rodents, small birds, snakes, discarded human food, and even goldfish in garden fish ponds.

But, are great blue herons becoming more abundant here in Western Pennsylvania? And, are they spending the winter here more frequently?

The answer is “yes” to both questions.

     A few years ago Mark Browning (a zoologist at the Pittsburgh Zoo) wrote an article about great blue herons for Pittsburgh-Tribune (July 10, 2009) in which he cited State Game Commission data that showed a 32% increase in great blue heron nesting sites in Western Pennsylvania since 2002. These nesting sites (called “heroneries”) include the large, four hundred nest site in Mercer County, the forty-one nests in Franklin Park, and the seventeen nests each on Twelvemile Island and along Deer Creek.  If you consider that each nest could house three to seven eggs, you can readily visualize a potential exponential increase in the local numbers of these birds!   

A great blue heron’s nest consists of a flat platform of tightly interwoven long sticks onto which softer materials (mosses, pine needles etc) are layered. The nests are usually located in the top branches of tall trees. Sycamore trees because of their branch strengths and open crowns (not to mention their typical proximity to wetlands) are especially popular great blue heron nest sites. If a heronery site runs low on tall trees, though, a great blue heron can build its nest lower to the ground on the tops of thick shrubs. A given heronery can be used continuously for many decades.

So, are more herons overwintering here? The classic pattern of a great blue heron’s yearly cycle describes their southward migration in the mid-fall. They may go to the southern United States (I remember seeing lot’s of them in the winter in Texas), or they may go down to Central America or even all the way to northern South America. They then return to their northern breeding sites (including Western Pennsylvania!) in late February or early March timing their arrival with the thawing of the ice on our ponds, lakes, and rivers. It is very likely that this pattern is carried out by a large number of our great blue herons. 

But, if a heron can find open water during the winter so it is able to hunt for food, or if it is able to utilize some of its alternative food possibilities, then at least some of the herons might be able to remain here in Western Pennsylvania all year round.

Comparing great blue heron’s winter distribution maps from the late 1970’s to more recent maps, it appears that these birds have significantly increased their local, overwintering numbers. Maybe climate change is keeping more surface water sites ice-free for more of the winter, maybe there are just more quality surface water sites available, maybe there have been changes in available foods, or maybe there is just a big enough local great blue heron population that now the small percentage of overwintering individuals is becoming more and more observable! 

It’s hard to say which explanation (or which combination of explanations) is accurate, but the presence of these beautiful birds softens the edges of the winter around us. They are a magnificent sign of spring no matter what season we see them.