I stopped by this Denton County office park late one summer afternoon. Least Terns were known to be nesting on the roofs of some of the warehouses, and I was here to attempt to determine which ones.

Least Terns typically nest on areas of sparsely vegetated, flat ground with a substrate of sand or gravel. Proximity to the ponds and small lakes these birds prefer to fish is also an important factor.

Least Tern with minnow

In a fortuitous twist, the flat gravel covered roofs of huge warehouse buildings found all around Dallas/Fort Worth closely mimic the Least Tern’s preferred nesting habitat. Further, in DFW business parks are often constructed close to the Trinity River bottoms or other similar flood plain areas. The small lakes and ponds found in these places are often perfect for the minnow-hunting Least Terns.

Indications are that this bit of fortunate serendipity has allowed the Least Tern to expand its presence in North Texas.

A pair of Least Terns hunting at a small Denton County lake

My plan on this afternoon was a simple one. I intended to sit and monitor a couple of the warehouses in this business park to see if I could notice the frequent coming and going of Least Terns delivering food to their hungry young.

The first building I watched was a bust. No terns there. Likewise with the second and the third. The day was rapidly drawing to a close as I moved into position to observe building number four. The sun had already dropped below the artificial horizon created by the long, tall buildings to the west. The last remnants of sunlight were fading fast as I began to watch the sky above this warehouse. Without a doubt, this would be the last stop of the day.

I was only minutes from calling it a day when a swirl of birds began to form up over the rooftop. Dozens of birds joined in flying around each other and calling out all the while. But, these birds seemed too large to be the Least Terns I was expecting, and I was not sure what I was seeing. I took an abundance of pictures hoping to get a positive ID with a closer look at the photos at home on the computer.

A swirl of noisy birds
These birds were flying just high enough to
catch a tiny bit of the last light of the day

At the house, I was pleased–and surprised–to discover that this grouping of birds was largely made up of migratory Black Terns–quite different from the Least Terns I was looking for. Nonetheless, this was a special sighting because this was a species of bird I had never seen before.

Migrating Black Terns

But the surprises were not over. A closer look at the grainy, low-light pictures revealed something more. There, mixed in with the larger Black Terns, were a number of more diminutive Least Terns. Brought together temporarily by some kind of tern-kinship, the Least Terns nesting on the rooftop joined in with the group of migrating Black Terns just long enough to reveal that this was the building they were nesting on. How about that!

Black Terns with Least Tern (bottom)
Black Terns and one Least Tern (bottom left)
Black Terns with two Least Terns (top left and second from bottom)
Black Terns
Black Terns. Least Tern in the middle