American Swallow Conservancy
For the past three seasons (2017, 2018, 2019), Purple Martins have formed a premigratory roost in the trees of the US Army Corps Lock & Dam #4 facility in Natrona, PA. This video was taken in late August 2018. The roost has grown to 5000 – 7000 martins, which gather in the evening into large swirling formations and eventually land in several large trees on the Corps grounds. The roost forms all through the month of August with peak numbers occurring in mid-august.
The martins begin gathering about an hour before dark, streaming in from all directions, landing on wires and and swirling in large formations several hundred feet above the Allegheny River, eventually landing in the trees in spectacular fashion just before dark (watch video above). They disperse in the pre-dawn hours, flying off in all directions to distances of up to 100 miles to feed and explore the countryside, then regather again in the evenings. Towards the end of August, individual birds or very small groups eventually depart for South America. This is the only known roost in the Pittsburgh area. The only other two known roosts are in Presque Isle, PA and in Waynesburg, PA.
There is a 20 pair Purple Martin colony on the grounds. I established this colony in 2005 with one breeding pair that was displaced from the Saxon Golf Course colony near Saxonburg, PA as part of a project intended to establish new breeding sites. There are several other articles about the Lock 4 colony and the displacement project on this website. I hope to publish more information and videos here about the roost in the future as time and energy permit.
I would find some dead martins under the main roosting tree every morning. Some had no visible marks but some were eaten. I wasn’t sure what had eaten them or whether whatever had eaten them was simply scavenging martins that had died and fallen or whether predators were climbing the roost tree and killing the sleeping martins. So in 2018, the Lockmaster gave me permission to place an electric fence around the main roost tree in an effort to discourage climbing predators. This did seem to deter most predators, but, as the one photo shows, an at least one raccoon got around the fence.
If you want to view the site with GoogleEarth, the address is:
1 River Rd., Natrona, PA 15065
There are also many articles with photos about the Lock 4 Purple Martin colony on this website.
Additional video of roost and Lock 4 facility:
video of roost (swirling formations) in August 24,2017:
(best video starts at 0:59 seconds)
video of martins bustling in the trees:
Below is GoogleEarth Photo of the Lock 4 facility, 1 River Rd., Natrona, PA on the Allegheny River in northeast Allegheny County. The red lines converge in the lawn area containing the martin housing and the roosting trees. PLEASE NOTE: This is a restricted U.S. Army Corps of Engineers Facility, not open to the public. Viewing must be done from outside the fencing. The large parking lot to the east is a good vantage point. This is a large private parking lot that is usually pretty empty but I’d stay with my vehicle. The other viewing area is a small parklet just downstream of the Lock 4 facility. This parklet can only be accessed by way of a dead end alley. The entrance to this alley is 0.36 south of the Lock facility off River Rd. The parklet viewing area is better for watching the large swirling formations that form above the river, but the parking lot viewing area is better for watching the martins as they come down into the trees, also a spectacular sight. Keep in mind this premigratory roost forms only in August. Mid to late August is the best viewing time.
Additional Photos of Lock 4 Roost site: (click on them to enlarge)
Update: On May 12, 2020, both unopened T-14 Purple Martin houses were removed from the Lock 4 facility, where they had housed breeding Purple Martins since 2005. The houses had remained because I was unable to enter the facility in the spring to remove the tarps due of the Covid-19 pandemic quarantine. One house was moved to the Aspinwall Veterans Hospital in Apinwall, PA. The other was moved to the Harrison Hills County Park Environmental Learning Center colony only 3 miles away. I do not believe the removal of these houses will affect the formation of the premigratory roost in August, since martins have been in the habit of gathering at Lock 4 since 2017.