New Purple Martin Roost in Natrona, PA on Allegheny River.

Ken Kostka
American Swallow Conservancy

For the past four seasons (2017, 2018, 2019, 2020), Purple Martins have formed a premigratory roost in the trees of the US Army Corps Lock & Dam #4 facility in Natrona, PA.  The roost begins forming in late July and lasts thru late August, although I have seem small numbers as late as early September. 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.  UPDATE: PLEASE NOTE: The roost did not form in 2021! These premigratory roosts are ephemeral, and so it would not be proper to say that the roost was abandoned. The martins could be roosting just about anywhere nearby, perhaps near another large body of water like Crooked Creek Lake, only about 10 miles upriver, or even in trees at Harrison Hills Park, about 1 mile upriver. Both sites have strong breeding colonies.

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 was an active Purple Martin colony on the Lock grounds from 2005-2019, which I believe served as a “seed” for the formation of this premigratory roost. 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.

DSCN3313I 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)


Cat under roost tree before electric fence
Electric Fencing erected around main roost tree with solar charger
Raccoon gets around electric fence
Looking east from River Rd. Roost tree is second from left.
Looking north. Main roost tree on right. Staging tower in background.
Ken Kostka adjusting electric fence around main roost tree. Notice martin houses in background.

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.

July 21, 2020   8:30 PM – 9:00 PM Lock 4 Roost site on River Rd. in Natrona, PA. I decided to check out the Lock 4 premigratory Purple Martin Roost site to see if any martins were yet present. I had texted one of the Lock workers earlier in the day to ask if he’d seen any evidence of roosting activity (droppings, dead martins, etc…) and he said yes there were quite a few droppings under some trees already and a few dead birds. So I was excited and when I arrived and parked in the parking lot next to the Lock yard, I began hearing martin vocalizations around 8:40 or so. I saw about 300 martins in loose groups, which eventually tightened. The first wave came into the treetops at 8:57 PM, followed by another wave at 8:58 PM. I saw a small group still in the air at 8:59 PM. I also saw several small waves of starlings (100-150) come into another nearby tree around 8:45 PM. A Harrison Township police officer stopped to check me out and I explained about the roost. He was very nice. He didn’t say I couldn’t park in that lot, so anyone wanting to view the roost would probably be OK to park there, I’d say, even though the lot is owned by ATI (Allegheny Ludlum Steel Corp.)  It’s a very big, largely empty lot.  But I’d stay close to your vehicle and “be aware of your surroundings.”  Large trucks sometimes enter and exit the lot.

60 of the 86 pairs at the 4 martin colonies I manage have already fledged, so considering each pair and their fledglings = 5 martins, and there are 60 pairs, the 300 number could just be “my” martins – I don’t know! Many if not most adult martins in southwestern Pennsylvania present at colonies other than mine were killed by the cold weather in April and early May, so it will be interesting to see how big the roost gets this season.

The martin housing was removed from the Lock 4 site back in early May, so I am relieved the martins are still gathering and roosting here even though it is not an active breeding site anymore.

Aug 09, 2020  Lock 4 Roost Site, 1 River Rd., Natrona, PA.  I estimate 1000-2000 martins present. When I first arrived, I noticed several hundred starlings lining the overhead wires just outside the Lock yard fence on River Rd. Around 8:00 – 8:30, they flew, in several groups, into the roost trees that the martins use. I used my phone to video a large flock of martins swirling in the sky above the river (150 feet?). The video (best viewed on a cell phone or hi-res. monitor because the martins are very small!) is at

There is also at least 1 Peregrine Falcon present. It is sitting on the tops of several telephone poles near the roost trees and going after martins as they come into the trees. I am standing just below one of these poles and am seemingly ignored by the falcon which is focused on the martins. The Peregrine goes after a martin in a small group of martins coming into the treetops. I do not see it make a kill but its getting a bit dark and I can’t be sure. The first (smaller) wave of martins comes in to land in the roost trees at 8:36 PM. It is nearly dark before the second, larger wave comes in at 8:48 PM. I wonder if the threat of the Peregrine is causing them to come in later than they normally would. The wooden telephone/utility poles that I saw the Peregrine perched on was just outside the fencing on the street side of the Lock yard, right along the sidewalk. I have also seen the Peregrine perched on the very tall metal tower (electrical?) near the river, just upstream of the Lock yard.

I also saw several other species of swallows at the roost – Northern or Bank, I think. It would be great to see Barn Swallows joining the roost, as I am trying to get a few pairs to nest in my sheds.

Aug 19, 2020  Lock 4 Roost Site, 1 River Rd., Natrona, PA. My friend Doug Falk calls me from the roost and report 1000-2000 martins coming into the trees. They report a falcon sitting on top of a phone pole and going after a martin, but failing to kill it. They also see a lot of starlings.

Aug 23, 2020  Lock 4 Roost Site, 1 River Rd., Natrona, PA. I arrive at 7:47 PM. At 7:54, I saw and heard the first martins at 100-150 ft. At 8:01, there are about 50 martins. They are starting to show up. At 8:04, there are about 100-200 martins. At 8:11, a LOT more arrive and they seem to be streaming in from a westerly direction at a height of about 200 ft., from above the town, but this may be an illusion as it is very cloudy and they may just have been circling very high. Not sure. At 8:17, there are 1000+ martins at 250(+)? ft. I took a video with my cell phone. Just before 8:20, the large swarm of martins seems to compress their swirling formation and lose altitude rapidly from a position slightly downriver. They come barreling towards and the trees, which they land in at 8:20.  This all happens very quickly. I did not see any Peregrine Falcons – or any starlings, like I did the last time. Perhaps the starlings have moved on, or perhaps they have moved because of the Peregrines. The martins seem to have stayed higher than in the past, and I speculate that perhaps this too was to avoid falcon attacks.

Aug. 27  Lock 4 Roost Site, 1 River Rd., Natrona, PA. I meet with two staffers from the Tribune Review (a reporter and a photographer) at the roost but very few martins come in to roost. I suspect it is because just 2 hours earlier, there was a very severe thunderstorm with rain, and the martins hunkered down and roosted where ever they were just before the storm hit. We did see a Peregrine Falcon fly over.