I tried rotating the lighting +-30 degrees in increments of 10 degrees. The black triangles sometimes changed slightly, but did not go away or diminish significantly. I rendered with a variety of Advanced Shading Rates from the default 1 to 0.1. I also varied the Pixel Samples between the default 4 and 12. Still no significant improvement. I also varied the Shadow Samples from default 16 to 24. Still no significant improvement.
The only experiments that got rid of the triangles were:
1) get rid of ray tracing (use no AO and no shadow at all, or use no AO and use deep shadow map)
2) set displacement strength to 0 on the Plumage region where the triangles show up
3) zoom in closer to the bird, which changes the image composition significantly
To me, turning off the displacement has the least impact on the appearance of the scene.