As mentioned, even if you manage a super-sharp long lens, in the 600mm to 1000mm range, and an excellent sensor, you're always going to be limited by the light and the atmosphere. If the light is very bright, clear skies, vivid sun, preferably slightly lower in the sky, and it's a cold or cool day, very low humidity, no haze, and you're in a pollution-free spot, then you could potentially take advantage of the really long lens to get clear details at high distances. Of course, if on that day you just have an average lens, or average sensor, or you don't use your lens' best aperture, or you're in any way unsteady or don't use a fast enough shutter speed, and so on...you could get poor results even in perfect atmospheric conditions...so you need the combination of very good lens, correct settings and technique, and a very cooperative atmosphere.
My best examples of decent sharpness and quality from farther distances - that I can verify through Google Maps - are the following:
From approx. 140 feet away, 600mm lens on APS-C sensor, and cropped about 60%:
https://pbase.com/zackiedawg/image/165515633/original
Not a bird, but a plane - flying at least 25,000 feet, shot from sea level, with a 400mm lens on APS-C sensor, cropped to 100% (1:1 pixel):
https://pbase.com/zackiedawg/image/166133734/original
Roughly 150 feet, 560mm lens on APS-C sensor (400mm + 1.4x TC), cropped about 40%:
https://pbase.com/zackiedawg/image/168333014/original
From 250 feet away, plus 150 feet up, with 400mm lens on APS-C sensor, cropped about 80%:
https://pbase.com/zackiedawg/image/168934824/original
Here's a rare case where I can show you an uncropped original, as it was framed in the camera at 400mm on APS-C sensor., from 400 feet away:
https://g4.img-dpreview.com/ABB6A3A2F19E4B17A333D2D186ED0D92.jpg
Then a 100% (1:1 pixel) crop from that original:
https://g1.img-dpreview.com/89524EF8FDCA44BBB3E1C501592183CA.jpg
All required not only a decent lens and sensor, and all the right settings, steady hold and stance, but also good atmospherics.