It does not matter what camera one has at dusk or low light, if they are using a Tamron SP 150-600mm f/5-6.3, as this is no a low light lens.
Of course it isn't a low light lens but there comes a point when you can get the shot at f6.3 ISO 6400. I've tried that with my D7100 photographing badgers and it's pushing it too far and IQ is really degraded. If I'd have had a D750 my results would have been better, reach wasn't an issue as the badger came very close, the images were just extremely noisy. Sure a f2.8 lens would have been better still but we've got to make the most of what we've got.