Exposure compensation or lower ISO?

I have a rather dark pond with full bright sunlight to shoot birds. Snowy Egrets are very challenging, even shooting at -1.0ev the highlights can be blown. Because of the dark water and heavily treed banks my auto ISO is usually 250-1000 even in the full bright sun. This is typically at 600mm.

My question is, if i lower the ISO purposely underexposing the shot, will my image quality be better than using exposure compensation with a higher ISO?

Thanks
Chris

EC.

Unless you're shooting film.

First off, I am still trying to wrap my mind around EC and have been for some time now, so take this for what it is worth.
A few questions.
Are you shooting in manual mode, or A priority or S priority? EC will have different results depending on your shooting mode.
To my understanding, shooting in Manual mode EC will only effect what you see on the light meter so you can adjust your exposure components to dial it in.
In shutter priority, the aperture will be changed with the EC unless it cannot completely adjust and then ISO will be changed.
In Aperture priority, the shutter speed will be changed and then ISO. (This is the way I understand it, but still am fuzzy on when to use it and how. Ha!)

All of this doesn't really answer your question as to bright sky vs. dark lake. To be honest, if you aren't already, you will be better off shooting in RAW and trying to use post processing to bring out the dark shadows. You will have to expose for the sky if you don't want it blown out and then work with the dark lake in post processing.

When I am walking around my place, most of the birds in flight I find are dark colored, so with the bright sky I shoot with shutter priority and +1 EC. The result is usually a blown out sky, but a bird that has some definition. I don't use EC much more than that. If shooting manual mode, I just adjust what I need (shutter, aperture, or ISO) to get what I want exposure wise. If I am lucky I can shoot a few shots and adjust as needed. Ha!

Long winded saying nothing. Ha! Sorry

I'm shooting in manual f7.1, ss640-1250 and auto ISO 250-1000.

I am shooting all white or partially white birds against a dark background. The birds are brightly lit.

I just wondered if lowering the ISO would result in better IQ.

I shoot bird in flight all the time. I generally shoot in Manual mode F8 @ 1/2000 sec with Auto ISO so that takes care of the changing light while shooting . Generally I am shooting against bright sky so I use Exposure Compensation +1 or +2 when against bright sky since the birds would be underexposed. Snowy Egrets are tricky though. Most of the time when I see them they are stalking around the pond edges so I have to change to -1 or -2 to compensate for them. I shoot the first one and look at the histogram quickly and compensate as I need to. I am lucky in that the D750 had a lot of room to bring up shadow detail.

To understand EC is to understand how a meter works.

And meters don't know anything other than 18% gray. Everything to them is 18% gray. Sunlit snow at high noon is 18% gray. A black cat in a coal mine at midnight is 18% gray. 18% gray is how the meter will try to render everything it sees.

If there are different brightnesses in a scene, it will average everything together and render the result as if it were all 18% gray.

Exposure compensation is used when your brain says, "Wait a minute..... here's a lighting situation I know perfectly well is going to fool my meter." That's when you start over-riding the settings your camera chooses for you.

You can underexpose the image using a lower ISO. If you are shooting RAW, there is more latitude when editing that may not be accessible if you shoot jpeg (you can recover some highlights in RAW if they are blown that you quite often can't recover with jpeg). Underexposing will make the dark or shadowed areas even darker, and while editing, you run the risk of increasing noise if those dark areas are made brighter.

My suggestion is to play around with different metering modes to see if that helps to get a better exposure especially if you are using matrix metering.