Dave,
I have been lucky enough to have been testing both the DRS4DNXT and DRS6AX over the past year with great results, so let me try and answer your question. Obviously many factors come into play and must be considered when looking for birds with a radar. On the boat side of things, the physical radar antenna size, radar antenna height and radar antenna power are very important. On the target side, the size of the target, height of the target and distance to the target are very important. The sea state is also a factor you must consider. If you vessel is bouncing around a lot you'll be getting more reflections from the waves and will need to adjust your Bird Mode settings accordingly.
The average height of a radar mounted on a center console boat is about 9 feet off the water. That alone limits your maximum distance to all but the largest targets from 10 miles and out. Storms can be seen at maximum range due to their height in reference to your radar. Since birds are a much smaller object to see, the bigger the antenna and the more power it has produces the best results and distances. I can tell you that I have seen even small flocks of birds on the DRS4DNXT out to four miles reliably. I test on the Chesapeake Bay, so we don't have large flocks of feeding birds. What we mostly see in my area are Cormorants, which flock in small groups. They are a diving bird species. Since they fly up to look for fish then dive quick when they find them, they make a great radar target even though the are small and found in small flocks.
I have found that looking for birds on the DRS4DNXT (or any radar) takes a little getting use to, but once you understand what to look for it starts to get easier. First, as you are probably already aware, like many Furuno radars the DRS4DNXT offers a Bird Mode with three automatic settings. Calm, Moderate and Rough. Having a mode you can turn on and off like the NXT has available takes away the guess work out of setting up your radar and allows you to concentrate on your target; birds. Pick the appropriate setting for your sea state and you're ready to start looking. If it's calm out, you can just turn Bird Mode to "Calm" and your settings are pre-done for you. Same goes for when it's a little rough (choose moderate) or very rough (choose rough). The other setting you need to turn on is Target Trails. This is a must. The reason for this is birds fly in a random fashion, while most other targets like other vessels have very gradual changes to their course. When you look at your radar screen you will start to see the target trails moving in an erratic fashion. This is a clear indicator of bird movement, so keep your eye out for this type of movement and you'll start to see that what you are seeing is birds from miles away. Like I said, it takes a little getting use to knowing what to look for, but once you see it you'll know what to expect. From then on it becomes easy to spot them.
The nice thing about the DRS4DNXT is its Doppler capabilities take seeing and tracking targets to a whole new level in a 24" dome size package. With the 25W output power that is comparable to a 4kW magnetron, you'll be amazed at what targets you can see (both birds and normal targets) and the clarity you see them (both at close range and long range). The DRS4DNXT is an amazing radar that I am confident you'd be happy with choosing. If you need any other information on the DRS4DNXT or bird mode questions, please just respond to this thread.