Why are Bald Eagle Heads White?

There are seven species of sea eagle who appear to have a common ancestor. Four of those species feature distinctive white heads, or nearly white heads (Bald Eagle, Pallas's Fish Eagle, White-bellied Sea Eagle and African Fish Eagle). The other species have heads which are much less white, being darker and more brown in coloration.

Could it be the species with white heads have less selective pressure to camouflage them as predator since they primarily prey on fish? Species who primarily hunt fish would benefit less from strong camouflage than species who primarily prey on fowl, mammals and other species.

One clue in favor of this hypothesis is comparing the White-bellied Sea Eagle and it's close cousin the Sanford's sea eagle. The Sandford's sea eagle seems to have recently divererged from the White-bellied sea eagle somewhere between 1mya - a few 100,000 years ago. The White-bellied Sea Eagle preys predominantly on fish, features a white head and belly, while the Sanford sea eagle has a much darker overall coloration. The Sandford Eagle seems to have specialized adaptations to the Solomon Islands where it's been observed to hunt fruit bats and other mammals in a dense rain forest canopy. In this case, was the white head and coloration selected against in favor of better camouflage more well suited to the Solomon Islands?

A number of the sea eagles have been well studied in terms of the overall diets. Four species in particular seem to have good data on them (Bald Eagle, Steller's Sea Eagle, White-tailed Eagle and White-bellied Sea Eagle). These studies looked in and around eagle nests at animal remains to try and get a quantiative count on prey species the eagles consumed (after the eagles left the nests presumably!). The eagles we have data for seem to support the general trend of white headed sea eagles being predominantly fish eaters while eagles with darker heads having more varied diets with more fowl and mammals mixed in.

* Diet studies not availible, estimates based on Wikipedia

A possible counterargument would be that camouflage is not that advantageous when hunting fowl, especially with the general approach eagles take when hunting water fowl. Avian eyesight is also very keen. It seems likley though that the camouflage would still help in setting up a general ambush. Once the ducks are in range of an eagle there's no practical way for them to escape through flight, which is why they seem to rely on diving to escape. This could setup a situation where ducks come to a lake or body of water to forage and don't notice the eagles perched on the water's edge. Eagles with white heads would be significantly easier to spot and might help the ducks skip a dangerous body of water.

Another counterargument might be that eagles are opportunistic and what they eat depends more on where they are located. A study done on Steller's Sea Eagle saw a significant difference in the diets of eagles nesting on rivers as opposed to in a coastal habitat. The river eagles observed percentage of diet for fish was 11% as opposed to the coastal eagle's 73%. It seems that with a fluid population where there's much intermixing the selection on the genes would average out such that advantages for a coastal habitat would still be found in eagles who predominantly live in a more river environment, something like gene swamping.

More data is needed to evaluate the correctness of this theory. Genetic data would be especially helpful if it could be used to identify which genes specifically modulate the whiteness of the head and when that mutation or set of mutations took place. Perhaps ancient DNA samples could be obtained which might've been perserved as far back as hundreds of thousands of years ago. This seems somewhat possible given eagle tendancies to dive into a body of water and get stuck, or even tar pits such as la brea (where they may have been feasting on carrion). Buried eagles would have a greater chance of having preserved DNA fragments which could be recontructed for a more complete picture.

Observational data could also be very useful, there doesn't appear to be good studies done on the diets of the African Fish eagle for example, although it seems to be predominantly on fish. Other observational data which would help would be comparisons between the fowl hunting success rates of Bald Eagles vs. White-tailed eagles or Steller's. If it could be shown that the White-tailed eagles or Stellers are more succesfull than the Bald Eagle and that the encounters feature the use of camouflage in someway, it would support the general idea.