Coots are the odd ones out in their family. Unlike other rails, they are bold, noisy and aggressive birds. Instead of searching for cover, they are all out, even their nests are usually placed in the open water and they are as prominent that you cannot miss them. Their squabbles amongst themselves and with other birds may carry out through the winter, as some birds keep their territorial behaviour year round. Coots have ashy-black bodies and black heads where a strikingly contrasting white frontal shield and beak is one of their most distinctive features. Unlike the long, thin toes of the Moorhen, coots have lobulated toes with side extensions - similar to a Grebe's - that allow them to dive in search of the algae and underwater plants that form a good portion of their diet.
The Coot "flippers"But lets go back to the frontal shield. The size of the shield changes through the year depending on the bird reproductive status. Both males and females have frontal shields although the males' are larger, but both change in parallel, getting bigger in size as the start of the reproductive season approaches, and peaking in February -March.
(from Visser 1988)
Birds holding a territory have thicker and larger shields than birds not defending one. Experiments on the American Coot, a close relative of the Eurasian Coot, showed that frontal shields enlarged when coots were injected with testosterone pellets, while they become smaller when injected with estradiol. At the same time, the birds climbed up in the pecking order when injected with testosterone due to an increasingly "pugnacious attitude" in the words of Gordon Gullion. The size of the frontal shield in European coots is also correlated with gonadal size. The frontal shield is apparently used for individual recognition and it is an important in territorial disputes or fights. When an intruder enters a territory, the resident bird will lower its head, raise its wings and prominently displaying their shield to the intruder. Like so...
A typical posture of an aggressive coot
I have much more on stock on Coots, but this will have to do for today.
Visser, J. (1988). Seasonal changes in shield size in the Coot Ardea, 76, 56-63
Gordon W. Gullion (1951). The Frontal Shield of the American Coot The Wilson Bulletin, 63, 157-166