the top is a Bird (鸟）
It is not very accurate, because island is a mountain surrounded by water. But in the ancient time, all islands had a lot of birds living on, much more than that on the mountain inland, which impressed ancient Chinese, so they used bird-on-mountain to be the character of Island.
The small shell script for Island
The top is small shell Bird, and the bottom is small shell Mountain.
The clerical script for Island
Traditional song typeface of the character is
Similar to the clerical script.
Simplified song typeface of the character is
岛Its Pinyin is Dao3.