Patrick I think you are mistaking the Indian Ocean Cocos Islands (also called Keeling) Lat 12S, Long 97E (see http://en.wikipedia.org/wiki/Cocos_Islands
) with Cocos Island 05°31′08″N 087°04′18″W off the coast of Costa Rica (see http://en.wikipedia.org/wiki/Cocos_Island
Laurie to in-part answer your question - we have here in the Gulf both Little and Saunders and there is considerable debate as to if the rule two Black primaries for Little over four for Saunder's holds true - and I don't know enough about your location to know if both species occur there. There are other features that can be used to separate them but its hard to tell from your photos - one point the extent of the white on the head ie. does it go behind the eye or not cannot be judge properly as the heads are all washed out by the sun.
This picture taken here early August shows three Saunder's in the front with a Little in the background.