Joe, I have have made 9 trips to Costa Rica at all times of the year. The birding is great year-round but there are trade-offs for each season. The rainy season is roughly May to December and weather can always interfere with birding. The trips I have done in the rainy season have not lost much time to the rain because it is often at times when you are not birding. In spring more birds are breeding and easier to find. Prices are also reduced in the rainy season. In late spring and summer you do not get the North American migrants, which some may consider a small loss, but I have seen more Golden-winged Warblers in Costa Rica than as migrants in Maryland. During the drier season (there is still some rain) prices are higher and many places may be crowded with tourists. Different parts of the country have different weather patterns and rainfalls. The Costa Rica field guide is very good and can give you more detailed info about climate. Bob Ringler