I want my data to be used for science and conservation as much as possible. If someone publishes a paper where my observations make up a large proportion of the data used, I’d certainly expect acknowledgment or even to be invited to contribute as a co-author. But if I am one of hundreds or thousands of observers adding one little piece to the puzzle, I just want researchers to be able to use my data as easily as possible. Disclaimer: I am also a researcher, so this may affect how I see this issue.
I’d support CC0 as being at a minimum the default observation license, with if possible clarification on the site as to whether the basic metadata (date, location, species) are included in this, or whether the license simply applies to observation notes and other ancillary information added.