I have point layers of bird observations, and I want to symbolize certain areas by the number of bird observations in each polygon. Each point in the point layer has a "observation count" field associated with it. I want to include this number in the join_count field that already includes the number of points in each polygon. Since I will use that field to symbolize the abundance in each area
When performing the join, you should choose the option "Each polygon will be given a summary of the numeric attributes that fall inside it, and a count field showing how many points fall inside it". (Sounds like you're already using this option.)
PLUS, you need to select the "Sum" checkbox in the Join Data dialogue window.
This way, you'll get a field for the count of points, PLUS another field for the sum of any numeric fields's values.
(Your question said that you wanted this sum IN the join field, not as well as the join field. If this really is the case, then you can follow up with a Field Calculator on the join field to equal the value of the sum field.)
If this does not resolve the question satisfactorily, then you'll need to provide more information, including all details of every step you're doing to perform the join.