Trouble with text search select statement

I am having trouble getting a multiple column text search select statement
to work.

I've setup the following:

CREATE TABLE tstest (col1 text, col2 text);
INSERT INTO tstest (col1, col2) VALUES ('bird', 'moon');
CREATE INDEX tstest_ts_idx ON tstest USING gin(to_tsvector('english',
coalesce(col1, '') || '' || coalesce(col2, '')));

I then execute what I believe is the correct select statement:

SELECT * FROM tstest WHERE to_tsvector('english', coalesce(col1, '') || ''
|| coalesce(col2, '')) @@ to_tsquery('english', 'bird');

If I remove the second column from the to_tsvector in the select statement
then it returns the row.

SELECT * FROM tstest WHERE to_tsvector('english', coalesce(col1, '')) @@
to_tsquery('english', 'bird');

I know I've missed something basic but I can't figure out what.

As a side note I don't understand the purpose of the || '' || between the
columns in the to_tsvector. I saw it in several examples. I've tried both
with and without it with no change.

Thanks in advance.

-Mont