Alders have two types of catkins (shown here). The long pendulous catkins are pollen producing and the rounded dark brown ones, which look similar to the cones on a conifer are the seed bearing ones.
Alder seed is borne profusely, and they have two small wings on either side of the flat seed- which help them disperse in the wind. Alder seed is a favourite of Pine Siskins however, and flocks of Siskins l. They are useful trees from a bird point of view, often they have heavy cateripillar loads and that makes them a good foraging area for songbirds looking for food for nestlings.
Rather embarrassingly, it was finding the Common Emerald (#142) that made us go look and see if we had Red Alders on the property this summer.