Today in Marshall County, Indiana, Betula nigra was blooming rather profusely. I don't believe that I'd ever noticed it in flower before.
Above are the erect pistillate catkins. The white stigmas can be seen protruding from the scales. The photo below shows several long dangling staminate catkins.
Betula nigra is a tree of swamps and floodplains of the eastern United States. It is primarily a southern species, but its geographical range stretches north along major river systems.