The term 'Birch Kin' is a crossword solution that can be filled in with the word 'ALDER, ' which is a five-letter word. ALDER is a type of tree that is related to the birch tree, hence the term 'Birch Kin.' ALDER trees are native to Europe and North America and are often found near water sources. They are deciduous trees that can grow up to 30 meters tall and have a lifespan of up to 200 years. The wood of the ALDER tree is used for a variety of purposes, including furniture, flooring, and even musical instruments.