I can spend countless hours watching and photographing swallows. These beautiful birds are mesmerizing as they perform aerial acrobatics to capture their insect prey on the wing. They also accomplish impressive feats of migration, with some species migrating thousands of kilometres from Central or South America to their breeding grounds as far away as northern Canada or Alaska. The Tree Swallow, pictured here, is a cavity-nesting species. However, natural holes in trees are increasingly uncommon, particularly around urban areas where dead trees (which tend to have the most cavities) are frequently removed by people. Fortunately, Tree Swallows have taken well to artificial nesting boxes, and as a result can still be seen nesting in urban parks and green spaces.