Because Spring has officially arrived, this week's Topic In Depth focuses on events that occur with the arrival of the new season.

Offered by the Howard Hughes Medical Institute, the first Web site (1) focuses on seasonal events like hibernation and migration, and the rhythms behind them. Next is a phenology site (2), where visitors can register to record their observations of Spring online or just learn what phenology is and why it is important. The Wisconsin Department of Natural Resources also has a phenology Web site for kids (3). The Missouri Botanical Garden (4) keeps a record of the plants in bloom each week, with corresponding photos and a choice of either a brief or more detailed description. Plantwatch (5), a site from the University of Alberta that encourages students and volunteers to register and report observations, has recently added a downloadable teacher's guide. Operation Migration (6), the organization that has successfully led endangered Whooping Crane migration by ultralight aircraft, provides daily updates as the birds begin preparing for their journey north. Users who want to begin identifying the birds that return to their yard this spring may be interested in the Peterson online identification guide (7). The final site (8) traces the life cycle of the Bumblebee, beginning with the queen emerging from hibernation and building a nest.