Penguin populations are declining around the world, and global warming may be to blame, researchers say. Warming ocean waters have been linked to toxic algal blooms, such as red tides, that have caused penguin die-offs. Adelie penguins may be taking a hit because warming temperatures are preventing the formation of winter sea ice that is essential to the life cycle of krill, a crustacean that is the penguins’ only source of food. Overfishing and oil spills are also causing big problems for the birds. Ten of the world’s 17 penguin species are already listed as threatened or endangered.