Anusha Shankar, Catherine H. Graham, Joseph R. Canepa, Susan M. Wethington and Donald R. Powers
How do hummingbirds manage to exist? They are tiny—different species weigh between 1.8g to 20g, or between the weight of a paperclip and 4 sheets of A4 paper. Because they need to stay light to fly, they store very little energy in the form of fat. But flying consumes a lot of energy, and without much of an energy larder, they are always on the border between life and death. We set out to understand what strategies a hummingbird species in Arizona, the broad-billed hummingbird, used to balance its daily energetic needs. We measured the total energy (in Calories) that wild hummingbirds spent in a day, and modeled how much of this energy was spent on thermoregulation (maintaining a high body temperature), daytime activity (perching, flying and hovering), and nighttime energy expenditure (including torpor, a version of nighttime hibernation). We assessed the relative importance of these different components across two seasons and two sites. In parallel, we studied how the hummingbirds’ primary food resources—flowers in their landscape—changed. We found that these hummingbirds could triple the amount of energy they spent in a day as their landscape changed with the advent of the monsoon. Before the monsoon, flowers were abundant and clustered, and hummingbirds could spend as little as 20% of their daily energetic needs on daytime activity costs, by largely perching (couch potato-ing), and spending the remaining 80% on thermoregulation and nighttime energetic needs. As the monsoons began, flowers became scarcer and more scattered. In response, hummingbirds increased their daily energy expenditure, by eating more but also spending more energy. They spent over 80% of their time and energy flying and hovering, searching for and feeding on flowers or insects (star athletes in search of their next high-calorie meal). So the answer to our initial question is that hummingbirds exist by being very flexible in how they manage their limited energy budgets. They adapt what they spend their energy on based on how their food landscape changes. Maybe it is alright to be a part-time couch potato, because somehow over 330 species of hummingbirds exist today.