Weighty Stuff: Answers

The following chart shows the weights (grams) of summer and winter Ruby-throated and Rufous Hummingbirds, and American Robins for comparison:

Average Weights in Grams
Winter Male
Breeding Male
Winter Female
Breeding Female
Ruby-throated Hummingbird

Think about what these numbers mean, and about the different problems robins and hummingbirds face as you answer these questions:

  1. Which are heavier: male or female hummingbirds? Females.
  2. Which are heavier: male or female robins? Males.
  3. Why the difference between hummingbirds and robins?
    For most territorial birds, males are larger than females. One explanation is that the larger males are, the more effective they are at defending their territories. Robins are typical of this. Hummingbirds are very territorial, too. But their tiny size is a good adaptation for them. The tinier they are, the easier it is to hover and feed at tiny flowers. But females can't afford to be as tiny as males because their bodies must produce eggs. They must also and share their body heat with their eggs and chicks. If hummer females were as tiny as males, they'd lose too much of their energy and have slimmer chances of survival.
  4. Which are heavier: winter hummingbirds or breeding hummingbirds?
    Breeding hummingbirds.
  5. Which gains more weight in spring: male or female hummingbirds? Females
  6. Why do you think this is?
    Females must produce eggs.
  7. Which are heavier: winter robins or breeding robins? Winter robins.
  8. Which loses more weight in spring: male or female robins? Females.
  9. Why do you think this is?
    Robins are large enough that the eggs are a fairly small percentage of a female's weight. Females wait to head north, so their bodies don't need as much fat as males. Males migrate early, when it is colder, and when they arrive larger size is helpful for them in obtaining and defending a territory. So males have an advantage if they don't lose as much weight.
  10. Why the difference between hummingbirds' and robins' weights between the seasons?
    To survive in cold weather, birds put on fat and extra feathers. These add weight. Winter robins are a good example of this. Hummingbirds also put on extra fat to survive cold temperatures. But they often spend winter in warmer places than they breed in, or at least places with similar temperatures. When birds are done breeding for the season, their gonads (reproductive organs) shrink in size. Hummingbirds are so very tiny that this shrinking makes a big difference in their weights.