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 |
Rufous
Hummingbird |
3.27 |
3.34 |
3.35 |
3.58 |
Ruby-throated
Hummingbird |
3.08 |
3.36 |
3.17 |
3.81 |
American
Robin |
86.2 |
84.8 |
83.6 |
75.0 |
Think
about what these numbers mean, and about the different problems robins
and hummingbirds face as you answer these questions:
-
Which are heavier: male or female hummingbirds? Females.
- Which
are heavier: male or female robins? Males.
- 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.
- Which
are heavier: winter hummingbirds or breeding hummingbirds?
Breeding hummingbirds.
- Which
gains more weight in spring: male or female hummingbirds? Females
- Why do
you think this is?
Females must produce eggs.
- Which
are heavier: winter robins or breeding robins? Winter
robins.
- Which
loses more weight in spring: male or female robins? Females.
- 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.
- 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.
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