Little Anna looks angry but I doubt she has a mean bone in her.
Why do some Hummers spend so much time driving other hummers away?

http://MichaelDanielHo.com

Hummers stretch their wings in different ways and that is a feel-good-look being displayed.

Instinct - they display aggressive behavior as soon as they leave the nest and that is what keeps them in great physical shape for hundreds of miles of migration flight.

Russ Thompson
http://e-hummingbirds.com
Louisville, Kentucky

Russ, Michael,

I might add that both male and female hummingbirds are territorial by nature, and they set up separate territories: the female in which to build a nest and raise her young, the male in which to attract a female for breeding.

Being aggressive about protecting one's territory is a strategy for reproductive success. If a male can keep other males out of his displaying territory, females which come into it may be more likely to respond to his "courtship" (such as it is), and he increases the chances of fathering young. If a female can keep other females out of her nesting territory, she gains a secure nesting spot hopefully within easy flight of food sources.

Any other time, aggression is also about protecting food sources. As Nancy's plant testing has noted, some plants have high-quality nectar, but the amounts in each bloom are small, so it takes a fair amount of work to gather lots of nectar to build fat for migration. Other plants have lots of nectar but it's not nearly as sweet, so high volumes don't translate to lots of fat-packing. Even feeders, which might appear to offer unlimited high-value nectar to the birds, can dry up, drip dry, or be taken down for whatever reason, so good food sources are fought over and protected - as a survival strategy.

Estimating fat reserves (part of Nancy's standard "measurements") has shown that birds pack on a LOT of fat during migration - we've seen birds double their weight in a week's time. Presumably, that's the strategy northbound Ruby-throats use in the spring if they're making a trans-Gulf crossing, in order to have the energy needed for a 20+ hour non-stop flight. In the fall, when the Ruby-throats seem to be skirting the Gulf around the western rim, we don't know if the birds fly as far as their fat reserves will take them, or if they fly a few hundred miles at a time and stop for food, or if they stop a few times a day whenever they see promising feeding options. My guess is that different birds may use different strategies. But in any event, that aggression is what helps them gather the resources needed to complete migration.

Kevin Morgan
Baton Rouge, LA