I sent the comment below to Rod and it's perhaps worth posting here
because too many make the assumption of simple pairs for breeding
(perhaps because it seems so common amongst passerines and other
common birds like waterfowl and perhaps because it's considered the
human trait) but quite a few alternate breeding systems are used by
different bird species.

The particular system in use is that which best fits the availability
of food and amount of predation that a species experiences.

My favorite odd one is the Magpie Goose (in Australia) where males
pair (triple?) with two females who breed a single brood with eggs
from both females in a single nest. The primary reason seems to
optimize against loss of chicks by predation from Fish Eagles when
the brood moves to it's feeding grounds. It turns out that three
adults do a lot better job than two defending the brood.

My comment was:

> As Anna's Hummingbirds are polygynous (as are all hummers) I rather
> doubt that would happen.
>
> The male defends a feeding territory and permits females to build
> nests in parts of that territory. So he guarantees food supply for
> himself and the females he breeds with. His sole job is to defend
> that territory (hence the aggression most hummers show). But she's
> on her own with the eggs and brood.
>
> Without insects to provide protein breeding too early will not
> raise a brood. I suspect these are just doing the preliminaries (I
> hope).
>
> On Jan 24, 2009, at 9:21 AM, Rod Gilbert wrote:
>
>> The Birder's Handbook says that only the female incubates the
>> eggs, but I was wondering whether they might take turns when
>> insect populations are scarce?

Note that in this case there is also female selection of the
territory i.e. a female will mate with a male if the defended
territory looks "good".

Non-territorial humming birds tend to use leks for mate selection.

On Jan 24, 2009, at 12:35 PM, Rod Gilbert wrote:

> A couple of people have emailed me to say that males have no role
> in nest building or anything to do with raising the young so my
> assumptions were apparently purely speculation. One of them stated
> that a male would mate with any female that is willing. But also
> that there is no pairing up, which certainly doesn't seem to
> correspond with what I appear to be witnessing. My male is very
> tolerant of only one of the females that I have at my feeder.
> However, there could be alternative reasons for that.

--
Kevin Purcell
kevinpurcell at pobox.com