Robins, tits and sex education

This morning at breakfast we were watching the birds in our garden.  There were a pair of blackbirds eating the bread on the lawn and interacting with each other.  There were two blue tits in the cherry tree eating the buds and jumping around together.  And then there were the three robins.

For most of the week we've seen three robins in the garden getting progressively more active.  There is definitely one female and then two males competing for her attention.  it's lovely to see, the males fly around so fast it's spectacular; whilst the female sits and watches and avoids their dive-bombing antics.

Rachel was thrilled to see all the birds and was asking what was going on so I told her they're pairing up for spring.  I hadn't thought this through, suddenly I had gone down a path I hadn't intended.

What's pairing up?
Why are they getting together?
Is that like you and daddy?
How are the babies made?

I actually don't know much about bird reproduction so I was able to be suitably vague which she accepted.  But she's definitely starting to realise that there's something I'm not telling her about how the sperm meets the egg.  I wonder how much longer it'll be before I have to explain it some more.  Perhaps if I just find out details of bird reproduction that'll be the best way forward.