Feathering One's Nest

This little chap and his or her partner are building a nest in the ivy around my garden fence right outside my kitchen window. I wanted, actually, to cut that ivy but daren't now.

One of them is a big fatty and the other is really slim. I wonder which is the male and which the female?

I watched them for some time and over a few days and they were very industrious. When they saw me lurking they changed tactics and worked a kind of pincer deception. One would perch on a fence post whilst the other would skit along next doors patio and then zip up to the nest whereas before they had flown in from above. Then they would swap over. I saw them at one stage exchange moss beak to beak so the other could make the final nest run. All very organised and clever.

There was also a pair of blue tits doing the same thing, but I couldn't see where they were nesting.