Here they both are on video. The female has been spending a minute or more each time checking out the box.
I am surprised she chose today to start building as it has been so wet and I thought she would wait for a dry day. This is the equal latest start day for nest building in the camera box. This photo shows the first few strands of nest material.
Once there is more material the female will start to fashion the cup of the nest at the back of the box by doing a circular dance while fluttering her outstretched wings. It was interesting to see her doing that today, even though there is hardly anything in the nest so far.
The box went up in 2009 and had blue tits in every year from 2010-2016. In the first year it had great tits and last year I put on a 32mm entrance plate and the box had tree sparrows. Although they were great to see, videoing the sparrows was very difficult because of the construction of their nest so I reduced the entrance plate to 25mm this year and I now have blue tits again.
I have records of the date of the start of nest building for 4 of the previous 5 blue tit nests and it varied between 09 March and 04 April, presumably influenced mainly by the weather. This has been a miserably cold wet spring and this is presumably why they have waited so long this year. First laying dates have varied from 23 April to 10 May with clutches of 2 to 11 eggs. The later the first egg the smaller the clutch so I expect this won't be a record year. In the 7 years of blue tit nests altogether there were 43 eggs, of which 42 hatched and all 42 fledged, so productivity has been high.
Data from the BTO Nest Record Scheme show that blue tis have been laying their eggs earlier over the past 50 years but there is very wide year to year variation (depending on the weather). My local laying dates have been about three days later than the national average. (01 April is day 91 so these are around 20 April - early May.)
More BTO data show that clutch sizes have been getting smaller,
and the number of fledglings per breeding attempt has fallen slightly,
but overall the blue tit population is doing OK.
As in previous years I will document day to day progress on a separate "Nestbox camera 2018" page accessed via a tab at the top of the blog, with occasional updates on the home page here. I just hope these two blue tits will evade the sparrowhawk.