BirdBox2018: our mixed species nest

Its been a very interesting season watching how the story unfolds in our mixed tit species nest.

The chicks have now left the safety of the box to take their chances in the big wild world.

So this post just reviews what we observed and what I think may have happened.

Back in March the blue tits started checking out the 32mm hole bird box which is mounted about 3m up a Canadian Maple tree. This box has been used every year by either blue tits or great tits. It is fitted with my original Raspberry Pi camera system designed in 2013 for the 2014 season.

The weather was pretty bad during much of March and most of April, but by the second week in April the female blue tit had made a nest and started spending each night in the box. By the end of the 3rd week of April the female blue tit had laid a few eggs.

On the morning of 23rd April the system captured a few images of great tits in the nest. The system does not run with the lights on, so it is possible that a great tit may have entered the box on other occasions, especially early morning when it can be light outside but pitch black inside.

Two days later the female great tit entered the bird box again and evicted the blue tits. There was very little violence except when the great tit popped out for a few minutes and the female blue tit made the mistake of going back in.

the female great tit checking to see if those pesky blue tits are still lurking outside

By this time I'd seen 6 eggs in the nest. The female great tit spent the next week acting as if she was expecting to lay her own eggs (i.e. she would sleep in the box at night, then stay away most of the day). However, the number of eggs remained at 6.

At the beginning of May she started to incubate the eggs, and after 2 weeks all 6 had hatched. Initially they all looked the same; little pink blobs. It was almost another 2 weeks before I noticed something odd; one of the chicks was a great tit! Only when they had developed a few feathers, and those feathers started to colour-up could we see that we actually had 5 blue tits and 1 great tit chick in this nest.

What I now think happened was that the great tits had already started their own nest and the female had laid a number of eggs. I suspect this nest was either raided or destroyed, so the female had to quickly find somewhere to lay her next egg, and she chose the blue tit nest. I think this must have happened either 2 days before the take-over (when seen in the nest) or possibly a few days even earlier. And by the time I noticed 6 eggs, the great tit had already laid her final egg.

You have to remember that most of the bird's breeding actions are not due to conscious thought. So her "genetic script" may have caused her to wait for 'the next egg' to arrive, or she could have been delaying incubation waiting for environmental conditions to improve; although the weather had already improved, so maybe it was about food supply.

The great tits have been excellent parents and don't seem to have shown any favouritism when feeding and caring for their mixed chicks. The blue tit chicks diet would normally consist mostly of green caterpillars. However, the great tit foster parents do not know that, so they have probably been fed on a much wider diet. We have seen a few green caterpillars brought in by the parents, but also a lot of spiders, flies, moths/butterflies and other unidentified crawling or flying bugs.

almost ready to leave the nest

As each day passed, the great tit grew bigger than the blue tits, but again there was no obvious signs of aggression between them.

In the meantime the original blue tit pair have been raising a family in a neighbours nest box.

So there will be some interesting competition around the bird feeders when these chicks also fledge.