Friday 29 November 2013

BirdBox Activity Counter; what can we expect?

Fitting a sensor to the entrance of a garden bird box can provide you with additional data.

But what can you expect this data to reveal about your little feathered friends?

Prompted by ConwayChallenger's recent Blue Tit Monitor, I've just been taking another look at some bird box data from spring 2011.

2011 was a pretty good season, and I went to the trouble of monitoring ambient temperature, box temperature and count for 2 boxes. We had blue tits nesting in our blue tit box, and great tits nesting in our nuthatch box (damn those great tits! They never read my emails!).

As I've said before, during March, blue tits seem to spend a lot of time checking out nesting sites. If you only have one box in your garden and a pair of birds come each day to peck around the entrance and pop inside for a look, you may think they are sure to nest there.

But they are almost certainly doing the same thing to any boxes or other potential nest sites in neighbouring gardens. Look real close, and you may find there are different pairs visiting at different times of the day.

Great tits also exhibit the same kind of behaviour, but seem to spend less time doing it than blue tits, and often just rock up at the last moment and start building. Blue tits will also give great tits a hard time if they are considered to be nesting too close (see my old website from 2009).

Let's see some graphs

The first graph shows the week 22-28th March 2011 (you can click on it to enlarge).

The yellow line (NH_temp) pretty much reflects the daily temperatures, because this box was almost permanently in the shade.

The blue line shows the blue tit box temperatures. You will notice that the over-night temperatures are very similar for the 2 boxes, as you would expect. But the day time temperatures are much higher. This is because the BT box was in full sun for most of the day.

This was a very pleasant, bright, sunny week. In fact it looks like a bit of cloud came rolling in on the morning of the 26th and the afternoon of the 28th.

The red line shows activity for the blue tits. They had already built most of their nest the week before, and just popped in during each day to add the final touches and ensure there were no squatters. Whereas the great tits (NH_count) had only just arrived.

Let's fast forward a couple of months

Here is a graph for the end of May, just for the great tits.

The ambient temperature was measured via a sensor close to this bird box, just under the roof & gutter of our garage. Just how much of the temperature difference is due to the birds, and how much due to the protection of the box or the heating effect of the camera, I'll leave you to decide.

The daily count (as detected by my photo-diode circuit) peaked at nearly 4100 on 22nd May. This may represent approximately 2000 feeds.

On the 20th May only 2400 counts were recorded. The temperatures look similar (and higher than the beginning of the week) so why the apparent drop?

So let's ditch the temperatures and focus on the counts.

Each count covers a 1 hour period. The parents got their best result between 9am and 10am on the 17th, with a count of 492. This possibly represents 244 feeds, or 3.75 feeds/minute. This rate is not impossible. The trees in our garden support a healthy population of caterpillars during the spring, and we have often "clocked" birds making return trips in under 20 seconds.

The limiting factor is possibly the availability of food, rather than the demands of their chicks, which appear to be in a constant "feed me" condition.

Now you may be wondering what happened on the 23rd May, or you may have already worked that out.

On the 4th May the chicks started breaking out of their shells. Less than 19 (full) days later they left the nest. The day started like any other that week, with a total count of over 750 from 6am to 8am. The chicks departure spread over a few hours, with all chicks away by 2pm. Then, at some time after 4pm, one of the adults returns to the nest, has a last look around, and then leaves.

Did They Jump Or Were They Pushed?

Up until a few minutes ago, I would have said that the parents decide when the chicks should leave, and that they do this by withholding food and calling them from the outside of the box. When I say withholding food, they do enter the box, sometimes with food, but mostly with nothing. And when there are a few chicks outside, they spend most of the time feeding those that are already in the trees.

However, there is now a small doubt in my mind. What if its the first chick leaving the nest that triggers the process?

I'm going to have to pay more attention the next time I witness this important event.

Conclusion (...or "Let's see how far I can stick my neck out")

First of all, you can't expect too much from your garden birds.
Your bird box may be in the ideal location, and birds may have nested there every year for the last 20 years. But there is no guaranty that it will happen next year. There has to be a spread of behaviour in our garden birds, otherwise they would not be able to adapt to changing conditions, and would never have started using man-made bird boxes. So next years birds may not view your efforts as favourably as last years did.

Is it worth adding an activity sensor/detector to the entrance of a bird box?
Yes. During the early weeks, it may indicate counts, which in turn will indicate that something is looking around. If you have been recording video, it will help you find it (without viewing everything). Counts recorded during the night may indicate mice or large insects like moths.

Is it worth recording temperature?
Maybe. I don't think there is much point in recording just activity and temperature. I don't believe there is a simple relationship between temperature and activity, anymore than between temperature and the start of nest building.

But wouldn't it be interesting to record lots of local parameters?

Here is a wish list:-
  • temperature
  • wind direction & speed
  • light level (sun/cloud)
  • humidity
  • barometric pressure
  • air quality
 It may also be useful to record:-
  • sound; internal/external, noise level (we live near a busy road)
  • images of immediate area (mostly tree cover)
  • predator count
I suspect the feeding rate is primarily limited by the availability of food (e.g. caterpillars, spiders & so on). This may be related not only to todays weather (e.g. temperature, rain-fall, wind speed/direction, humidity & so on) but also the weather leading up to this period. How quickly do conditions recover after snow or an extremely wet or dry period?

However there are other factors which slow down the feeding rate, including the presence of people, vehicles and predators (especially cats).

1 comment:

  1. You might like my take on an activity counter: