Friday, 22 September 2017

Tawny Frogmouth update

The pair of Tawny Frogmouths which roost (most of the year) in our garden have yet again built a nest in the Big Yellow Box - Eucalyptus meliodora, not something from Playschool.

They were the latest starting of any year since I started recording full details in 2010.  There are a range of observational snafus (I found that link really helpful) for several years:

  1. I was actually aware of the nest in 2008, but the male was already in situ by that time.  
  2. For 2009 I noticed the skinny platform of twigs before a cloaca was placed upon it, but as brooding started 3 days later I had clearly missed the commencement. of the process.  
  3. 2011 marked a shift to a different tree.  I only realised this when the male went AWOL and I found him sitting on a nest about 40m away.
  4. In 2014 they were back in the old tree and I thought twigs started to be gathered very early.  However brooding started "on time" so I think the early start was just some windblown twigs falling into the designated fork.
So of the last 10 years I actually have 6 observations of the date of starting nest building which I regard as reliable.  I have been able to come up with some estimates of dates of starting nest building for the missing years as explained below, but don't regard them as anything more reliable than anecdotes.
  • I have looked at the set of dates for 2008 and the chicks didn't leave the nest until 27 November, far later (approximately 3 weeks) than any subsequent year except 2016 when fledging seems to have been delayed by the very wet Winter (which was still 9 days earlier than 2008).  Counting back from that 2008 date it  would seem likely that the male started brooding about 27 September 2008 which is even later than this year's record.  
  • Looking at 2009, 2011 and 2014  and counting back from the commencement of brooding gives possible dates for commencing nesting in those years in the 'normal range' of mid-late August.   
In contemplating what might be causing variations in the date of starting nest building, rainfall seems to be an obvious contender since it affects the supply of food.  I have looked at two measures of rainfall: total rainfall year to date (YTD); and Winter (June -August) falls.  The outcome is summarised in this chart (where Day of the Year is number of days since 1 January in that year).
I was a tad surprised at the poor performance of the Winter rainfall series.  It appears that an interpretation is that the record-breaking sogginess of 2016 didn't delay the start of nest building as much as could be expected.  (Just for fun I changed the 2016 Winter rainfall to see what happened: reducing it from 291mm to 210mm raised the value of R2 to that of the YTD series.  That way lies insanity.)

There are many other possible measures of triggering rainfall (eg: June plus July; May to July etc etc) but I don''t have the time to invent and test every combination. I also have a niggle that the variation in rainfall is relatively great compared to the variation in starting date which implies that something else (eg some measure of temperature) might also be involved.  Excel isn't up to that.

The next milestone in the breeding cycle comes when the male "assumes the position" on the nest,
... ending the nest building phase.  This next graph shows the dates of starting the two activities measured from the start of the year.

For the previous years for which I have good records nest building averages to about 18 days.  This year they took 14 days, equal to 2012. (That year the nest was not as visible since it was in the different tree so it is possible I missed one or two days with little material in the nest.)

Thus I have 5 reliable observations of the start and finish of nest building (2010, 2013 2015-17).  Again, I wondered if the various lengths of the nest building phase correlated with rainfall for those years.  Looking at total rainfall for the year to date the correlation with number of days to build the nest is strong at 0.76.  Restricting the rainfall to June - August gives an even stronger relationship at 0.94.  (Of course 5 observations isn't really enough to make any big calls.)  Adding in the least dubious value for 2014 improves the correlation with year to end August rainfall to 0.84 but reduces that with Winter precipitation to 0.88.  (If I assume I only missed the first day of nest building in 2014 the correlations rise to 0.91 and 0.95 respectively: but making changes like that leads to wearing a wrap-round jacket!).  I have summarised this in a graph (including 2014).
I have no idea why the values of R2 appear somewhat inconsistent with the correlation coefficients cited in the text, but do note the small number of observations.  The key point is that there appears to be a pretty good correlation between the length of the nest building exercise and the amount of Winter rainfall.

This is really an exercise in getting a baseline for looking at the timing of nesting behaviour of the birds.  5 - 6 observations isn't much data.  Inshallah I'll add to the data next year and or other folk who regularly monitor individual nests will be able to do their own analyses.

Following my posting a link to this post on Facebook a friend asked if I had looked at the effect of temperatures on these dates.  I hadn't done so but have now looked at a few correlations.  I decided that I would look at average maximum and average minimum temperatures (for June and July) against day of starting and length of nest building.  Of these the only correlation coefficient greater than +0.2 was average maximum temperature against length of nest building.  The coefficient was -0.39 which is not great, but does suggest that nest takes longer when the maximum is lower.

My data isn't robust enough to try to muck about with multiple regressions involving rain and temperature.

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