Friday, June 30, 2017

The possible influence of light and temperature

As Matthew points out in the preceding blog, the conditions have not been conducive to intensive iris activity.
However, in certain Bucks/Oxon woods during the last week, the numbers have been as good as in the heat wave of the previous week. This last seven days or so have been characterised by leaden skies interspersed with short spells of sunshine, some rain, and temperatures between 17 and 20.
Today, in the best Bucks wood, was instructive: the local forecast indicated temperatures slowly rising from about 16 to 17 at 10 am to 20 in mid-afternoon; the light on the other hand was best in the morning, deteriorating according to Mick Campbell's light meter, from 14.00 pm, which is when I arrived!
From midday until 13.45, Mick saw 24 individuals, mainly high up. He also watched a female ovipositing and we found the egg later. Together, from 14.00 until 16.50, we saw only 15 individuals.
So, from 12.00 until 13.45, temperature 18 degrees, 24 were seen = one every 4.4 mins; from 14..00 until 16.50, temperature 20 degrees, poorer light, 15 were seen = one every 11 minutes. This is not a scientific study, but you could possibly conclude that light is more important for activity than temperature, in the temperature range experienced today.
As Matthew also notes, it was fascinating to stare at the tops of the trees for long periods of time in dull conditions without seeing any movement, and then, suddenly, as the sun shone through for just a couple of minutes, four males took to the air, clashing, separating and gliding before disappearing again as soon as the sun disappeared.
Nevertheless, for this past week in several woods, it was reassuring to note that one could venture out in dull and even breezy conditions and yet see a reasonable amount of activity at temperatures > 17 degrees.

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