The graphs represent the percentage of records for a species in each week, as a proportion of the number of records for the same species over the whole year.

Why not use data from more years?  To answer that, here is an example:

​

​

​

​

​

​

​

The two lines in the first graph have a similar shape but with the prominent peaks and troughs out of phase.  When the two are merged as in the second graph, detail is lost.

Why percentage?  The later period is shorter than the earlier period.  I made the split at 2009/2010 because either side of that point the number of records is most similar but still different in every case; absolute numbers of either individuals or records wouldn't give a worthwhile comparison, hence percentages.

Note also that the area under each line adds up to the same, that is 100% of all records for the period.  Push down on a peak and the sides will bulge out.  The graphs don't tell that there were more Large Red Damselfly records during week 19 in later years; merely that a larger proportion of the records are from that week.

There is inevitably some wiggle in the lines, resulting from randomly inconsistent observer effort especially where the number of records for a species is low.  However, I am confident that where there are sufficient records, the prominent peaks are valid representations of actual flight times ... near water!  Observers focus their attention on areas around water yet most immature adults keep away, the females for longer while their eggs develop.  The time spent away varies from species to species and also with the weather since feeding opportunities are reduced in wind and rain.  I will continue referring to "flight period", with the understanding that it is observed flight period.  Whatever the effects of observer effort and time spent maturing, underlying it all are the different strategies towards emergence.  Simplifying those strategies (a lot): some species spend their last winter as final instar larvae and are ready to emerge as soon as spring conditions allow;  these tend to have a well synchronised emergence.  Others, in spring, still have some larval development to do or might even have overwintered as eggs.  These emerge later in the year and are less well synchronised.  Even within a species, the strategy can be adjusted depending on how quickly the larvae developed during the preceding year.  Some individuals of the first type overwinter without reaching final instar and give rise to a second emergence peak.  In "A Biology of Dragonflies", Corbet describes a second Emperor peak occurring three or four weeks later than the first peak - conveniently for me, the same as the interval between the two highest peaks shown for both periods here.  The second emergence is much smaller but I suppose adults from the first are still active so the numbers are simply boosted.  Scarce Blue-tailed Damselfly can have two generations per year in continental Europe and probably now in southern Britain too (David Chelmick, referenced in the distribution pages).  The 2010-2024 line hints at evidence of this in the New Forest, from where most records used in the graph originate.  It might also show that this is a recent development since the earlier period doesn't have the same shape in August-September.

Southern Damselfly and Scarce Blue-tailed Damselfly graphs have a different period split, avoiding the first decade of this century. There was a lot of research into these two species undertaken by Liverpool University and mostly conducted in June and July.  As a result, the record counts would have a huge spike in those months, unrepresentative of the actual flight period spread.

Whatever the shape,  it is obvious in several cases that flight periods have changed in recent years.  Where there is a change, without reference to the earliest and latest outliers the bulk under the lines of early species has shifted to the left and for late species the shift is to the right.  Large Red Damselfly and Common Darter are the most obvious examples.  Changing climate is presumably the cause but I don't want to even guess at why different species respond differently.

For a shorter recent span of years (2015-2024) I compared the graphs for the three coastal vice counties with three Midland vice counties around Rugby, expecting to see a difference in many cases.  In fact, only two species showed much difference - Small Red-eyed Damselfly (earlier in the south) and Southern Hawker (later in the south).  I should have gone further north.