Tuesday, January 16, 2018

Year to date in the yard

Apart from keeping records of birds seen in the area generally I also maintain a COG Garden Bird Survey site.  The principle of this is to record:

  1. the maximum number of birds of each species;
  2. seen in a week;
  3. within 100m of a house.
Practically the third criterion is a bit flexible since if a bird-attractive feature is nearby human nature will include the birds seen there.  So as long as the site is no larger than the area of a circle with radius 100m (ie 3.14 Ha) it is acceptable.  It is also helpful in maintaining some rigour if the boundaries are physically recognisable (eg fences, roads etc).  Birds overflying the site are included.

My site is shown below.
The polygon function of Google Earth shows the area as 2.8 Ha which is pretty close to the required limit.  Of course the area is not covered evenly: the section visible from my study window (see below) is monitored for 4 hours a day minimum while the dam and Whiskers Creek get checked for perhaps a couple of minutes every second day (depending on what I'm doing around the block).
The Survey operates on a year beginning 3 July so that breeding seasons falls within a single year.  It starts on 3rd July because the original proponents felt it important to keep the week numbers consistent with desk diaries (!!!! - but we are dealing with a Survey that starts before personal computers had been invented!) in which the the start of week 1 is January 1.

The reason for compiling this post is that the results for this GBS year so far are a bit strange in a number of ways. Since starting the site in February 2007 I have recorded 111 species within the site.  This chart shows the pattern through which that total has been arrived at.   
The dashed line is a logarithmic trend line which seems to be usual form of trend followed in such activities.  I added two species last GBS year:  Channel-billed Cuckoo (an unusual species in the Southern Tablelands) and Flame Robin (common on the Hoskinstown Plain but not up here - they are renowned colonisers after fire).  So far this year I have added one species: Rufous Songlark- again quite usual on the Plain but not so common up here.

One of the major unusual features of the year is the pattern of total diversity.  The next graph shows the cumulative total number of species seen as the current year progresses and the equivalent average over the 10 previous years..
We were seriously underperforming until the week ending 27 September, when 7 species were added to the tally.  The total got a further boost, of 6 species, in the first week of October.  Additional species have continued to be added.

I am currently at 81 species for the year.  I did an extrapolation of a trend line fitted to the data up to 16 January and that indicated about 85 species would be the total.  As there are 30 species seen in previous years which I have not yet recorded this year that seems to be quite achievable, and there are at least 10 MIAs which are regular in the Gazette area.

Another way of looking at the current year is that the number of species already recorded is equal to or greater than the number recorded in any full year except 2009-10.

Four species in particular had caught my attention as appearing more frequently than in the past.  To give a little rigour to the analysis I compiled a table of species seen by week compared this year (so far) with the average of other years.  This added a few more "interesting" species.

I have written a specific post about White-winged Triller so will not say more about them here.

The second species is Dusky Woodswallow.
So far this year I have recorded them in the GBS site in 19 weeks, whereas the average is 2.78 weeks per year.  In other years they have been very common on Whiskers Creek Road (about 300m away) but much less so this year.  They have also been very aggressive to other birds - including this Kookabura.
Note the hurtling Woodswallow  lower right in the photo.  (This image was rejected by ebird because the Kooburra dominated the depiction of the Woodswallow.)  They have also given Magpies similar treatment.  This made me wonder if they were nesting in the area and on 15 January I  saw a Dependent Young Woodswallow.

The third species is Red-rumped Parrot.  .
I had only recorded this species once before in the site, although they are very common on the (relatively) nearby Hoskinstown Plain.  This year I saw a flock of 4 fly over just after New Year's Day and since then have been seeing and hearing them almost daily.  It has been suggested that the bush fire has opened up the vegetation and allowed some plants attractive to the parrots to come to seed.

The fourth species is Tree Martin.  These are not widespread in the area but a remnant Brittle Gum Woodland - about 8km away - with lots of hollows is a common nesting site for the species.  This year I have recorded them in 6 weeks compared to an average of 3.2 weeks in other years.  Their call is distinctive and pretty much continual as they hawk overhead.

An addition thrown up by the quantitative analysis is Sacred Kingfisher.  I have recorded them in 7 weeks this year, which I thought "usual" but in fact I have only recorded them on average 2.3 weeks per year in the past.  They appeared to be trying to excavate a nest hollow at one point

but didn't pursue this endeavour.

I realised that only 29 weeks of the year had passed so then expanded the number of weeks seen by 52/29.  (This gave a few false positives for migrants as a much higher proportion of their period of presence has occurred, so I ignored them.)  The main additional species drawn to my attention by this step was Magpie Lark, which has been recorded 9 weeks (after adjustment 16 weeks) compared to an average of 6 weeks.

The analysis also drew my attention to some species which have dropped significantly in their reporting rate.  The two most obvious, before adjustment, were Eastern Yellow Robin (average 28 weeks and even after adjustment only 5 weeks) and White-eared Honeyeater (average 34 weeks. 3.5 weeks after adjustment).  The adjustment process also drew attention to reduced recording of Common Blackbird, Yellow-rumped Thornbill and Common Bronzewing.  For the last species, they are commonly attracted by the fallen seed of Acacia dealbata which shrubs largely got incinerated in the fires.  I have not yet come up with a theory about the reason for the disappearance of the other species.

No comments:

Post a Comment

I am very happy to receive constructive comments. However anything I deem offensive will not be published.