A recent post by another blogger commented in a
Note towards the end of the post, on the relative frequency of recording
sightings of Nightbirds across Australia on the Birdata system. This showed that in Birdata the Tawny Frogmouth was the most commonly reported Nightbird in Australia, with Southern Boobook in second place.
That led me to do a similar analysis for
eBird records for East Gippsland (as I have the data readily to hand). This showed a rather different outcome with
Southern Boobook being the most commonly reported “Nightbird”. Tawny Frogmouth was second. In correspondence, the author of the original
post subsequently provided Birdata information for the Birdata East Gippsland Region
(East Gippsland and Wellington Shires) again showing Boobook as the most common species.
These three sets of data are shown in the page ‘cf Birdata’ in this
Google Sheet and are summarised in
the chart below. Note that some of the
species in the page are omitted from the chart as having relatively few records.
I concluded at the time that the difference in frequency between
Birdata and eBird was probably a reflection
of the fact that a high proportion of reports of the Boobook are based on
hearing the distinctive and far-carrying onomatopoeic call of the species. The typical call of Tawny Frogmouth is a
quieter and less far-carrying “Ooom”.
Once a Tawny Frogmouth is spotted at a site – not easy due to them being
camouflage specialists - repeat sightings are easy, especially in urban areas,
as they roost in exposed positions, often in a very tight area within their territory. (Boobooks tend to roost in dense vegetation so
are much harder to see in daylight.) I
will return to this discussion at the end of the note.
In discussion of this Frances commented that she has seen
Tawny Frogmouths many times but has never seen a Boobook (although she has heard
them many times). This led me to look at
the timing of sightings of Nightbirds in eBird for East Gippsland (Shire). For simplicity I defined “daytime” as from 0600
hours until 1800 hours. A more rigorous approach would consider the
change of sunrise through the year – at Mallacoota from 0736 in June to 0537 [0437
excluding DST] in December. (If being obsessive,
it could also adjust for the range of
times in East Gippsland: on 24 July sunrise in Mallacoota is 0707 while at Paynesville
it is 0716.) Efficient implementation of such
refinements is beyond my computing skill, and would not, I suspect change the
outcome in a significant way.) The
following chart shows the number of eBird Nightbird sightings by the hour in
which the eBird survey was started.
I have then calculated and plotted, in the following chart,
the % of records for each species that were started in “daytime”. Tawny Frogmouth stands out as having by far
the highest proportion of daytime records.
I have hinted above at the need to bear in mind observer
behaviour in interpretation of these data.
I have not attempted a rigorous analysis of the possible behaviours
involved (and suspect that even for the eBird data that would be a major
project) but the following anecdotes might be relevant.
As a general comment I am relying on the moderators of the Birdata and eBird system having satisfied themselves that records of Barking Owl are in fact those birds and not dyspeptic Jack Russells!
As noted above most Nightbirds are hard to spot in daylight. Some species roost in dense vegetation (eg
Boobooks) or hollows (Barn Owl and kin).
Others are very hard to find due to excellent camouflage when roosting (White-throated
Nightjar). Even the Frogmouth, Powerful
Owl and Owlet‑nightjar can be difficult to spot, but once a roost site is known,
often on a relatively exposed perch, they can often be seen there repeatedly. For example:
- At my previous place (near Canberra) a pair of
Tawny Frogmouths had a territory in our rural yard and I included this species
in every list I completed for the site over 8 years.
- Another Tawny Frogmouth roosted in a suburban
garden clearly visible from a well birded urban park (and I am fairly sure
included in many lists from the park).
- A Powerful Owl – a species of interest to many
observers – took to roosting in an easily accessible site in the Australian
National Botanic Gardens (ANBG). Staff
of the ANBG estimated that the presence of this bird brought an extra 400
visitors to the Gardens over a 2 month period of residence.
- Also in ANBG – one of the most popular birding
sites in Canberra - Owlet-nightjars roost in hollows in trees, but emerge to
bask in sunlight. They use the same
hollow for some time and people visit those places to add the species to their
lists.
- For Southern Boobook the data collection for the
Atlas of ACT Birds was interesting. A
first tranche of data showed the bird to be entirely urban – apart from one
site in the Brindabellas where some birders had camped! This was resolved to some extent by special night-time visits to the Brindabellas in peak calling season.
While I have not recorded the details, my observation of eBird records when compiling regular reports on the birds of Mallacoota is that a high proportion of Nightbird records arise when people are driving to Mallacoota after dark and see Owls fly across the Genoa Rd. These are logged as ‘incidental’ records.
Most other sightings of nightbirds in Mallacoota arise from targeted outings by birders going out after dark to places where the hunting birds perch on infrastructure. Examples are the powerlines along Watertrust Road and the fence around the Airstrip. (Even those are not guaranteed to sight the birds: on an outing during one of our Big Weekends 20+ birders – including some nightbird experts – traversed Watertrust Rd over an hour and didn’t see or hear a single bird.)
In terms of comparison between the Australian Birdata information and the Gippsland data it must be noted that the environments covered can have an interesting effect (which I have not come close to sorting out). Taking the example of Powerful Owl (Powl), until the bird took up residence in ANBG, on the edge of the CBD, it was considered that, near Canberra, they were only found in the forest gullies of the Brindabellas. A Powl also appeared later, reducing the Ring-tailed Possum population of Inner North Canberra for some 18 months (and leaving the remains on pavement as an easy guide to where it was currently roosting) . I – and many other birders - listed a bird in Carlton Gardens on the edge of the Melbourne CBD (guano again giving a good hint) while in Mallacoota most reports of the species come from a forest gully only ~500m from the town centre.
Summary
My summary of this note is that it again illustrates the risks of single-number statistics for natural phenomena.
For serious consideration of nightbird statistics information about the times at which data were collected is essential. If the number of nightbird records drop is it because the number of birds has dropped or because there are less night time records?
I would also regard information about the number of sites at which records have been made is very important. If numbers of Powerful Owl records have dropped in Mallacoota is it because people are not checking the Pumping
Station Track? If the number of Tawny Frogmouth records rises, particularly for a small area, is it because one observer is studying the birds and reporting them every day from one site?
