Some Musings on Mallacoota Migrants

Not only is Spring the time that birds start breeding in Mallacoota, but also the time that birds begin to migrate back to the area (or in some cases back to where they came from).  So I thought I'd see what I could dredge out of the eBird data set.

Methodology

I decided that it was better to look at frequency of observations rather than the number of birds seen.  In contrast to the British folk-saying about one swallow doesn't make a Summer, looking at frequencies might mean that one Koel does make a Summer (swallows are around most of the year).

My next step was to sort out a concordance between the taxonomies used by:

• eBird (which follows international rules, and includes some special codes to allow for uncertain identification to species level); and
• Birdlife Australia (BLA) which is designed for Australia and (in the elements I have used) is restricted to good species.

This would allow me to access the data about family membership (eg group all Cuckoos together) from my list derived from BLA.

I then removed the species with less than 5 observations from the analysis, since it seemed daft to try to spread 4 records across 6 (or worse, 12) months.  Possibly I should have removed those with less than 12 observations but those with 5-11 observations don't seem to upset too many apple-carts.

The penultimate step was to express the number of observations for each species in each month as a proportion of the number of checklists submitted in that month.  (As an aside, the highest value is 0.941, showing that Superb Fairywrens  were included on 94.1% of checklists entered for May.)  These proportions were then added across the year and for the periods {April - September} which I'll call Winter (for convenience) and {October - March} which I'll call Summer (ibid).  I then calculated the ratio of Winter - Summer proportions as my basic index of migrantness (hereafter, the index).

Where a species has not been recorded in Winter this amounts to a division by zero, so no answer is possible,  It doesn't matter, as such species would seem at first glance to be die-hard Summer migrants.  7 species fit this bill.  (4 others with no Winter records appear to reflect difficulty in locating the birds as they are not noted migrants.)

I have used 6 classes to look at the results.

1. Strong Summer Migration: no records in Winter
2. Weaker Summer Migration: ratio >2, so frequency at least twice as high in Summer than Winter.  In many cases this is due to a few records being in April or September - they extend the season a little.
3. Strong Winter migrant: no records in several months of Summer.  This distinction is not as sharp as with the Summer migrants.
4. Weaker Winter Migration: ratio <0 .5="" a="" again="" an="" extended="" few="" frequency="" half="" in="" less="" li="" mainly="" march="" nbsp="" observation="" october.="" of="" or="" period="" records="" so="" summer="" than="" that="" winter="" with="">
5. Not migrants: ratio between 0.5 and 2
6. Uncertain: Species with ratios indicating either form of  migration but the species is not known to migrate.

Results

I have put a list of the species classified to the above classes (except #5 and 6) here.  Discussion of the species originally classified to Status 6 is here.

Discussion of Groups

Rather than go though the 6 classes of migrants seriatim I have decided to cherry pick some groups and  families and comment on them.

Strong Summer Migrants

Of the 6 species in this group most are rarely reported.

The commonest species is the Pacific Koel which has  only recently started to be reported regularly in this area.  It has also spread inland and the view has been expressed that in Canberra the birds time their arrival to match the laying of second broods of their host species (in Canberra, Red Wattlebird).

Strong Winter Migrants

Only five species are in this group.   

Two (White-fronted Tern and Double-banded Plover) are species which breed in New Zealand and over-Winter in Australia.  

The others are:

• 2 Albatrosses which appear just off-shore in foul weather.  These are the Black-browed Albatross and the Shy Albatross (noting that the taxonomy of Thallasarche Albatrosses is more than usually open to debate).  In Summer both are further South.
• Arctic Jaeger - very uncommon trans-equatorial migrant from ..... the Arctic!

Weaker Winter migrants

In several cases it seems as though a non-trivial number of birds remain in the area over Summer, so attributes such as month of arrival are as clear cut as for the Summer migrants.  At present the species classified here only reflect the outcome of the mindless calculation of ratios.

Families of interest

In this section I will comment on a few of the more interesting families.  To try to give more detail would delay the report until after all the Summer migrants have left.

Cuckoos

6 species including 2 of the strong Summer migrants.  With the other 4 species it appears a good proportion return in September.  There are few reports in Autumn - which may be due to reports being based on the calls which are distinctive, and typically far carrying.   The species are less vocal out of the breeding season.

Honeyeaters and Chats 

I include this family as the Autumn movement of Red Wattlebirds and Yellow-faced Honeyeaters along the Inlet is so obvious.  However some birds from both these species remain over Winter so the ratio gives a migrant status of 5.  Scarlet Honeyeater and Noisy Friarbird are absent for several Winter months, while Crescent, White-eared and Yellow-tufted Honeyeater are more common in Winter.

Monarchs and Flycatchers

Leaden and Satin Flycatchers, and Black-faced Monarch are Summer arrivals.  The Restless Flycatcher comes to the area for Winter (although some remain over Summer) while the Magpie-lark is very common through the year.

Plovers, Dotterel and Lapwings

One of the two larger families of shorebirds.  In includes a Winter migrant (Double-banded Plover) and a Summer arrival - of low frequency - Pacific Golden Plover,  The other 4 species are reliably seen year round and breed in the area.

Snipe, Sandpipers, Godwits, Curlew, Stints and Phalaropes

The second larger family of shorebirds.  Most members of the family are Summer migrants (breeding in the Arctic) but a few Bar-tailed Godwit and Red-necked Stint often overwinter.  Possibly they are younger birds not yet afflicted with the biological imperative to breed.

Expected Month of Arrival for Summer migrants

I have compiled a classification of migrants according to the expected month of their arrival in the Mallacoota area.  (That list now also includes the Winter birds.)

It is based on the first month in which there are birds recorded but in a few cases were there seems to be an outlying record I have taken the first month with a large number of records. 

The list is here.