Magpie-lark migration

In commenting on my observation of a flock of 25 Magpie-larks at Mallacoota Rohan Clarke said "Magpie-larks are partial migrants.". Rohan followed up with comments about observing the species in the far NW "  ... we get them at Ashmore Reef in small numbers demonstrating migration to Timor."

This surprised me as I have never thought of them as migratory, so I have done a little delving into eBird and HANZAB.  

I started with two range maps from eBird (they are pretty consistent with the maps in the New Atlas of Australian Birds).  The various shades of mauve indicate relative abundance with a grey shade indicating the area was birded but the species no reported.  A white area was not visited in the season.

April - September

For some reason - possibly weather related -  many of the areas in the Great Victorian, Gibson and Great Sandy Deserts weren't visited in Summer!

October - March

It is difficult to pick out a clear pattern in these maps.

Sticking with eBird for a while, I looked at the data for East Gippsland Shire.  My first effort was to look at the total number of birds reported and the total number of data sheets on which the species was reported.

 At first glance the shape of the graphs (very similar with a correlation coefficient of 0.95) suggests a typical summer migrant pattern.  However that shape is also very similar to the chart of number of sheets submitted (correlation o.95) so the pattern possibly just reflects the number of boots on the ground (or at least bin's in the bush).  So I expressed the numbers as rates of the number of checklists submitted each month. 

The resulting graphs were not - to my mind - indicative of a typical migration pattern.

Fortunately HANZAB (vol 7A) resolves the issues. The key phrases are:

• Partly sedentary and resident , partly migratory  but varying geographically.
• In s. Aust adult breeding pairs sedentary ... while juveniles, immatures and non-breeding birds appear to form mobile (but probably resident) flocks
• Resident or sedentary in much of n. Aust. but seasonal migration to coastal and subcoastal n. Aust . in dry season.

I'm not sure how well that is reflected in the crude maps from eBird, but certainly explains the difference between East Gippsland data and Rohan's observations.