Migration of Yellow-faced Honeyeaters

The catalyst for this post was a message to the COG email discussion group reporting a small flock of Yellow-faced Honeyeaters (Lichenostomus chrysops) heading North on 13 March and wondering if this was early (for the start of the migration).

This species is notoriously difficult to handle in the COG Garden Bird Survey (GBS) as a result of a range of procedures being used at various times by observers (and deemed acceptable by the various Coordinators of the Survey).  Most of the difficulty comes about by some observers - mainly in the early years of the GBS - accumulating the number of migrating honeyeaters seen throughout a week rather than reporting the largest single flock.  To mitigate these problems I decided to create an indicator by counting the number of reported observations of flocks of 5 or more birds of this species by GBS week around the "migration period".  (Outside the migration period the species is typically reported as single birds or flocks of 2 or 3.)

In what follows I have defined the study period as GBS weeks 9 to 24 (on the calendar, 26 February to 17 June).  The current week - and that of the catalysing message - is week 11.  As always, click on an image to see a larger version.

This first chart covers all 29 years since the start of the GBS. 

I have plotted both the raw count of the number reports and adjusted that to allow for the fact that some weeks may have had greater or lesser amounts of observer effort (probably related to the dates of school holidays).  The 'adjustment' was to divide the count of sightings by the number of sites active in the week giving a result similar to the indicator F cited in the Annual Bird Reports).

Looking at this Chart suggest that the number of larger flocks has increased a little by week 11 but starts to really kick up (sorry about the technical term) after week 12.  I would suggest that for this species, the "migration period is closer to weeks 12  to 20. 

Thus my answer to the question "is this early?" would be "possibly a little, but one flock doesn't make a migration".  It also shows that when looking at a single data series the level of observer effort is quite stable and certainly is unlikely to outweigh the "noise in the system" from other sources.

There have however been some events in the Canberra area over the last 29 years which have been suggested as likely to affect, significantly, the migration.  The two that I have considered are:

• the building of the new suburbs of Southern Tuggeranong around the key migration point through Point Hutt Crossing on the Murrumbidge River; and 
• the bushfires in December 2001 which burnt the pine forests to the North of Cotter Road.  (I had a feeling that these, rather than the more catastrophic fires of January 2003, disrupted the honeyeater migration as recorded in the GBS).

I took the first of these as affecting the migration after year 10 (ie 1991 - 92) when the suburbs of Conder and Banks commenced.  The second possibly affected migration after year 21.  We thus have a 10 year period to be compared with an 11 year period and an 8 year period. Obviously the indicator of frequency is required to balance this different number of years.   Here is a second Chart.

The broad comment is that the three series show a pretty consistent overall pattern. So in terms of TIMING of the migration I don't think the events suggested have greatly influenced the outcome.  (I make no comment, because the data wouldn't support it, on the magnitude of the movement.)  The higher rate of reported observations of larger flocks of this species apparent in the earliest series could well be explained by a change in observer behaviour away from accumulation.