Arrival dates

This table shows the average month of first return of migrant species as recorded in the COG Garden Bird Survey (over 30 years with about 65 observers per year) and the Carwoola project (over 5 years with 1-10 observers per month).  It should be noted that 2 of the GBS sites are in the Gazette Catchment area so the series are not totally independent.

Common Names	GBS	Carwoola	Difference
Collared Sparrowhawk	7	7	0
Australian Hobby	7	9	2
Dollarbird	9	11	2
Sacred Kingfisher	9	10	1
Rainbow Bee-eater	10	11	1
White-throated Needletail	11	12	1
Pallid Cuckoo	8	8	0
Fan-tailed Cuckoo	8	8	0
Brush Cuckoo	10	11	1
Horsfield's Bronze-Cuckoo	9	8	-1
Shining Bronze-Cuckoo	9	9	0
Eastern Koel	10	12	2
Channel-billed Cuckoo	11	11	0
Tree Martin	8	8	0
Fairy Martin	9	10	1
Grey Fantail	7	8	1
Leaden Flycatcher	10	10	0
Rufous Whistler	8	8	0
White-winged Triller	9	10	1
White-throated Gerygone	9	9	0
Western Gerygone	10	9	-1
Brown Songlark	10	11	1
Rufous Songlark	9	11	2
Australian Reed-Warbler	9	10	1
White-browed Woodswallow	9	10	1
Dusky Woodswallow	8	9	1
Spotted Pardalote	7	7	0
White-naped Honeyeater	7	8	1
Yellow-faced Honeyeater	7	8	1
Noisy Friarbird	8	9	1
Olive-backed Oriole	8	8	0
Striated Pardalote	7	8	1

Summarising the column headed 'Difference':

• 2 species show the average for Carwoola is earlier than for the GBS;
• 13 species there is no difference;
• 12 species show the average for Carwoola is 1 month later than for the GBS
• 4  species show the average for Carwoola is 2 months later than for the GBS

I can envisage two reasons for the overall later arrival in Carwoola:

1. Due to the greater number of observers in the GBS the earliest arrivals have a greater chance of being reported;
2. As Carwoola is on average about 200m higher than Canberra the birds are a little slower to arrive, awaiting the food resources (nectar, invertebrates).

Both reasons are probably contributors, and I am not going to suggest which of these is the more important (mainly because I can't at this stage work out how to test this).  It is however interesting that the average 'delay' is about 2 weeks (ie .5 of a month): we have concluded that this is the usual amount by which weather-influenced events in Carwoola lag Canberra.