Age-ratio scans on bar-tailed godwits
If you are out looking at godwits in October-November, can you help us and measure the proportion of juveniles in flocks along the East Asian-Australasian Flyway? Distinguishing juveniles from adults is generally easy (see below), and will help piece together answers to two questions: how many young godwits are there, and where are they? In recent years there has been concern about apparent low productivity of Bar-tailed Godwits, based on the low numbers of juveniles seen on the staging grounds in Alaska. In September 2004 in Alaska, while some sites had the ‘typical’ low numbers of juveniles, another site had about six times as many juveniles as adults! But what is not clear is whether this means that many juveniles routinely choose different locations to adults, or early departures of adults from a much bigger flock at that site left only juveniles remaining.
In order to understand better the demographics and migrations of godwits, we need to enlarge the area over which we estimate godwit productivity. The most practical way to estimate how productive godwits have been is to count the numbers of juveniles and the numbers of adults in flocks on the non-breeding grounds. This is best done by visually scanning large numbers of birds.
We are interested in age-ratio data on godwits anywhere along the Pacific Rim between Alaska and New Zealand (our interest is really in the subspecies baueri). While we believe adult godwits may migrate direct from Alaska to their non-breeding grounds in New Zealand and Australia, there is nothing to say that all juveniles do the same thing. By making scans through Australasia and Asia, we will learn a lot about how godwits migrate along the East Asian-Australasian Flyway.
Identification of adults and juveniles
Ageing of post-breeding godwits is done largely on the patterns and condition of the contour feathers on the wings, shoulders and back. Juveniles have strongly patterned mantle (‘back’), scapular (‘shoulder’), tertial and wing covert feathers, which are dark with extensive pale ‘notches’ coming in from the edges (Figure 1). This results in a spangled look. All the body and wing feathers are the same age, and the scapulars are slightly smaller than on adults, so juveniles look ‘neater’ than adults. Juveniles are also quite buffy in their overall colour, including on the breast, though this fades with time.
FIGURE 1: Juvenile godwits, showing contrasting upperparts. The distinctive tertials are circled in the bird on the left. Note that some adults may still retain notched tertials after breeding.
Adults can be told by the wear on their feathers, their colouration, and their stage of moult. Postbreeding adults typically undergo some body moult on the staging grounds (sometimes a lot). Adults newly arrived on the non-breeding grounds can have a mixture of very worn, old breeding plumage feathers (often just the dark central part may remain on the upperparts) and new, grey-brown feathers with crisp edges (Figure 2). Some reddish plumage may be present on the underparts. The wing coverts of some adults can look at first like those of a juvenile, but closer inspection will show them to be frayed and worn. Juveniles have much fresher feathers in good condition.
FIGURE 2: Post-breeding adult godwit, showing a mix of worn breeding (alternate) plumage and non-breeding (basic) plumage. Note the ‘toothed’ appearance of the worn wing feathers.
How long can the ages be distinguished?
Juveniles moult into their first non-breeding (basic) plumage some time after arriving on the non-breeding grounds. Experience in New Zealand indicates that juveniles can be easily distinguished throughout October and into November. Beyond mid-November ageing becomes more difficult, and while some birds can be told by retained juvenile tertials, age ratios can be unreliable if some first-year birds get overlooked. The best age ratios are when all juveniles have arrived but before the early-moulting juveniles start to look like adults. Total counts of juveniles, even without accurate age ratio data, can help determine when numbers have levelled off.
What data should be collected?
The basic data are counts of ‘adults’ (including previous years’ subadults if present) and counts of juveniles. Counts can take two forms:
- Complete count
If the flock is small enough, or laid out in such a way that all birds can be accurately counted and aged, count all adults and all juveniles.
- Samples of a larger flock
If the flock is too large to age all birds in it, repeatedly subsample different parts of the flock. Sample 50 birds from one part of the flock, then shift to another part and sample another 50 and so on. Aim for 10 non-overlapping samples of 50 birds. This becomes one scan. If time and birds permit, repeat the scans on the same flock (particularly if birds have flown and resettled) as many times as you are able/willing to do. Scans can be numbered by flock then scan (e.g. 1.2 equals flock 1, scan 2).
NOTE Juveniles often end up together at one end of a flock. This is a problem for scanning when the whole flock does not get covered (e.g. if birds fly up part-way through a scan). If the ‘juvenile end’ has been scanned, the juvenile ratio is inflated; if the other end was scanned, the reverse will occur. It is crucial to spread subsamples throughout the flock, ensuring that the ends, edges and inner parts are all sampled appropriately. A field-sketch in your notebook may be useful for showing whether the flock has been sampled well enough, and whether juveniles are bunched (e.g. Figure 3).
FIGURE 3: Example of a field sketch showing age ratio samples across a flock of godwits. Numbers are the count of juveniles and count of adults per subsample. In this flock, 900 birds were scanned. Note that most juveniles were found towards one end of the flock. Data from Bob Gill.
What other infomation is required?
Observer details, site, date, how many birds were present in total (even roughly), and any general comments. If scanning a flock somewhere we are unlikely to know the location of, please give latitude and longitude.
Big Sand Island, Tapora, Kaipara Harbour, New Zealand (36deg 22’53.9”S, 174deg 14’578.8”E). 25 October 2004. Observer: Feel Badly. Total number c. 1600, in two flocks. Flocks worked for 55 mins at high tide (1330-1425) in good light.
Flock 1 (c. 900 birds)
Scan 1.1: 12 juv; 488 ad.
Scan 1.2: 5 juv; 495 ad.
Flock 2 (c. 700 birds)
Scan 2.1: 7 juv; 375 ad (flock flew when harrier passed by).
We would also welcome any information about godwit numbers anywhere along the Flyway on southward migration. Please feel free to contact us if you have any questions, comments, or information.
Many thanks for your assistance!
Sending the records in:
Ecology Group, Massey University
Private Bag 11-222
Palmerston North 4442
06 356 9099 ext. 2605