Survival rates of long-distance migrants

Some birds make long migrations to hospitable destinations; others travel only a short distance but have to survive a cold winter in the non-breeding season. Is this trade-off between migration length and non-breeding conditions reflected in different seasonal or annual survival rates? We are using individual marking and mark-recapture (resighting) methods to estimate survival rates in different subspecies of godwit and knot. By studying populations with different migratory strategies, we hope to tease apart the relative roles of migratory flight and overwinter survival in these species. At a key site in New Zealand with high resighting rates, I aim to accurately estimate the annual mortality of godwits on the non-breeding grounds and while away on migration and breeding. Over time and with sufficient relocations of birds on staging grounds, it may be possible to estimate the direct mortality associated with migratory flights.

Individual variation in migratory strategies

Why don't birds migrate synchronously? Why does one individual leave in full breeding plumage while another leaves with very little? This variation in migratory preparation and behaviour provides a fertile ground for studying 'decision-making' in birds. While it is difficult to address the real fitness consequences of such variation in migratory birds, establishing when individuals leave on migration, and in what condition, is an important first step in understanding the constraints in reaching the breeding grounds at the optimal time. Work I have been involved in has included using radio-telemetry to determine individual departure dates in Great Knots and Red Knots, and resightings to track the consistency of migratory schedules in individual godwits in different years. With colleagues in Alaska studying breeding godwits, and access to key stopover sites around the Yellow Sea, Bar-tailed Godwits provide one of the few model systems in which all stages of their migration can be studied.

Habitat networks on varying scales

In terms of the total distance birds fly in a year, distances between sites within New Zealand are trivial. Yet godwits in New Zealand seem to be quite site-faithful, staying at just one site for most of the non-breeding period. Knots are far more mobile, moving around between sites much more frequently. How strong is this tendency? Do individuals use networks of sites within New Zealand, and is there any consistency between birds? This is being studied as part of an Ornithological Society project from 2004-2007.

Body composition

The body composition of migrant birds changes as birds fuel up for migration, make long flights, change diet, increase food intake in winter - in other words, birds' bodies are often changing internally. Changes in overall amounts of fat are well documented in migrants, but there are also dramatic changes going on with the lean tissue, as organs grow and reduce as demands change. Guts may increase during refuelling, only to shrink again before migration starts. During flight itself, unavoidable protein turnover causes most organs to get smaller, leaving a bird much lighter when it arrives than it was before it started fuelling. By using accidental casualties or collecting small numbers of birds at key times, much can be learnt about the biology and flight ranges of migratory shorebirds.

Intertidal benthos

Intertidal birds depend on intertidal invertebrates for food, but surprisingly little work has been done on relationships between shorebirds and their prey in New Zealand. The diet of godwits, knots, and Pied Oystercatchers has been described at some sites, particularly Farewell Spit, Northwest Nelson, but no study has looked at how tightly the distribution of birds reflects the distribution of their prey, at least on a large scale. This is partly because it is difficult to map mobile bird abundance on a large scale. It is much easier to look at the distribution of potential prey on tidal flats, and this can provide important baseline information for monitoring the ecological condition of an area. Recently, we conducted the largest-scale assessment of intertidal benthos in New Zealand, on the 10,000 hectare flats of Farewell Spit.

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