Global migrations of up to 14,000 km are one of the world’s biological wonders. Individual flights of 8,000 or even 10,000 km are energetically demanding and birds may reach their physiological limits. Consequently, mortality associated with migration may be higher in birds making long migrations than in birds making short migrations. The corollary of this is that birds migrating to the tropics or southern hemisphere encounter much more amenable conditions than those settling closer to the Arctic breeding grounds. The evolution of different migratory strategies should be reflected in different seasonal patterns of mortality. What is unknown is whether absolute mortality levels also vary between populations. Comparing demographic characteristics of different subspecies of shorebird that have different migratory strategies will discern the nature of survival trade-offs in these migratory birds.

We aim to compare seasonal and annual mortality patterns in different subspecies of Bar-tailed Godwits (Limosa lapponica) and Red Knots (Calidris canutus) around the world (see tables below).These issues are of urgent conservation as well as scientific importance. Globally, shorebird populations are declining, but for most populations, current monitoring is insufficient to diagnose causes or predict future trends. For rufa subspecies knots migrating through Delaware Bay, USA, however, over-harvesting of Horseshoe Crab eggs has led to a catastrophic decline in knot numbers, and this decline is predicted to continue. For that population, an increasing proportion of birds has been unable to accumulate the nutrient stores critical for successful migration. ‘Protective’ internal organs such as the intestine were not enlarged during fuelling when birds were time-stressed, possibly decreasing immune function. Increasingly tight timing along a migration route and physiological costs during migration mean that changes in habitat quality and availability (e.g. reclamation, natural or human-induced food shortage, hunting pressure) put shorebird populations under extreme pressure. The use of only a few, essential, stopover sites means that entire populations or subspecies can be at risk.

There is a real need for detailed demographic monitoring of shorebird populations. While counts are the basic census tool for biologists, they contain errors of unknown magnitude and direction, and do not allow modelling of the causes of population changes or prediction of future numbers. Comparing demographic parameters of different subspecies of bird is the key to understanding both the biology and the conservation status of the populations. Because the underlying biological features of the populations are similar, differences in demographic characteristics can be related to features of their migrations or habitats. Large deviations from the predicted relationships between seasonal and annual mortality would indicate that other factors are impacting on survival; knowledge of the timing, and therefore the location, of the impacts can be used to target conservation efforts. Constructing population models will reveal whether population processes are similar for different populations. This type of monitoring on the non-breeding grounds is critical for managing multi-national species. Given the rates of intertidal reclamation in Asia (China and South Korea have already developed 25% and 40% of their intertidal habitat), ongoing monitoring of key species that winter in New Zealand is required.

Table 1. Bar-tailed Godwit subspecies: distribution, migration characteristics, and status of studies

subspecies	breeds	winters	migration distance (km)	non-breeding conditions	predicted migration mortality	predicted non-breeding mortality	current colour-banding?
lapponica	Scandinavia	Europe	2,000	cold-temperate winter	low	high	yes
taymyrensis	central Siberia	West Africa	8,500	tropical	moderate	low	yes
menzbieri	eastern Siberia	NW Australia	10,000	tropical	moderate	low	yes
baueri	Alaska	NZ, east Australia	11-14,000	warm-temperate summer	high	low	yes

Table 2. Red Knot subspecies: distribution, migration characteristics, and status of studies

subspecies	breeds	winters	migration distance (km)	non-breeding conditions	predicted migration mortality	predicted non-breeding mortality	current colour-banding?
islandica	NE Canada, Greenland	Europe	4,000	cold-temperate winter	low	high	yes
canutus	central Siberia	West Africa	8,500	tropical	moderate	low	yes
piersmai	New Siberian Islands	NW Australia	10,500	tropical	moderate	low	yes
roselaari	Wrangel Island	Gulf of Mexico	7,500	warm-temperate winter	moderate	moderate	no
rogersi	eastern Siberia	NZ, east Australia	14,000	warm-temperate summer	high	low	yes
rufa	northern Canada	South America	12,000	cold-temperate summer	high	moderate	yes

This work is being undertaken in collaboration with Theunis Piersma of the University of Groningen, The Netherlands, through the Global Flyway Network.

Individual recognition is the key to these studies. This godwit was colour-banded in New
Zealand, and photographed in Japan two weeks after it was last seen in New Zealand
in March 2005. Photo: Kouji Takenawa.