Ecology of migrant shorebirds in New Zealand, focussing on Farewell Spit, north-west Nelson : a thesis presented in partial fulfilment of the requirements for the degree of Masterate in Ecology at Massey University
Migratory shorebirds are a dynamic component of New Zealand's coastal fauna, alternating between distant breeding and non-breeding grounds. The Red Knot Calidris canutus, Bar-tailed Godwit Limosa lapponica and Pied Oystercatcher Haematopus ostralegus finschi were studied on Farewell Spit, North-West Nelson. The first two species breed in the Arctic and migrate to New Zealand for the non-breeding season. Over the southern summer they experience low thermostatic costs and generally improving prey conditions over the summer. Oystercatchers are resident over autumn and winter, so experience rising costs and declining prey quality in some species. One bivalve species, Macomona liliana, shows seasonal depth changes in the sediment, and so is largely inaccessible even to a long-billed bird such as the oystercatcher. Despite this, oystercatchers feed for less time than is available, and achieve intake rates sufficient to cover estimated needs. The energy needs of the Arctic waders rise as they prepare to migrate, and they achieve at least part of this by increasing the duration of feeding. Knots during spring tides in the premigratory period feed for the entire low-water period. Godwits are apparently less stressed, underutilising nocturnal feeding opportunities over summer. They are thought to increase feeding time by using this night-time feeding. The high energy demands for migrating birds come from the need to deposit nutrients for migration, and knots around the Auckland region are estimated to increase in mass from 115 to 185 g prior to migration. Fat deposition is not the only physiological preparation, however, and a sample of knots shot from Northland (illegally, recovered by DoC) revealed complex interactions between organs. Large amounts of fat were deposited, mostly in a subcutaneous layer but also in the abdominal cavity. Muscle protein was also deposited in flight and heart muscles, presumably to prepare for the extreme effort involved in trans-oceanic flights. At the same time, digestive organs decreased in mass. This is interpreted as freeing up muscle protein which is then deposited in organs for use during flight. Knots and godwits migrated from Farewell Spit in March. Most departures occurred in the evening and on rising tides. The former probably allows for the use of multiple navigational cues, while the latter may maximise feeding opportunities immediately before the flight. Most departures occurred after the passage of a low-pressure system or with the approach of a high-pressure system. This enabled favourable winds to be gained, so that the mean wind vector was a small tailwind. Thus, while departure directions were intermediate between the expected directions for flights to either Australia or northern New Zealand, it is probable they were able to fly across the top of a high-pressure system and gain wind assistance for a direct flight to Australia. However, the variability in flight range estimates depending on assumptions of travel-speed and protein deposition makes predicting migration routes difficult. Numbers of godwits have increased on Farewell Spit over the past decade, while oystercatchers have remained static Knots have shown a slight decline. Knot numbers are independent of national census counts so are presumably determined largely by factors operating on Farewell Spit itself. A possible mechanism that could give to a slowly declining population could be if a certain sector of the population fails to deposit sufficient nutrients to successfully migrate and return. If site-fidelity is very high (as it generally is in waders) then a long-term decrease could ensue.