He believed in transmutation, one species changing into another and swapping plumages - as one winter migrant arrived and another went southwards! Other crazy notions reigned - birds wintered on the moon, small birds hitched lifts on larger birds. Early Irish natural history texts - Topogaphia Hiberniae (1188) by Giraldus Cambrensis and Ireland Naturall History (1652) by Gerard Boate peddled ancient mythologies about the migration of birds in Ireland.

The 1880s saw the first scientific investigations in Ireland - systematic visual observations of migrants. Richard Barrington (1849-1915), Bray farmer and ornithologist, organised from 1881 to 1897 a "viz mig" - visual migration - network, based on the 58 Irish lighthouses and lightships.

Ideally placed to track migrants that follow coastlines, light keepers reported bird movements, recorded dead birds found below the lights (many birds, dazzled by the light, strike the glass and die), and sent over 2,000 dead birds or wings to authenticate identification. Barrington collated and interpreted over 30,000 separate observations, producing a remarkable book, The Migration of Birds as observed at Irish Lighthouses and Lightships (1900).

Fifteen migratory species, previously unknown in Ireland, were recorded. Also, the blackbird, song and mistle thrushes, goldcrest, meadow pipit, skylark, chaffinch, greenfinch, rook and the water rail were shown to be regular migrants, a revelation at the time.

Viz mig techniques became more scientific with the establishment of bird observatories on Great Saltee, Co Wexford (1950-64, operating ad hoc today); Copeland Islands, Co Down (1954, going strong) and Clear Island, Co Cork (1959, going strong). Tory Island and Malin Head, Co Donegal, together with Annagh Head, Co Mayo, have acted as temporary bird observatories.

The early 1960s saw viz mig watching extended to off-shore waters, from prominent headlands or islands, tracking and trying to make sense of movements of shearwaters, petrels, auks and other seabirds that brush the Atlantic and other fringes of Ireland. Having once spent 110 hours plugged into the end of a telescope, scanning the Atlantic waters of Co Mayo during one spring, diligently recording every bird and naively thinking that I would crack the mystery of their migrations and movements, I must admit total failure.

Ornithologists have recently rekindled their sea watching enthusiasm, driven mainly by the hope of "twitching" a rarity such as a black-browed albatross or a Fea's petrel reported recently from Kilcummin Head, Co Mayo, and the Bridges of Ross, Co Clare respectively.

Tracking migration by radar began when strange echoes or "angles" were seen on screens during the second World War - the "angles" were flocks of birds. Radar can be used to scan a large area to pick up the broad movements of birds, or more specifically to follow the track of a bird across the sky by "locking" on to it or by the use of Doppler radar to determine the speed of migrants. While British ornithologists have monitored movements of birds into and from Ireland I do not know of any Irish-based migration studies employing radar.

Approximately 28,000 wild birds are ringed every year in Ireland with a small metal leg band carrying a unique number and return address. The birds are first trapped, using ultra-light mist nets, so that damage is extremely limited. Recovery of these - usually found dead or re-trapped by other ringers - provides information on migration routes, travel speed, wintering areas, age and cause of death. About 500 birds are recovered each year in Ireland, either ringed here or in other countries. The recovery rates for small birds is low, perhaps 0.1 per cent, but for big birds such as the cormorant, of which I have ringed 11,000 young chicks on Little Saltee, Co Wexford, during the past 43 years, the rate rises to about 13 per cent.

New types of markings have made migration studies easier. Large plastic leg bands, each with a unique alphabetic code, are used mainly on swans, geese, ducks and wading birds. Once caught and ringed each bird can be visually recovered by reading the ring code through a telescope up to 350 metres away, allowing multiple sightings by observers in numerous locations.

These rings have provided previously unobtainable insights into the life histories, movements and migrations of mute and whooper swans, barnacle, brent and Greenland white-fronted geese in Ireland. Inscribed plastic collars are used on Icelandic-ringed whooper swans wintering in here and on Irish-wintering Greenland white-fronted and brent geese. Plastic wing tags are fitted on the introduced golden eagles at Glenveagh National Park, Co Donegal.

The ultimate migration tracking technology is satellite telemetry - a satellite transmitter such as a PTT100 (weight 20-30 gms) with an antenna 1-2 mm long on a short-base spring, is fitted with an elastic harness on to a largish bird such as a Greenland white-fronted goose (at Wexford Slobs), other transmitters are placed on golden eagles (at Glenveagh), other birds of prey (common in Britain) or Irish wintering brent geese (Wildfowl and Wetlands Trust, Strangford Lough).

In the case of the white-fronted geese the PTTs send regular signals on temperature and bird activity, picked up by a polar orbiting NOAA satellite and downloaded by Argos CLS in Toulouse. Position, distances and directions travelled by each goose are calculated.

There is something both magical and unsettling about watching the progress of a radio-tagged bird on the laptop as it traverses, at great physical cost, oceans and continents to and from its wintering grounds and breeding grounds - the ultimate in bird voyeurism. Although I am sure to use this technology myself sometime I am currently happy just to wander along the coastline and islands, while watching and listening for migrants as they come and go in their wonderful and mysterious ways.

David Cabot is a zoologist, film-maker, broadcaster and writer. His Irish Birds (expanded version 2004 HarperCollins) is designed for people with a general interest in birds.