It could have been a dragonfly, its wings a blur of iridescent dark blue as it hovered among the leaves. But the soft whirring sound was unmistakable and a second glance identified it as a hummingbird.
Watching this tiny creature in suspension as it poked its beak into a flower was thrill number 86 - an unexpected delight at the end of five days bird-watching.
We were in northeast Argentina, in a wetland reserve called the Esteros del Ibera where huge rhea, herons, giant storks, wading birds of the same family as the pukeko, woodpeckers, three species of kingfisher, and smaller gems like the scarlet-headed blackbird or the crested cardinal paraded before our binoculars in an astonishing array.
Birds were not the only wildlife to admire. Capybara (pig-like, they're the largest rodent), howler monkeys, caiman (freshwater alligators) and marsh deer are plentiful and easy to see. If you're lucky you might also spot an otter or an anaconda.
These treats made the discomfort of getting there worthwhile. We took the overnight bus from Buenos Aires to Mercedes, the nearest town - as hick as any remote village on the edge of the pampas with its dilapidated adobe houses, scarcely paved streets, and dogs.
Next morning our driver was waiting before seven to take us the 120km of dirt road into the esteros (meaning "marsh" in Spanish) across an immense plain with herds of cattle, occasional sheep and horses, and paddocks ploughed ready for rice. The only traffic was a gaucho cantering by on a loose-reined horse.
Posada Aguape, the lodge we'd booked, was a collection of low white buildings set on a wide lawn on the shore of lake Ibera.
The birds were waiting and hardly welcoming. Two angry lapwings dive-bombed us as we crossed the lawn to our room. Ten metres from our door we saw why: they were guarding a third sitting on a low mound. It was, after all, spring. Lapwings stay in family groups and share brooding duties and the third bird was an earlier offspring.
After breakfast we were whisked away in a flat-bottomed boat, along with six Argentinian tourists. The day was glorious, the waters of the lake (Ibera means sparkling waters in the indigenous language) truly sparkling in the sun which warmed us to a comfortable 17 or 18 degrees.
The first sight to make us catch our breath was a great egret - garza blanca in Spanish, similar to a white heron but taller. Minutes later our guide Sebastian pointed, then cut the motor as we glided through reeds towards one of many small islands.
An even taller white bird, long neck crooked back on itself, yellow beak held horizontal, head and wings black - another egret? Garza mora, Sebastian announced and we found it in our guide book translated not as egret but white-necked heron.
With a long bamboo pole, Sebastian punted along the edge of the island. "Jacares," he said nudging the bow into the bank. The caiman, or alligator, was so close we could have touched it. Several more basked in the sun, ignoring our presence.
Close by, a family of fat, hairy pig-like creatures were also enjoying the sun. These were capybaras and they, too, remained insultingly oblivious to the boat-load of camera-clicking admirers.
Sebastian indicated a cormorant, or shag, in stick-like position on a tree. But this was no ordinary shag. "Aninga," Sebastian told us, "like a snake."
Through our glasses we could see its pale gold neck and as it flew off, the narrow snake-like shape of its body.
By the time we returned to the lodge for an excellent three-course vegetarian lunch we'd seen a dozen more new birds of which the most endearing was the little jacana, a swamp bird with black body and terracotta wings. It has exceptionally wide feet which enable it to walk across the foliage of the marsh. We'd seen a relative of the same name in Australia.
After a siesta we set off in search of the family of howler monkeys that frequent the bushy area around the park's information centre. When they didn't show we watched an excellent DVD on the wetland ecology.
The Esteros del Ibera reserve was created in 1983 and is the second largest wetland reserve in the world, after Pantanal in Brazil. Extending over 13,000 square kilometres and with 350 bird species and 500 vertebrates, its international significance was recognised in 2002 when it was made a Ramsar (wetlands of ecological importance) site.
Its many flat islands are floating areas of consolidated vegetation which vary in thickness from a few centimetres to more than a metre. The lake is fed by a network of rivers and by an average rainfall of 1500ml. But there is concern because this year only 300ml has fallen, lowering the level so some islands no longer float, inhibiting the natural currents and making the water brown instead of clear.
Deer live on the islands - large-eared, their legs black, so that as they browse the plants that grow in the water the flesh-eating piranha fish mistake them for branches. The herbivorous capybara, of which we saw hundreds in five days, have webbed feet and swim from island to island.
Among the most numerous birds were the Southern Screamers. Resembling large turkeys, they live in families and announce their presence by... screaming. They're monogamous and according to our guide, if one dies the other does too, within a few days.
Spring is the best time to visit (the heat in summer would be unbearable) and we saw several screamers sitting on nests - enormous piles of uncomfortable-looking sticks - their mates on guard nearby.
Each day we set off for a two-hour outing at 10am and again at 4pm. One morning I opted for a horse trek. On that trip my list of birds included white woodpeckers, a black ibis, a long-necked whistling heron, squirrel cuckoos and a lovely scythe bill, its curved beak as long as its dark body.
On another afternoon's walk through a patch of scrappy forest we found the monkeys. Weighing 50-60kg, the male was shiny black, the female and their youngster pale brown. Like the other wildlife they were totally unperturbed, even coming closer as we watched them.
The best bird-watching experience came on our last afternoon when we went out by truck with guide Naldo, a fourth generation inhabitant of the village at the Esteros, who modestly claimed to know most of the 350 species. By now we were able to name many ourselves, like the jaburu, a magnificent stork, and the unremark able hornero, Argentina's national bird whose nest is a round football made of mud with an entrance like a door opening inwards and which we saw on telegraph poles, fence posts, chimney pots and in trees.
The three hours with Naldo brought two prizes. The first was a group of pink spoonbills, or "spatulas rosadas" which let us admire them from a distance before spreading their wonderful pink wings and flying off.
The second came thanks to Naldo's persistence and acute eyesight. We walked through a strange landscape of trees, cacti and anthills where a group of rhea gave the place an eerie feeling of prehistory.
We were looking for the great horned owl and had almost given up when Naldo saw it.
Obscured by leaves, turning its head this way and that so that we saw its round brown eyes and its black beak, it looked more like a large fluffy cat than a bird.
The next afternoon was our last. We'd logged 85 birds over five memorable days, our hosts at Posada Aguape had been warm and helpful and while we waited for the driver to take us back to Mercedes we wandered beneath the trees beside the swimming pool.
There we spotted our last prize - the tiny hummingbird. The prospect of another night on the bus seemed insignificant.
Getting there: The Esteros del Ibera is 800km northeast of Buenos Aires. Fly to either Posadas or Corrientes and arrange to be driven about six hours in to the reserve by 4x4 vehicle. Or take the overnight bus from Buenos Aires to Mercedes and drive two hours on a dirt road.
Where to stay: Posada Aguape is one of several lodges that also arrange transport from the nearest towns. Cost for meals and excursions for two people for five days was $1500.
Further information: The website for information on the reserve is iberaesteros.com.ar.