Yasuni National Park is arguably the most biodiverse place in the world, although its approximately 10,000 square kilometers of humid forest are severely threatened by oil production, illegal logging, and colonization. While avian diversity here is spectacular with close to 600 species present in the park, tree, amphibian, and bat diversity is even more impressive, especially on a small scale, where in just one hectare of forest there are more tree, shrub, and liana species than anywhere else in the world. Interestingly enough, the park is also home to several indigenous groups, namely the Huaorani, including small groups of uncontacted people. Visiting the edges of the park is relatively easy from a base at one of the many jungle lodges along the lower Napo River; exploring the park at length is considerably more complicated as access is limited to a handful of research stations and villages located within the park boundaries.
Given its size, the Napo River functions as a barrier for some birds, with certain species only found to the north and others only to the south of the huge river, such as the Chestnut-Belted and Ash-Throated Gnateaters. In addition, much of the forest along the southern side of the Napo is terra firma, while the forest on the northern side where most of the lodges are located is varzea forest, or forest that is seasonally flooded. This creates an even stronger impression of there being different avifauna on either side of the river. Although I had visited the southern side of the river several times, birding excellent terra firma forest for several days, most of the birds I was hoping to encounter on my trip to the eastern lowlands were more likely to be found here within the park boundaries, especially along the trails behind the park's famous parrot and parakeet clay licks. Both Sacha and Sani Lodges include full-day excursions to the park in their birding programs, so on four separate days my guides and I took a motorized canoe and box lunches to see what we could find. 
On our first day here, Oscar and I briefly stopped at one of the river's many islands on our way to the trails behind the parakeet clay lick. Here we found many island specialists that I was still missing from my country list, including the Lesser Wagtail Tyrant, Fuscous Flycatcher, and White-Bellied Spinetail. We also encountered a Small-Billed Elaenia, an uncommon austral migrant, in the tall grasses along the edge of the island. In the park itself, it was disappointingly quiet, and it took us less than ten minutes to walk a stretch of trail that had taken us two hours to walk several years ago on a previous visit as there had been so much bird activity. Again, there was no response from either the Great Jacamar, Collared Puffbird, Rufous-Capped Antthrush, or Striped Manakin at their many territories along the trails, and we didn't find any army antswarms either. Nevertheless, we pushed on deeper into the park searching for the Ochre-Striped Antpitta in the hilly terrain, finding the Yellow-Billed Jacamar, White-Eyed Tody-Tyrant, Banded Antbird, and Black-Bellied Cuckoo along the way. Perhaps the best bird seen on the day was a group of Gray-Winged Trumpeters that were headed towards us along the trail, the iridescent feathers on the chest of one individual clearly visible as it paused in front of us before dashing into the forest on its powerful legs.
Our itinerary was more or less the same on our second day in the park, and we stopped again at the same river island, this time to connect with the diminutive Gray-Breasted Crake, which we successfully spotted in dense ground cover. Along the same trails in the park, we first located a silent Brown Nunlet waiting motionless for its prey to pass by. Then, we had nice views in the scope of another birding group that had lined up a calling Sapphire Quail-Dove. The thrill of the morning, though, was certainly discovering a huge group of army ants, forming a swarm that wasn't well-attended by birds but was definitely exciting. Several scorpions were chased out of their cover in the roots of a tree as ants attacked them, and a variety of unusual birds came by to capture insects fleeing from the swarm, including the Yellow-Billed Nunbird, Wire-Tailed Manakin, and Rufous-Tailed Foliage-Gleaner. We also noted several antbirds in the area, although they were very wary of our presence, including the Sooty, Bicolored, and White-Plumed Antbirds. Although this wasn't a classic antswarm, we were heartened by our luck in at least finding ants, and perhaps our positive attitude yielded one final good bird before the rain started for the day, a pair of massive Great Jacamars responding nicely to playback.
On our third day in the park, Oscar and I headed to a different network of trails, located significantly upriver from the clay licks and called Providencia. We found a few good birds along the way as we wound through flooded forest in our motorized canoe, including the Olive-Faced Flatbill and Cinnamon-Throated Woodcreeper. Once on the trails, bird activity seemed low yet again, until a Rufous-Capped Antthrush finally responded to Oscar's repeated trolling. Calling aggressively as it perched in the open at several locations around us, the bird yielded superb looks and even some photographs, although a digiscope would have captured the moment better. Buoyed by this sighting, we then encountered another antswarm, getting exquisite views of the White-Plumed Antbird as it surveyed the scene intently. Amazingly, a male Striped Manakin was heard calling nearby, which we located with a bit of effort and watched feeding on berries in the understory as it hopped up from below. Now that we had established some momentum, the birds seemed to just keep coming, and I picked up several new antwrens in quick succession with Oscar's help, Dugand's and Long-Winged Antwrens.
Our attention was then drawn to a pair of calling birds in some dense ground cover that turned about to be the impressive Undulated Antshrike, a massive and beautifully patterned bird. Both the male and female were seen out in the open as they pumped their tails and called in irritation while responding to Oscar's deft use of his recording equipment. This encounter was certainly one of the big surprises of the trip, as the antshrike is described as being rare and local in the field guide. Next, while tracking down a group of male Blue-Backed Manakins, we heard an Ochre-Striped Antpitta calling in the distance. This large and strikingly colored antpitta was the only antpitta species I had yet to see or hear in Ecuador (I have now seen them all except for the Bicolored Antpitta, which was heard-only along the La Bonita Road), and so we set off to find it immediately after hearing its call. Oscar and I had discussed the challenges of actually seeing this bird at great length already, as it runs along the forest floor without pause making it extremely difficult to spot. As it turns out, a pair of birds were calling and responding well to Oscar's recordings, indeed covering a lot of ground between calls. We set up low on a hill where the undergrowth wasn't too dense and waited for the antpitta to run by, which it did, pausing uncharacteristically on a fallen tree trunk to look around. Then, it absconded into dense ground cover, where I amazingly found it after scanning for a few minutes through my binoculars. I watched it call for a while, admiring its rich orange chest and black-striped belly, while Oscar marveled over a group of White-Bellied Spider Monkeys that were causing a commotion in the canopy above us.
Buzzing from this much sought-after encounter, I had little time to reflect as Oscar directed my attention towards a pair of Lunulated Antbirds that were calling nearby. These antbirds are rarely found away from antswarms but responded aggressively to playback, and I had great looks at a male calling at close range. At this point it was well past midday, and I was in danger of missing my ride to Sani lodge, so we hustled back to the river along the trail that had yielded so many terrific birds in just a few hours. Indeed, it had been one of my best mornings ever birding in Ecuador and easily made up for an entire week of relatively low bird activity. While there had been no sign of the Hairy-Crested Antbird, Collared Puffbird, or Reddish-Winged Bare-Eye, we had witnessed many of the avian treasures of Yasuni National Park, where I would still spend yet another day with Domingo, my guide at Sani Lodge.Notable birds seen: Yellow-Headed Caracara, Black Caracara, Spix's Guan, Gray-Winged Trumpeter, Gray-Breasted Crake, Rufous-Sided Crake, Collared Plover, Sapphire Quail-Dove, Blue-and-Yellow Macaw, Scarlet-Shouldered Parrotlet, Black-Bellied Cuckoo, Ladder-Tailed Nightjar, Great-Billed Hermit, Straight-Billed Hermit, Gould's Jewelfront, Yellow-Billed Jacamar, Great Jacamar, Brown Nunlet, Yellow-Billed Nunbird, Golden-Collared Toucanet, Cinnamon-Throated Woodcreeper, Rufous-Tailed Foliage-Gleaner, Black-Tailed Leaftosser, Undulated Antshrike, Plain-Throated Antwren, Ornate Antwren, Long-Winged Antwren, Dugand's Antwren, Banded Antbird, Spot-Winged Antbird, Warbling Antbird, Sooty Antbird, White-Plumed Antbird, Lunulated Antbird, Bicolored Antbird, Scale-Backed Antbird, Rufous-Capped Antthrush, Ochre-Striped Antpitta, Ash-Throated Gnateater, Golden-Headed Manakin, Blue-Backed Manakin, Striped Manakin, Wire-Tailed Manakin, Amazonian Umbrellabird, Small-Billed Elaenia, Lesser Wagtail-Tyrant, White-Eyed Tody-Tyrant, Olive-Faced Flatbill, Fuscous Flycatcher, Drab Water-Tyrant, Musician Wren, Long-Billed Gnatwren, Slate-Colored Grosbeak.