The Reserva Natural del Bosque Mbaracayú is 64,406 hectares of Upper Paraná Atlantic Forest and cerrado in Canindeyú, and it belongs to the Fundación Moisés Bertoni, which has managed it in perpetuity since Ley 112/91 made it Paraguay's first private reserve in 1991. It is, by the foundation's own claim and most other people's, the best-managed protected area in the country.
The numbers are the reason birders come: 440 bird species, 62 of them endemic to the Upper Paraná Atlantic Forest, plus 93 mammals, 1,269 plants and 64 fish, and more threatened species than any comparable area in Paraguay. Nineteen natural communities have been mapped inside the boundary, from tall forest through campos cerrados and marshes to caves and river headwaters, all of it in the Jejuí catchment.
Everything at Mbaracayú goes through the foundation — the guided walks, the guiding, the field station beds. There is one phone number and one WhatsApp, both answered from the Asunción office on Avenida Santísimo Sacramento, and no booking engine, no published rates and no email address on the site. Write to the WhatsApp; that is what it is there for.
A caveat worth passing on: their website is visibly mid-rebuild — page-editor artefacts are showing in the markup, several social links are unfilled placeholders, and the reserve page describes conservation rather than how to visit. The contact details are current; the visitor information is not there yet.