FAUNA PARAGUAY BIRD MIGRANTS IN PARAGUAY

The movement of birds through the interior of South America is an understudied and poorly known phenomenon. With its location in the dead centre of the continent, the situation in Paraguay is particularly complex, with two broad classes of migrant, Nearctic migrants (those that migrate from northern latitudes to spend the austral “summer” in South America) and Austral migrants (those that migrate within the continent of South America). This situation is further complicated in that some Austral migrants breed in Paraguay, while others do not, and some resident species have their populations augmented by movement of birds from other countries, either from the north or the south. A brief and necessarily simplified explanation of the basic mechanics of migration through Paraguay is given below.

Nearctic Migrants
Nearctic migrants breed in the northern hemisphere and are likely to be very familiar to North American birders. The vast majority of Nearctic Migrants (NM) are waders that visit Paraguay on passage to coastal wintering grounds in Argentina and Chile. Small numbers of some Nearctic Migrant waders are present all year, so it is not unusual to see groups of waders (especially first year birds) at times when they really should be elsewhere. Observers have noticed that migrant wader numbers during the southbound migration (late Aug to early Dec) are considerably higher than during the northbound migration (late Jan to early April) and have speculated that this discrepancy is related to changes in the hydrological cycle. High rainfall during the Austral summer (late Nov to early Mar) causes extensive flooding of wetlands during this period, considerably decreasing the suitability of habitats for waders. However the drier weather that follows and continues through the Austral winter creates a more conducive mosaic of sandbars, lagoons and mudflats in time for the southbound migration, hence the higher wader counts. This phenomenon holds true even during dry years when considerable suitable habitat remains available during the time of the northbound migration, suggesting that it is an established pattern. It has been hypothesised that northbound birds follow the Atlantic coast of South America to their breeding grounds, or perhaps fly non-stop overland to wetlands in Venezuela and northern South America, where the opposite pattern seems to occur.

Austral Migrants
The vast majority of Austral Migrants are passerines, and the pattern of Austral migration is rather more complex. True Austral migrants (AM) are those that breed in the southern cone of South America during the Austral summer, and spend the Austral winter enjoying the rather milder Paraguayan climate or passing through on route to wintering grounds in the tropics. However there exist two other classes of Austral migrant which complicate the issue. Breeding Northern Migrants (BN) which breed in Paraguay during the Austral summer but move north to the tropics during the Austral winter (at which time they are far less common or even absent in Paraguay) and return the following spring, and Breeding Southern Migrants (BS) which breed in Paraguay during the Austral summer, but have their numbers considerably augmented by an influx of birds from further south for the Austral winter. However not all species fit neatly into these categories. Some species can be classed both as Breeding Northern and Breeding Southern Migrants, and in the case of the Peregrine Falcon Falco peregrinus as both a Nearctic and an Austral migrant depending on the subspecific origin of the bird.