An Overview of Austral Migration
"Migration is less pronounced among land birds of South America
than among those of North America, but the small amount of recent
information indicates that more southern birds migrate than was
thought."
-
E.O. Willis (1988)
Fuscous Flycatcher (Cnemotriccus fuscatus) is a common austral
migrant in eastern Bolivia (click
for larger picture).
The following is a brief overview of South American austral migration,
illustrating that although it is a major migratory system, it has largely
been ignored.
Basic information on the migration of virtually all austral migrant
species is extremely poor. Information that does exist is taxa - or locality-specific.
Chesser (1997), for example, provided details on breeding and wintering
ranges of migrant flycatchers. It is not exaggerating to state that basic
topics such as diet, habitat use, and migratory routes are poorly known
for effectively all austral migrants, regardless of family.
Austral Migration: An Overview
Austral migrants are birds which breed in the temperate latitudes of South America and move northwards towards the Amazon basin for the austral winter.
Although different estimates exist on the actual number of austral migrant species (for example, Stotz et al. (1996) estimate 237 species, and Chesser (1994) estimates at least 220), there is a general consensus on the approximate number: between 220-240. Estimates will change with increasing research on these birds. In contrast, the Nearctic-Neotropical system is comprised of approximately 420 species ( Stotz et al. 1996).
It has been generally assumed that austral migrants originated in the tropics of South America, then migrated to the temperate latitudes of the continent to breed, taking advantage of the seasonally abundant resources at those latitudes. Joseph (1997) advances the idea of tropical origins for austral migrants which breed in temperate latitudes and winter as far north as the tropics or subtropics, though he gives no solid reasons for this belief. Chesser and Levey (1998) conclude that austral migrants originated from Neotropical residents having a tendency to occupy edge, canopy or open habitats.
Most austral migrants do not winter as far as the tropics - only 32 species reach Amazonia and 14 more winter north of the Amazon basin. In over two thirds of migrant species, birds on winter ranges overlap with the ranges of conspecific residents (Stotz et al. 1996). There are a couple possible explanations for this pattern.
First, there are no apparent barriers to austral migration on the continent. The Andes mountain chain runs North-South, and there are no large bodies of water or other apparent physical barriers to separate breeding and wintering ranges. Because South America narrows towards the south, there exists a low ratio of breeding to wintering habitat (Chesser 1994). For this reason, as well as a lack of geographic barriers, there is less of a segregation between wintering and breeding grounds as opposed other migratory systems (Nearctic-Neotropical, Palearctic-Paleotropical). Competitive effects in winter may therefore lead to winter range segregation between similar species, forcing some to continue further northwards towards Amazonia (Chesser 1994). Additionally, Hayes et al. (1994) cited the possibility that a smaller land area at higher latitudes may provide “less potential for hosting migrant species”, so that those which do migrate to temperate latitudes do so for only a relatively short distance from tropical latitudes.
Useful in determining the reasons behind patterns observed in the austral migratory system is a comparison of it to the better-understood Nearctic-Neotropical system. Both are widely thought to have originated in the Neotropics, although Cox (1985) proposed that the bulk of Nearctic-Neotropical migratory species, mainly Parulids, originated in the seasonally wet-dry monsoonal Mexican plateau of northern Mexico and southern Arizona.
As mentioned previously, a large proportion of Austral migrants exhibit overlapping breeding and wintering ranges. North American breeding migrants, however have the Caribbean Sea to cross, and as a consequence over half have completely disjunct breeding and winter ranges, while only 7% of austral migrants have disjunct ranges between seasons (Stotz et al. 1996). The taxonomic makeup of each system differs considerably. About a third of Austral migrants are Tyrannids (33.2%), with the Emberizinae (9.6%) and Anatidae (7.4%) the next most common taxa - no other major migratory system in the world is as dominated by one family as the austral system (Chesser 1994). In contrast, the most common subfamily in the Nearctic-Neotropical system, the Parulinae, is virtually nonexistent in the austral system.
Stotz et al. (1996) found no tendency for migration within families in one system as opposed to the other. They point out, however, that birds which depend on open water (ie. Anatidae and Scolopacidae) and those which forage on active insects (ie. Tyrannidae, Vireonidae) are almost completely migratory in both systems, illustrating a common ecological thread. Chesser (1994) concurs with this observation in a comparison between the austral, Nearctic-Neotropical and Palearctic-African migratory systems, showing parallelism between the systems in the tendency of the same families to migrate. For example, he shows the Hirundinidae, Cuculidae, Anatidae and Laridae and Charadriidae to be highly migratory in all three systems. Additionally, he shows the Tyrannidae to be common migrants in the Nearctic-Neotropical and austral systems. He cites the related foraging ecologies within families as the reason behind their similar migratory tendencies.
Chesser (1994) examines the foraging behavior in the austral migrant flycatchers, concluding that perch-gleaning is the most common foraging technique among migrants in this family (24% employ it). He points out that it is also the predominant foraging mode of the Parulids of the Nearctic-Neotropical system, showing possible convergence between the systems. Overall, he finds no one predominant foraging technique among austral migrant flycatchers, aerial hawking being used by 18%, ground-foraging used by 14% and fruit/hover-glean used by 14%.
A final comparison between the migratory systems, that of habitat use, highlights similarities in winter habitat use but not in breeding habitat. Stotz et al. (1996) conclude that both austral and Nearctic-Neotropical migrants have relatively low habitat specificity on their winter ranges, finding that only 25% of austral migrants use humid forest in winter, although they caution that very little is known of wintering habitat use among these birds. Some species are known to have specific habitat requirements in the winter. More of a difference in breeding habitat use seems to exist between the systems. Approximately 65% of Nearctic-Neotropical migrants breed in forest and woodland while a similar proportion of austral migrants breed in open or scrubby habitat (Chesser 1994).
When breeding and wintering distributions of austral migrants are looked at, a strong latitudinal gradient on the breeding grounds for a tendency to migrate is evident in temperate South America. Fifty percent of breeding species in Tierra del Fuego of southern Argentina migrate (Humphrey et al. 1970) as opposed to only 10% at the Tropic of Capricorn in Sao Paulo, Brazil (Stotz, pers. obs).
Chesser (1998), considering only austral migrant flycatchers, found results consistent to the hypothesis that tendency to migrate at any given location is dependent on resource abundance during the breeding season and severity of the winter season. He found a 100% tendency for flycatchers in southernmost temperate South America to migrate and a figure of 20% in the temperate latitudes of southeastern Brazil.
While most austral migrants are known to breed in open, scrubby habitats (Chesser 1994), winter range distributions and habitat use of austral migrants are poorly known (Stotz et al. 1996, Joseph 1996). Attempting to elucidate patterns of winter distribution among these migrants, Joseph (1996) looked at winter range patterns between species as related to temperature. He found that austral migrant passerines basically divide into two groups in the winter: one in warm humid areas with daily mean temperatures over 20 degrees Celcius, and another with winter ranges in an area with daily mean temperatures of less than 20 degrees Celcius. He sees a possibility to learn about austral migrant patterns by generating bioclimatic models generated from statistical distributions of bioclimatic variables which would be measured throughout known species distributions.
Very little information exists on habitat use during migration for these birds. This would depend on the migratory flight strategies (i.e., short-hop vs. long haul) these migrant species employ, a virtually unstudied field as well. If they employ a strategy which would take them from breeding or wintering grounds via one long flight with few or no stops, habitat use during migration may be insignificant. Conversely, different strategies may be employed by different populations, such as observed in North America where many species employ “leapfrog” migration (Greenberg 1980). In this strategy, austral migrant populations breeding further south may winter further north than more northerly breeding birds of the same species. Working in North America, Rappole and Warner (1976) hypothesize that, within species employing this strategy, populations migrating a shorter distance may not be as dependent on fat reserves during migration as the populations migrating longer distances. Whether this applies to any austral migrant populations has yet to be known. Until this information becomes available, conservation action for austral migrants during migration will be less than efficient.
Literature Cited
A list of literature cited in this website on austral migration. Additionally, there are included citations on Neotropical biology and ecology in general.