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496 Short Communications and Commentaries [Auk, Vol. 111 Remsen et al. (1991) recently named it as Circum- classification employed by Nores, thus violating one Amazonian. important assumption of his hypothesis. However, I Therefore, some disjunctions pointed out by Nores regard this as only a minor problem. If one excludes exist only in his rather limited study area in Argen- the problematic species from the analysis, both distina, but were not evident when the Chaco region tribution patterns discussed by Nores appear to be and Yungas forests were studied as a whole. Besides, supported by other taxa. one of his disjunct species does not occur in the Par- Secondary contact zones for birds in central Chaco.-anaense region at all (Dendrocolaptes picumnus; see Short Nores (1992) indicated that the region of the Pilco- 1975, Pinto 1978). In another case (Picumnus cirratus), mayo river should be considered as a secondary conthe two subspecies (thamnophiloides and temmincki), tact zone for his nonforest birds. This proposal conpointed out by Nores as disjunct, are separated by trasts with that of Short (1975), who suggested that another subspecies inhabiting the Chaco region (pil- this region should be considered mainly as a zone of comayensis). Short (1982) has acknowledged that this primary intergradation (Short 1975:fig. 82) rather than latter species needs revision, but Nores did not pre- as a secondary contact zone (Short 1975:fig. 81). It has sent any evidence to modify this taxonomic arrange- been suggested that without detailed knowledge of ment. In fact, he included Short (1982) as one of his the evolutionary histories of populations involved it basic taxonomic references (Nores 1992:table 1). The is difficult, if not impossible, to decide whether these remaining 30 species appear to represent genuine dis- zones are results of primary or secondary contact (Enjunctions between the Yungas and Paranaense regions. dler 1977, 1982, Barton and Hewitt 1985). However, Similar critical remarks apply to Nores' nonforest some suggestions for explicit tests have been probirds. If the secondary contact zones between them posed (e.g. Thorpe 1984, 1987). Since Nores' hypothare a result of expansion and retraction of the gallery esis is more recent, he should have presented much forests along the Pilcomayo and Bermejo rivers, one more evidence to support it as well as falsify Short's could expect that this type of habitat would still be hypothesis (e.g. by using more accurate maps to show an important barrier to these nonforest species today. the distribution of the different populations in the Nores used 10 nonforest species to support his hy- area [e.g. see Haffer 1974, Ford 1986] or by analyzing pothesis. However, as he noted, some of these (e.g. the geographical variation of some species [e.g. Haffer Thamnophilus caerulescens and Phacellodomus rufifrons) and Fitzpatrick 1985]). Instead, he used 10 rough maps, also inhabit certain forest habitats. This information 8 of which were based on maps published by Short is upported by my personal experience with these (1975), and did not present new detailed information. species in the Cerrado and Caatinga. These morpho- Consequently, we have two hypotheses, little declimatic domains form, with the Chaco, the "open tailed information available, and no critical tests. vegetation diagonal" instead of the "arid diagonal" When Nores (1992:354) proposed that "The distrias incorrectly quoted by Nores (1992; for exact defi- bution patterns of nonforest birds ... also are connition, see Ab'Saber 1977). The Cerrado is not an "arid" sistent with the former existence of a forest belt along region. the Bermejo and Pilcomayo rivers," he implicitly as- Thamnophilus caerulescens is mainly found in gallery sumed that the present-day location of this contact or other types of evergreen and semideciduous forests zone is in the same position as the barrier that sep- (the same pattern was also found in southern Bolivia arated the populations in the past. This likely is a [J. Fjelds pers. comm.] and in the Paranaense forests false assumption because there is no evidence that IF. C. Straube in litt.]), while P. rufifrons is found pri- the courses of the Bermejo and Pilcomayo rivers have marily in semideciduous and deciduous forests, but always been the same as today (see below). Thus, I also in gallery forests. Two more of Nores' nonforest suggest that only with more detailed data and robust species should be able to live in forest habitats. For tests would it be possible to evaluate the Nores' hy- Campylorhamphus trochilirostris, one of the subspecies pothesis that there is a secondary contact zone for listed by Nores (1992) (lafresnayanus) inhabits gallery nonforest birds in the central Chaco. and dry forests in southeastern Mato Grosso (pers. Questionable assumptions.--Since the reality of secobs.). Also, Pseudoseisura cristata inhabits both Caatin- ondary contact zones still needs better support than ga woodland and high deciduous forest in north- that offered by Nores, only one genuine pattern of eastern Brazil. This latter species is able to cross wide bird distribution is suitable for biogeographical intracts of gallery forests, through the canopy, without terpretation in Nores' study area: the disjunct distriproblems (pers. obs.). I think that these four species bution of the 30 taxa of humid forest birds between cannot be used as evidence for Nores' hypothesis, the Yungas and Paranaense forest regions. Nores prosince their ranges would not necessarily be inter- posed that the Yungas and Paranaense forests were rupted by a continuous gallery forest along the Pil- connected many times in the past due to expansion comayo and Bermejo rivers. The remaining six species of gallery forests, thus allowing the dispersion of the can be regarded as nonforest (open vegetation) spe- forest bird species. cies. To interpret the different speciation levels shown In short, at least some species do not fit the habitat by these taxa, Nores (1992:353-354) argued: "species
April 1994] Short Communications and Commentaries 497 that crossed during the last connection would not yet Bermejo rivers is closely connected with the stability have had time to differentiate. Another group of spe- of the river course. However, the current drainages cies that show differences at subspecies level presum- of these rivers are so unstable that according to Adaably crossed during an earlier connection .... A third moli et al. (1990): "a river may abandon its present group could have crossed even earlier and differen- course and start to flow an ancient bed, all within a tiated to megasubspecies level. Finally, pairs of al- period measured in days." lopatric species probably represent differentiation that Because of these remarkable dynamics, there are began very early .... " intense changes in composition and structure of gal- In this quite simple hypothesis, there are three ba- lery forests that can go forwards and backwards, in sic assumptions. The first is that the time between terms of centuries, from the xerophytic forests with two cycles of humid-gallery-forest expansion was suf- Chaco elements to humid forests with the composificient to permit the development of reproductive tion dominated by foreign elements (Sennhauser isolation and competitive superiority so that upon a 1991). It follows that the humid gallery forests in the new meeting, the different populations would serve Chaco region can be considered as too unstable, at as self barriers to the expansion of the other. This least in the time scale required by Nores' hypothesis, assumption is necessary to ensure that the two other to function as biogeographical corridors such as those possible results of the contraction-expansion of pop- currently found in the Cerrado region (Redford and ulations (i.e. fusion or sympatry) would not take place. Fonseca 1986). The second assumption is that the amount of differ- In fact, this conjecture is well supported by the entiation is proportional to the duration of the dis- comparison between gallery forests of these two junction and, consequently, that the rate of change is regions made by Adamoli et al. (1990): "Gallery forequal for all characters and taxa (see discussion about ests in the Chaco region grow mainly on river leveethis subject in Cracraft 1985). The third assumption banks, that is, on top of a positive relief structure with is that different taxa showing this distribution pattern respect to the surrounding flat land .... In the Cerhave different propensities for dispersal, since once rados region, in contrast, gallery forests grow at botthe connection existed some species allegedly dis- tom-of-the valley position, benefiting from the adpersed while others waited for another opportunity. ditional water supplied by the higher water table .... While it is possible to find some evidence for the Consequently, Cerrados and all gallery forests growfirst assumption, the second and third have no em- ing at the bottom of valleys are more stable in time pirical or theoretical justification. Thus, Nores' ar- than those growing on elevated river levee-banks." gumentation can be regarded as insufficient. Besides Under these conditions, it is expected that a system these logical flaws, Notes' hypothesis is also a two- of wide and continuous gallery forests such as that taxon statement and, as such, it is not particularly proposed by Nores would be almost inconceivable, informative either from the standpoint of systematics even in more humid conditions. In fact, if during an or biogeography (Cracraft 1985). interglacial period (such as the one we are now in) Paleoecological background.--Nores' hypothesis rests the Chaco region was more humid than it is today on two main paleoecological assumptions: (a) that the (because of its poor-drainage system and flat topogcourses of the Bermejo and Pilcomayo were constant raphy), there would be a trend to great and perhaps during all of the Quaternary; (b) that the gallery for- disastrous floods of rivers. These floods would be even ests along these rivers were stable (at least during the more severe if we also consider the melting of the interglacial periods) and, thus, could function as fau- Andean glaciers and raising of sea level. Thus, the nistic corridors between the Yungas and Paranaense region could be somewhat like a hyperseasonal saforests. vanna, dominated by grasses, palms and perhaps with There is good evidence that the river drainage in mosaics of unstable patches of humid and dry gallery the Chaco region was completely different from the forests in some points of rivers. Ramella and Spipresent one (see review in Ramella and Spichiger chiger (1989) suggested that such an environment, 1989). In the case of the Pilcomayo River, it only found currently only in the wet Chaco region, is a reached its present-day course after the deposition of relict of a more widespread situation. sediments from the Andes in the western Chaco re- Nores also suggested that the presence of relict forgion. This event has to be recent, but as Sennhauser est patches in dry riverbeds and the upper parts of (1991) commented: "These rivers (including the Pil- the channels of the Bermejo and Pilcomayo rivers comayo and Bermejo rivers) have not yet reached their could support his hypothesis, since such forest patchequilibrium profile, and consequently the fluvio-dy- es would suggest the ancient extension of the humid namic processes are still at work." gallery forests. I examined the list of plant species If one assumes that all events proposed by Nores presented by him and found that many species listed took place after these rivers have reached their pres- occur also in different types of dry forests (for lists, ent-day courses, there still are difficulties. Sennhauser see Ratter et al. 1978, 1989, Ramella and Spichiger (1991) has pointed out that, currently, the persistence 1989). These still poorly known forests are currently of humid gallery forests along the Pilcomayo and distributed as islands of variable size in northeastern
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