This ecological dimension was explored further by Handford and students in the highly diverse habitats of northwestern Argentina.  They showed that the ecological ordering of dialect variation  [2,3,4, 6] over a huge geographical space (1200 x 350 km) and across a dramatic sweep of structurally-distinct habitats (puna scrub, grassland, desert scrub, thorn woodland, and drought-deciduous forest (see Figure) was largely consistent with the previously-established picture.  This work also demonstrated that these spatial patterns show temporal stability of at least 20 years (now known to exceed 30 years), and stability on the order of centuries is implied by the persistence of certain habitat dialects long after the native vegetation has been removed by agriculture [3].  This massive demonstration of acoustically rational habitat-based song variation strongly supports what is now known as the Acoustic Adaptation Hypothesis [10].  However, the work also provided a basis for a final evaluation of the GAH on a similar geographical scale [5].  This study showed that the substantial genetic variation shown by the species is organised largely by distance;  dialect songs impose no further structure:  it seems that for this species the GAH has no explanatory value.

The most recent work on this species confirms that the clear ecological segregation of acoustically rational vocal dialects in Argentina extends from 22ºS at the Bolivian border south to 42ºS in northern Patagonia.  Across this vast space, the greatest song diversity is concentrated in the vegetationally-diverse north west;  in the ecologically more uniform central and southern regions, great song uniformity is encountered;  finally, island habitats, such as montane grasslands, are represented by repeated islands of the specific song dialect. Other recent work suggests, however, that tropical population (Ecuador) do not show this pattern:  instead, individuals show repertoires (from 1-7 trill-types; mean = ~4) and local populations can show nearly as much trill variation as is known from all Argentina.