The most obvious choice within range is a young Great Pampas-finch, which is streaked above, has a similar head pattern and a variably dark bill, with pale at the base. The only thing against this is that juveniles have a streaked breast, but perhaps this bird has replaced those feathers?
Macaulay Library ML556565671; © Hugo Valderrey; Maracó, La Pampa, Argentina
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Macaulay Library ML583685321; © Jodi Boe; Ituzaingó, Corrientes, Argentina
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Macaulay Library ML578797041; © Martín Toledo; Rió Segundo, Córdoba, Argentina
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Macaulay Library ML567899101; © Eugenia Boggiano; Pocho, Córdoba, Argentina
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Brian