Study in Italy Reports that Bilingual Infants can…

When infants are given the opportunity to simultaneously learn two different regularities of language, bilingual infants learned both, while monolinguals learned only one of them. Hence, bilinguals may acquire two languages in the time in which monolinguals acquire one because they quickly become more flexible learners.

Also summarized in a Spanish newspaper:

Los niños bilingües son más flexibles en el aprendizaje
Un estudio indica que los bebés de doce meses 'bilingües' son, petencialmente, aprendices más flexibles que los bebés monolingües, cuando se enfrentan a estímulos lingüísticos.