A fiber formed from a thermoplastic composition that contains a thermoplastic starch and an aliphatic-aromatic copolyester is provided. The copolyester enhances the strength of the starch-containing fibers and also facilitates the ability of the starch to be melt processed. Due to its relatively low melting point, the aliphatic-aromatic copolyester may also be extruded with the thermoplastic starch at a temperature that is low enough to avoid substantial removal of the moisture found in the starch. Furthermore, the aliphatic-aromatic copolyester is also modified with an alcohol so that it contains one or more hydroxyalkyl or alkyl terminal groups. By selectively controlling the conditions of the alcoholysis reaction (e.g., alcohol and copolymer concentrations, temperature, etc.), the resulting modified aliphatic-aromatic copolyester may have a molecular weight that is relatively low. Such low molecular weight polymers have the combination of a higher melt flow index and lower apparent viscosity, which is useful in a wide variety of fiber forming applications, such as in the meltblowing of nonwoven webs.