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 facilitates the ability of the starch to be melt processed. Due to its relatively low melting point, the copolyester may also be extruded with the thermoplastic starch at a temperature low enough to avoid substantial removal of the moisture in the starch. Furthermore, the copolyester is also modified with an alcohol to contain 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 relatively low molecular weight. Such low molecular weight polymers have the combination of a higher melt flow index and lower apparent viscosity, which is useful in a variety of fiber forming applications, such as meltblowing nonwoven webs.