While there are plenty of invasive earthworms in North America, primarily in the northeast and areas that have been relatively recently lost their glaciers, about 2/3rds of the earthworms in North America are natives and could have fed American Robins before transatlantic shipping became common:

"Earthworms are shifting their ranges northwards into forests between 45°<https://en.wikipedia.org/wiki/45th_parallel_north> and 69° latitude<https://en.wikipedia.org/wiki/69th_parallel_north> in North America that have lacked native earthworms since the last ice age<https://en.wikipedia.org/wiki/Last_glacial_period>.[4]<https://en.wikipedia.org/wiki/Invasive_earthworms_of_North_America#cite_note-Frelich-4> The worms in question are primary engineers of their environment. They are considered keystone species<https://en.wikipedia.org/wiki/Keystone_species> because, as detritivores<https://en.wikipedia.org/wiki/Detritivore>, they alter many different variables of their ecosystem.[4]<https://en.wikipedia.org/wiki/Invasive_earthworms_of_North_America#cite_note-Frelich-4> Of the 182 taxa of earthworms found in the United States and Canada, 60 (33%) are introduced species.[5]<https://en.wikipedia.org/wiki/Invasive_earthworms_of_North_America#cite_note-Blakemore-5> "

From: Invasive earthworms of North America - Wikipedia<https://en.wikipedia.org/wiki/Invasive_earthworms_of_North_America>