Townsend's Warbler gleaning insect larvae from leaf

Here is a Townsend's Warbler appearing to glean an insect larvae from a cottonwood tree leaf. It looks like this infestation of insect larvae (or maybe pupae?) have provided a veritable feast for at least those warbler species for which gleaning this larvae is compatible with their food habits. (most of the Wilson's and Orange-crowned Warblers in this park were mostly foraging low to the ground in shrubs) A check of Birds of North America online shows Townsend's Warbler is known for gleaning insects. It also states that the Yellow-rumped Warblers that were in abundance "Primarily gleans insects from leaves and branches." Researching this I found that leafminers are insects that feed inside of leaves and that they produce large 'blotches' that are irregularly round. I found one leafminer, the Phyllonorycter apparella, that "feeds in an underside mine on cottonwood" and other willow family trees causing a yellowish blotch (called a 'mine'). Maybe this is the insect larvae in the cottonwood tree leaves that is a wonderful source of food for migrating warblers or one like it. SeEtta