Streaking is strongest on female Red-winged Blackbird
Bill shape separates cowbird from the rest
Flock and habitat context can finish the job
Sources and references
These references support the bird-identification logic used in this guide and are useful for cross-checking field marks.
The 4 Keys to Bird Identification (Official, Cornell Lab of Ornithology) - Foundational bird-ID framework centered on size and shape, color pattern, behavior, and habitat.
Common Grackle Identification, All About Birds (Official, Cornell Lab of Ornithology) - Official Cornell Lab identification page used for structure, color-pattern, and behavior checks.