Are pellets or seed mixes better for pet birds?

Across r/parrots and r/budgies, the dominant view is that formulated pellets should make up the bulk of a pet bird's diet, with seeds treated as a supplement or treat. Multiple specialist-community threads flag seed-heavy mixes (including popular Kaytee Fiesta blends) as too high in fat for long-term daily feeding, while pellets like Harrison's, TOP's, and ZuPreem are repeatedly recommended by users citing avian-vet guidance.