This is the key sentence:
Despite a robust increase in eagle abundances in the northeast United States over that period, we estimated that deaths arising from ingestion of Pb was associated with a 4.2% (females) and 6.3% (males) reduction in the asymptotic long-term growth rate (lambda).
They had a “robust” increase in population but we feel like there should have been more, so we need you to outlaw lead bullets.
Good catch!