Jody Victor: Over the last two decades, the population of Bald Eagles in the Great Lakes region has been on the rise. Productivity trends, closely monitored since the early 1980s, indicate that reproductive rates are sufficient for population growth. The eagle population's overall health is improving.
Studies demonstrate that eaglets hatched today carry a less significant burden of toxic chemical concentrations than 25 years ago. In 2000, Bald Eagle pairs were able to fledge their young at a 90 per cent success rate. Still, with all of this good news, an unresolved question remains: Is the Great Lakes Bald Eagle population truly self-sustaining?
Significant problems affecting the health of local aquatic ecosystems may interfere with the long-term success of the eagles. Concerns that are not yet alleviated include: the progressive loss of suitable habitat for nesting and feeding; high turnover rates of breeding birds; the potential damage from long-term exposure to persistent organic contaminants and possible harmful effects of exposure to heavy metals.
So, what is causing the high turnover of breeding birds? Bald Eagle pairs generally return to the same breeding territory, and often the same nest, every year. Recently, turnover rates were calculated for eight nests in southwestern Ontario. The nests in the study were occupied for an average of 11 years before one or both of the breeding birds were replaced. While this may sound like a long time, a population of healthy adults would not be expected to have such a rapid nest occupancy turnover rate.
One result of the study that warrants particular concern is the age of the replacement birds. Four of the 11 observed turnovers involved replacement birds that were juveniles, or "sub-adults". The opportunity for juvenile birds to breed may indicate that there are not enough breeding age birds in the population. Inexperienced breeders may be less successful in fledging their young, which, over time, would have a significant negative affect on the population. Mortality rates are high for young eagles, even in healthy populations.