Brenda Hanley
January 13, 2022

Long before I was aware of the problem, professionals of veterinary medicine and pathology treated, rehabilitated, or necropsied ill, dying, or dead bald eagles. The wild birds had been presented for care after they ingested lead fragments from spent ammunition, discarded upon on the landscape in the carcasses of legally harvested big game and other upland wildlife. As evidenced by necropsy records, time and again, veterinary professionals saw otherwise healthy birds succumb to the effects of lead, sometimes with desperately little they could do to help.

I was hired as the quantitative modeler to conduct an analysis of their data. Our interdisciplinary team set out to identify the combined impact of these individual deaths to the wild eagle population from whence they came. We discovered that despite increasing abundances of wild bald eagles in the northeast US over the last three decades, deaths associated with lead poisoning reduced the growth rate of eagle populations by 4.2%.

In some scenarios, 4.2% can feel like a big number. Imagine being offered a 7.2% interest rate on a mortgage when a different bank can offer you 3.0%. In that case, a 4.2% difference feels enormous! On the other hand, a 4.2% difference can feel desperately small. After all, statisticians regularly use a 5% p-value as the threshold of significance in scientific studies. In the context of eagles, what does 4.2% mean to you? And to our wildlife legacy?

 

Bald eagle mated pair preparing their nest for their young.

Our analysis of the veterinary data further revealed that in the face of deaths associated with ingestion of lead, the wild population has reorganized itself to ensure population growth despite these mortality events. On the one hand, that reorganization speaks to the incredible ability of a population of eagles to overcome obstacles and persist. But on the other hand, that restructuring depleted the resilience of the population to withstand additional mortality from other causes, such as emerging infectious disease or climate change.

If we look around, we see evidence of climate change impacting the natural resources that we depend on. What does climate change mean for the future of bald eagles? I don’t know. Does anyone know? The evidence suggests that climate change is happening. And in this uncertain future, does the practice of using lead-based ammunition outweigh the cost of the depletion of resilience to the eagles?

In this study, the i’s have been dotted, and the t’s have been crossed; the data are what they are, and the math is what it is. But natural resource management is not just about the resource, the data, or the math; it is also about people and the choices we make (Balmford and Cowling 2006). So, here is the question: given that alternatives do exist, does an activity that leads to a 4.2% reduction in growth and an alteration to the life history of eagles constitute a reasonable decision in the wise use of our natural resources? It’s our choice.

 

Here is the link to the article: https://wildlife.onlinelibrary.wiley.com/doi/10.1002/jwmg.22177 and full citation is below.

Hanley, B., J, A. A. Dhondt, M. J. Forzán, E. M. Bunting, M. A. Pokras, K. P. Hynes, E. Dominguez‐Villegas, and K. L. Schuler. 2021. Environmental lead reduces the resilience of bald eagle populations. Journal of Wildlife Management 1–18. https://doi.org/10.1002/jwmg.22177  

Literature Cited:

Balmford A, Cowling RM (2006) Fusion or failure? The future of conservation biology. Conserv Biol 20:292–255