September 23, 2014
In Sickness and in Health: A Tale of Two Eagle Studies
Two studies of lead levels in eagles were released in recent weeks. With two very different methodologies and findings, a quick look at both studies shows the importance of asking the right questions and not assuming to already know the answers.
The US Fish and Wildlife service published a study of lead exposure in bald eagles in the Upper Midwest. The researchers collected the livers of 168 eagles found dead. While only 21 percent had lead concentrations that would be considered lethal, a little less than half had some detectable levels of lead exposure.
Rather than attempting to determine the actual sources of the lead exposure, the researchers decided that traditional ammunition was probably to blame and focused their attention on proving that assumption. They did this by collecting gut piles from managed deer hunts, then x-raying the gut piles to show that only one-third of the piles contained one or more lead fragments. Between the two separate studies – one showing less than a quarter of dead eagles had high lead levels – and one showing that a third of gut piles contain at least one lead fragment – the authors proclaimed “discarded offal piles from deer shot with lead ammunition can be a potential source of lead exposure for bald eagles.” Our tax dollars are hard at work.
The second study, out of Iowa State University, took issue with the fact that all of the studies of lead exposure in eagles use only dead eagles or eagles treated in raptor rehabilitation centers, as test subjects, then extrapolate out to the entire population. The author instead tested fecal samples from nest areas for lead levels.
They found detectable lead levels in most samples, but in the majority of samples, the levels were low and within the range of birds in lead-free sites, and within background environmental lead levels. There was no statistically significant difference in samples collected in the winter versus the spring or in those collected near the river (meaning plenty of food sources) or far from the river (hypothesized to be more likely to scavenge gut piles). There was also no correlation between lead exposure and the number of deer harvested in an area.
As a second part of the study, they also compared lead levels between eagles brought to rehabilitation facilities with free-flying eagles. Not surprisingly, those in rehab had significantly higher levels of lead. Overall the study found, “that the majority of free-flying nesting and wintering Bald Eagles in Iowa experience low levels of lead exposure and that lead levels in rehabilitation Bald Eagles are not representative of lead exposure levels in free-flying Bald Eagles, but rather representative of a small subset of the population.”
So it turns out that when studying how many eagles are sick, it matters if you only look at sick eagles, not the broader population. With raptor populations soaring in recent years, the Iowa study finally takes a meaningful look at the anti-hunting groups’ argument that traditional ammunition poses a threat to populations.