Lead in species of greatest conservation need: free-flying bald eagles as indicators
Stephen J. Dinsmore
In Iowa, Bald Eagles (Haliaeetus leucocephalus) have increased from one nest in 1977 to more than 220 nests in 2012. However, many aspects of their breeding biology, behavior, and the factors that influence their survival and reproduction have yet to be studied. Recently, there has been a growing concern about whether lead ammunition is linked to lead exposure in Bald Eagles. Raptor rehabilitators throughout the U.S. continue to report cases of lead exposure in Bald Eagles and other raptors and have also noticed temporal correlations between the incidence of lead exposure and the timing of upland and big game hunting seasons. Yet, little is known about the magnitude of lead exposure in free-flying Bald Eagles and whether those levels vary temporally or spatially. In addition to concerns about lead exposure in Bald Eagles, there is also interest in understanding factors associated with their reproductive success. The objectives of this study were to: 1) Characterize lead levels in the feces of nesting and wintering Bald Eagles in Iowa and identify temporal and spatial factors related to elevated lead exposure as a function of diet, 2) Compare lead exposure in free-flying eagles with eagles admitted to rehabilitation centers, and 3) Characterize and identify factors that influence nest survival of Bald Eagle in Iowa. Nest sampling was spatially and temporally stratified to enable us to test two specific hypotheses about lead exposure in Iowa. We documented detectable amounts of lead (>0.11 ppm) in 93% of feces. In the majority of samples, fecal lead levels were low and within the range of fecal lead levels found in other bird species inhabiting known non-lead contaminated sites and also similar to background environmental lead levels. Lead levels from nests sampled during the winter were higher than nests sampled in the spring (1.63 ppm vs. 1.09 ppm) and lead levels from non-Mississippi River nests were higher than Mississippi River nests (1.81 ppm vs. 0.90 ppm), but these differences were not statistically significant (P > 0.05). Lead levels in samples from nesting eagles were higher than samples from wintering eagles (1.37 ppm vs. 1.26 ppm), but these differences were not statistically significant (P > 0.05). We also found no statistical difference between fecal lead levels from nest sites on public versus private land (1.89 ppm vs. 0.98 ppm) and no correlation between lead exposure and land ownership, watershed, proximity to water, proximity to potential environmental sources of lead, or number of deer harvested or lost due to wounding. A second component of the study compared lead levels in eagles brought to rehabilitation facilities to free-flying eagles. We documented lead in 100% of the feces collected from rehabilitation Bald Eagles. Fecal lead levels in rehabilitation Bald Eagles were significantly higher than lead levels in free-flying Bald Eagles (20.36 ppm vs. 1.34 ppm, P < 0.001). We also compared blood lead levels with fecal lead levels taken from rehabilitation Bald Eagles and found that fecal lead levels were higher than blood lead levels (21.55 ppm vs. 2.87 ppm), but these differences were not statistically significant (P > 0.05) because of the high variation within each sample. We created a linear regression examining the relationship between fecal and blood lead levels from rehabilitation Bald Eagles and found that fecal lead levels were a good predictor of blood lead levels in rehabilitation Bald Eagles. From this regression we created a model to estimate unknown blood lead levels from known fecal lead levels. Lastly, we monitored eagle nests and modeled their daily survival using Program MARK, and included disturbance and other environmental parameters collected during our fecal sampling visits. We did not monitor nests from clutch initiation to fledge and instead modeled nest survival from the onset of incubation to when nestlings were 9 weeks old (98 days total), which is shorter than the typical fledging age of 10-12 weeks. Bald Eagle nests in Iowa had high survival rates for the time period we studied. In addition, we found that nest survival was primarily influenced by nest location (survival of nests along the Mississippi River was lower than all other nests), land ownership (survival was higher on public land compared to private land), and proximity to roads (survival was lower near roads). We also estimated that nest initiation dates for Bald Eagle nests found active in the spring ranged from 19 February to 21 March, with a mean estimated nest initiation date of 4 March. Estimated hatch dates for Bald Eagle nests found active in the spring ranged from 26 March to 25 April (mean = 8 April) while estimated fledging dates ranged from 11 June to 11 July (mean = 24 June). Estimated survival was greatest for non-Mississippi River nests on public land (0.98, 95% CL [0.88, 1.00]) and private land (0.92, 95% CL [0.80, 0.97]) and lower for Mississippi River nests on public land (0.67, 95% CL [0.52, 0.79]) and lowest for Mississippi River nests on private land (0.19, 95% CL [0.00, 0.74], there were only two nests in this category). Our short-term nest monitoring suggested that our sampling visits had little to no short term-impact on the behavior of incubating adults and caused no known cases of nest abandonment. We further infer that our non-invasive sampling method in which we spent <30 minutes within 100 m of the nest had a minimal impact on the nest survival of this species. Our results indicate 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. Our results also suggest that feces are a promising method for non-invasively measuring lead exposure when blood is difficult to collect such as in some rehabilitation Bald Eagles or in large-scale field studies involving free-flying raptors or other large birds in the wild. Overall we documented some of the highest nest survival rates reported for Bald Eagles and found that nest survival was lower for nests along the Mississippi River, higher for nests on public land, and slightly lower when nests were closer to roads. This study improves our understanding of Bald Eagle nest survival and other vital nesting parameters, as well as our understanding of the magnitude of lead exposure in free-flying Bald Eagles in Iowa.