Habitat use and population status of Yellow-billed and Pacific loons in western Alaska, USA

Effective conservation of sympatric avian populations depends on unbiased estimates of population size, distribution, and habitat use. For populations of Yellow-billed Loons (Gavia adamsii) and Pacific Loons (G. pacifica) co-occurring in Arctic wetland communities in Alaska, USA, such data are limited and difficult to obtain, hindering population assessments and decision making. The Yellow-billed Loon is also under consideration for additional protections under the Endangered Species Act due to small global population size, specific habitat requirements, and low fecundity, further increasing the need for information at the landscape scale. To help evaluate the population status and habitat use of both species, we used repeated aerial surveys and a dynamic multistate occupancy modeling approach to jointly estimate 1) probability of lake use and 2) probability of use for nesting for Yellow-billed and Pacific loon populations at the landscape scale on the Seward Peninsula and Cape Krusenstern, Alaska, in 2011 and 2013. We also estimated state-specific transition probabilities and degree of interspecific competition to assess population stability and degree of species interactions. We found that probability of site reuse (ϕYₑₗₗₒw₋bᵢₗₗₑd = 0.73 [0.44–0.94]; ϕPₐcᵢfᵢc = 0.86 [0.72–0.98]) or reuse for nesting (ϕYₑₗₗₒw₋bᵢₗₗₑd = 0.72 [0.46–0.97]; ϕPₐcᵢfᵢc = 0.59 [0.38–0.85]) in 2013 was high, as was overall use of lakes >7 ha by loons (>80%). These results suggested that lake habitats may have been saturated, and that populations of both species were stable over the two-year interval between surveys. Our estimates indicated that nesting populations in western Alaska were much larger than previously thought for both Yellow-billed (∼2.5 times larger) and Pacific loons (∼1.5–2.0 times larger). Together our results indicate that Arctic wetlands in western Alaska are important for both species and that loon populations in this area warrant additional consideration for conservation.