Population dynamics of double‐crested cormorants in two interior breeding areas

Abstract

Because of rapid population expansion, conflicts between double‐crested cormorants (Phalacrocorax auritus) and humans have increased, particularly at aquaculture facilities in the Lower Mississippi River Valley (LMRV), United States. Although key demographic information on the Interior population is sparse, management decisions for population reduction are already in place across their breeding range. New evidence suggests the Appalachian Mountains may act as a migration divide for Interior cormorants, causing some birds to migrate between LMRV and the western Great Lakes, whereas others migrate between Florida and the eastern Great Lakes. Most of the banded cormorants recovered near LMRV aquaculture facilities were banded as nestlings at colonies from the central Great Lakes west to the northern Great Plains. We selected 2 geographically distinct Interior cormorant breeding areas: west of the Great Lakes in Lake of the Woods (LOW) and eastern Lake Ontario (ELO), located on either side of the migration divide in the province of Ontario, Canada, to study population dynamics. We used age‐specific classes for our mark‐recapture analysis (Program MARK) on data collected from 9,498 color‐banded cormorants during the breeding seasons of 2000–2008. Adult survival was the same for both areas (LOW = 0.84 ± 0.09 SE, ELO = 0.83 ± 0.05 SE). Finite rates of population growth (λ) were also similar (LOW = 1.01, ELO = 0.97) and sensitivity analyses indicated that adult survival was the most influential vital rate contributing to λ for both breeding areas. However, young‐of‐the‐year survival estimates were distinctly less in ELO (0.19 ± 0.02 SE) than in LOW (0.45 ± 0.10 SE) and sensitivity analysis revealed that young‐of‐the‐year survival played a key role in ELO population growth. Moreover, ELO breeding area fidelity (F) was much greater (0.94 ± 0.05 SE) than the F estimates for all age classes in LOW (between 0.68 ± 0.19 SE and 0.80 ± 0.13 SE). Considering these regional demographic differences, we recommend that future management efforts be based on migratory flyways within the Interior population. © 2013 The Wildlife Society

Modelling effects of nonbreeders on population growth estimates

“…; Chastant et al . ) results in potentially large overestimates of λ when the population contains nonbreeders (Fig. ).…”

“…Accounting for demographic differences between breeders and nonbreeders is clearly important for estimating population growth rates, but what happens if the presence of nonbreeders is unknown or ignored? The common practice of estimating demographic parameters and growth rates based solely on the breeding population (Gr€ uebler et al 2008;Katzner et al 2011b;Chastant et al 2014) results in potentially large overestimates of k when the population contains nonbreeders (Fig. 2).…”

Morphometric Variation During Chick Development in Interior Double-Crested Cormorants (Phalacrocorax auritus)

“…Larger clutch sizes may also contribute to higher fledge rates, particularly when coupled with superior foraging, because larger clutches facilitate the opportunity to raise more young. Likewise, the cause for reduced fledge rate in western breeding areas is unclear, but may be linked to poor nest site quality or inadequate local foraging opportunities as the birds nesting in the west exhibit weak colony site fidelity (Chastant et al 2014). Colony differences in chick growth and development may reflect age and experience of the adults.…”

“…spring migration, and catfish comprises 87% of their diet (Glahn et al 1999). These piscivorous birds have subsequently increased their overwinter survival by improving body condition through catfish exploitation (Glahn et al 1999;Chastant et al 2014). Using sulfur isotope analysis from feathers grown during winter months, Hebert et al (2008) also suggested that if cormorants spent at least a portion of the winter foraging at aquaculture facilities, they returned to the breeding grounds in better physical condition than cormorants that solely fed in marine environments.…”

“…A recent study of cormorant movement suggests the Appalachian Mountains may serve as a migratory divide for cormorants across the Great Lakes; western populations migrate to winter in the Gulf of Mexico region of the southeastern United States (Alabama, Arkansas, Louisiana, and Mississippi), and eastern populations overwinter in Florida (Guillaumet et al 2011). Moreover, population models reveal survival disparity within the cormorant's range; first-year survival rates in western populations more than double the first-year survival rates of their eastern counterparts (Chastant et al 2014). If overwinter foraging on aquaculture facilities enhances body condition of western cormorants, and distinct eastern and western populations truly exist and are differentiable, we predict that reproductive performance of western cormorants will exceed that of the eastern population.…”