Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis

Annual counts of migrating raptors (Accipitriformes, Falconiformes) are used as indices of population size. Variation in the proportion of the raptor population counted may decrease precision of trend estimates, thereby reducing power of inference. The proportion counted is the product of sample cov...

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Main Author: Nolte, Eric G.
Format: Text
Language:unknown
Published: ScholarWorks 2012
Subjects:
Online Access:https://scholarworks.boisestate.edu/td/718
https://scholarworks.boisestate.edu/context/td/article/1754/viewcontent/Nolte_Eric_G_thesis_May_2012.pdf
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spelling ftboisestateu:oai:scholarworks.boisestate.edu:td-1754 2023-10-29T02:35:41+01:00 Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis Nolte, Eric G. 2012-02-15T08:00:00Z application/pdf https://scholarworks.boisestate.edu/td/718 https://scholarworks.boisestate.edu/context/td/article/1754/viewcontent/Nolte_Eric_G_thesis_May_2012.pdf unknown ScholarWorks https://scholarworks.boisestate.edu/td/718 https://scholarworks.boisestate.edu/context/td/article/1754/viewcontent/Nolte_Eric_G_thesis_May_2012.pdf Boise State University Theses and Dissertations raptors migration detectability double-observer Zoology text 2012 ftboisestateu 2023-09-29T15:11:08Z Annual counts of migrating raptors (Accipitriformes, Falconiformes) are used as indices of population size. Variation in the proportion of the raptor population counted may decrease precision of trend estimates, thereby reducing power of inference. The proportion counted is the product of sample coverage and probability of detection. It is possible to improve the power of trend analysis by the adoption of techniques, such as double-observer or distance sampling, which estimate the probability of detection. I used a dependent double-observer method to estimate detectability at the annual fall raptor migration count at Lucky Peak, Idaho, in 2009 and 2010. I used Huggins closed-capture removal models and information-theoretic multi-model inference to describe important factors affecting detectability. The most parsimonious model included effects of observer identity, distance, wingspan, genus, and day of the season. Competitive models also included wind-speed, cloud cover, and hour of the day. These results demonstrate the importance of controlling observer effort and training at watch-sites, and the potential utility of adjusting daily counts to account for differences in flight distance. I used model-averaging to account for selection-uncertainty in estimating coefficients, and used the resulting equation to simulate 30 years of counts of Sharp-shinned Hawks (Accipiter striatus) and Northern Harriers (Circus cyaneus) with heterogeneous detectability, a known population trend, and a degree of unexplained random variation in the number of available birds. Imperfect detection did not substantially bias trend estimation, but did increase variance in counts, decreasing power. Correcting for detectability did little to improve power to detect long-term declines when there was a realistically high variation in the number of available raptors (CV ≥ 0.26). Detectability-correction by means of double-observer or distance sampling may, in the case of raptor migration counts, not be warranted for the purpose of long-term ... Text Circus cyaneus Boise State University: Scholar Works
institution Open Polar
collection Boise State University: Scholar Works
op_collection_id ftboisestateu
language unknown
topic raptors
migration
detectability
double-observer
Zoology
spellingShingle raptors
migration
detectability
double-observer
Zoology
Nolte, Eric G.
Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis
topic_facet raptors
migration
detectability
double-observer
Zoology
description Annual counts of migrating raptors (Accipitriformes, Falconiformes) are used as indices of population size. Variation in the proportion of the raptor population counted may decrease precision of trend estimates, thereby reducing power of inference. The proportion counted is the product of sample coverage and probability of detection. It is possible to improve the power of trend analysis by the adoption of techniques, such as double-observer or distance sampling, which estimate the probability of detection. I used a dependent double-observer method to estimate detectability at the annual fall raptor migration count at Lucky Peak, Idaho, in 2009 and 2010. I used Huggins closed-capture removal models and information-theoretic multi-model inference to describe important factors affecting detectability. The most parsimonious model included effects of observer identity, distance, wingspan, genus, and day of the season. Competitive models also included wind-speed, cloud cover, and hour of the day. These results demonstrate the importance of controlling observer effort and training at watch-sites, and the potential utility of adjusting daily counts to account for differences in flight distance. I used model-averaging to account for selection-uncertainty in estimating coefficients, and used the resulting equation to simulate 30 years of counts of Sharp-shinned Hawks (Accipiter striatus) and Northern Harriers (Circus cyaneus) with heterogeneous detectability, a known population trend, and a degree of unexplained random variation in the number of available birds. Imperfect detection did not substantially bias trend estimation, but did increase variance in counts, decreasing power. Correcting for detectability did little to improve power to detect long-term declines when there was a realistically high variation in the number of available raptors (CV ≥ 0.26). Detectability-correction by means of double-observer or distance sampling may, in the case of raptor migration counts, not be warranted for the purpose of long-term ...
format Text
author Nolte, Eric G.
author_facet Nolte, Eric G.
author_sort Nolte, Eric G.
title Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis
title_short Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis
title_full Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis
title_fullStr Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis
title_full_unstemmed Raptors Present but Unobserved: Detectability at a Western Migration Watch-Site and Its Effect on Trend Analysis
title_sort raptors present but unobserved: detectability at a western migration watch-site and its effect on trend analysis
publisher ScholarWorks
publishDate 2012
url https://scholarworks.boisestate.edu/td/718
https://scholarworks.boisestate.edu/context/td/article/1754/viewcontent/Nolte_Eric_G_thesis_May_2012.pdf
genre Circus cyaneus
genre_facet Circus cyaneus
op_source Boise State University Theses and Dissertations
op_relation https://scholarworks.boisestate.edu/td/718
https://scholarworks.boisestate.edu/context/td/article/1754/viewcontent/Nolte_Eric_G_thesis_May_2012.pdf
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