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Interpreting Evidence of Depredation of Duck Nests in the Prairie Pothole Region

Appendix B, Table 8


Table 8. Stepwise application of specified criteria to estimate the proportion of destroyed (by predators) incubated duck nests of ≥6 eggs in uplands of 48 federal waterfowl production areas in Minnesota, North Dakota, and South Dakota with destruction caused by striped skunks.
    Number of nests
Criteria Indicatora Affected Remaining in sample
Nestsb     1,108
Destroyed by predator   636 636
≥6 eggs present when last visited   564 564
Eggs incubated when last visited   421 421
Visit interval of ≤14 days   395 395
Excluded becasue of missing datac   6 389
Evidence hen was killed N 19 370
Evidence egg cached N 13 357
Eggshells of >50% of depredated eggs P 118 118
>50% of eggshells have small holes N 12 106
>50% of openings in eggshells with
small or large holes were in an end
N 12 94d
>25% of eggshells >1 m from nest N 7 87d
Dug area N 7 80d
Whole egg(s) at nest N 5 80d
No eggshell fragments N 6 75d
No nest material displaced N 9 69e
a Positive (P) or negative (N) indicators of depredation by striped skunks. For each category of evidence, all nest depredation records remaining in the sample are examined for presence or absence of that evidence. Records with evidence are retained (P) for possible assignment of destruction to skunks. Records without that evidence (N) are removed from further consideration of possible assignment of destruction to skunks.
b Total nests found that were suitable for analyses (excludes nests with eggs broken of hens injured by investigators, nests abandoned within a few days after discovery, and nests parasitized by another species).
c Nests excluded because data were absent for ≥1 examined variable.
d Nests likely to have been destroyed by striped skunks.
e Nests with cause of destruction assigned to striped skunks.

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