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

Appendix B, Table 9

Table 9. 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 raccoons.

		Number of nests
Criteria	Indicator^a	Affected	Remaining in sample
Nests^b			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 data^c		6	389
Evidence hen was killed	N	19	370
Evidence egg cached	N	13	357
Dug area	N	21	336
Eggshells of >50% of depredated eggs	P	110	110
>50% of eggshells have small holes	N	12	98
>50% of openings in eggshells with small or large holes were in an end	N	67	31
>25% of eggshells >1 m from nest	N	3	28^d
>30% of nest material displaced	N	8	20^d
Whole egg(s) at nest	N	3	20^d
No eggshell fragments	N	0	17^e

^a	Positive (P) or negative (N) indicators of depredation by raccoons. 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 raccoons. Records without that evidence (N) are removed from further consideration of possible assignment of destruction to raccoons.
^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 raccoons.
^e	Nests with cause of destruction assigned to raccoons.

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