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Techniques for Studying Nest Success of Ducks in Upland Habitats in the Prairie Pothole Region


Appendix B

Instructions for Recording Data and Calculating Nest Success


Glossary

Nest. A scrape or bowl containing one or more eggs. The terms "nest" and "clutch" are often used interchangeably in this and other duck nesting study reports. Only nests tended by hens (not destroyed or abandoned) when found are used to compute nest success by the methods described.

Age when Found. Number of eggs plus incubation stage in days. (Assume that one egg is laid each day.)

Exposure Days. The number of days a clutch of eggs is under observation and vulnerable to loss to predators or other decimating factors.

Full Clutch. Clutch size of incubated nests that have no history of destroyed or missing eggs.

Mean Age at Hatching. Mean clutch size plus mean incubation term in days.

Terminated Nests. Nests, successful or unsuccessful, no longer tended by a hen.

Nest Fate. The success or failure of a nesting attempt.

Abandoned. Intact clutches that are deserted by the hen. True abandonment rates are difficult to estimate because some untended clutches result from the death or injury of hens and some abandoned clutches are destroyed before their true fate can be determined. Abandoned clutches (other than those abandoned because of investigator disturbance on the day found) are combined with destroyed clutches to compute survival rates.

Destroyed. A nest in which one or more eggs are missing or destroyed and none hatched.

Fate Unknown. The fate of the clutch was not determined because of inadequate evidence at the nest site or because the nest was not revisited.

Nonviable. All eggs are infertile, addled, or contain dead embryos.

Successful. One or more eggs hatched even if young are found dead at nest sites.

Nest Success. The probability that a nesting attempt will result in the production of one or more ducklings, as opposed to hen success - the probability that a hen will succeed in hatching at least one duckling in one or more nesting attempts.

Mayfield Method for Computing Nest Success. A method that uses the interval during which a nest is under observation and exposed to decimating factors (see Exposure Days).

Shortcut Method for Computing Nest Success. A quick method for obtaining preliminary results or for checking calculations used for the Mayfield estimator.

Traditional Method for Computing Nest Success. The number of nests in which one or more eggs hatched divided by the total number of nests of known fate that were found; reported as apparent nest success. Estimates by this method are almost always extremely biased.


Sample Field Form for Recording Nesting Data

The sample Nest Record (Fig. B-1) contains space for entering the data necessary for computating nest success rates, and supplementary data on vegetation at the nest site and the fate of hens and eggs. Some investigators will need other data to meet the objectives of their studies. If so, additional data can be systematically recorded on a supplementary form cross-referenced to the basic record described here.

A computer program is used at Northern Prairie Wildlife Research Center to calculate exposure days for each clutch and to generate tables of clutch survival for such variables as species, study area, and habitat. The Nest Record was designed to facilitate direct computation of exposure days by those who want to process small batches of data without a computer.

Columns 2-19 are used for data control, columns 27-47 are used to compute nest success with a computer, and columns 20-26 and 48-51 are used to record supplementary data on the nest site, hen mortality, unhatched eggs, and parasitism. Uncoded space is provided to record observations made at each visit to the nest (Nest Visitation Record) and for calculating exposure days when a computer is not used. Entries are made in columns 2-27 on the day the nest is discovered. Data in the Nest Visitation Record are used to complete columns 28-51 after the last visit to the nest. Instructions for making entries needed to compute nest success (columns 27-47) follow.

Col. 27
Nest Status on Day Found: Nests coded greater than 0 are usually not used to compute survival rates. Nesting attempts disrupted by investigators after the first visit may be used in nest survival computations.

0 - Normal
1 - Eggs destroyed or hen injured or killed by investigators
2 - Some eggs cracked or broken by investigators
3 - Any other major disturbance by searchers that would jeopardize nest survival
4 - Nest abandoned because of investigator disturbance on day found; determined on second visit
5 - Partially destroyed when found
6 - Terminated when found
7 - Parasite eggs present when found

Cols. 28-29
Number of Eggs When Found: Record number of whole eggs. If nest contains parasite eggs, record only the number of eggs of host species.

Cols. 30-31
Incubation Stage When Found:

nn - Number of days incubated (00 = laying stage)
44 - Pipping
55 - Hatched (young in nest)
77 - Nesting attempt terminated when found
88 - Unknown
99 - All eggs addled or contained dead embryos

Cols. 32-33
Full Clutch: If nest was not observed during incubation or has a history of parasitism or destroyed or missing eggs, code . (dot) in Col. 33. Do not assume Full Clutch from broken eggs or membranes in terminated nests.

Col. 34
Fate:

1 - Successful
2 - Abandoned - if abandonment was caused by observer on the day the nest was found, code 4 in Col. 27
3 - Destroyed
4 - Nonviable eggs
5 - Unknown

Col. 35
Cause of Loss: Data in this column are not used to estimate nest success therefore codes are not presented.

Cols. 36-38
Date Found:

Cols. 39-41
Date of Last Visit on Which Nest Was Still Viable: Same as Date Found when nest was terminated between the first and second visit, date of second visit when the nest was terminated between second and third visit, etc. May be left blank if nesting attempt was successful.

Cols. 42-44
Date Fate Determined: Usually the last visit to the nest. Code . (dot) in Col. 44 if fate not determined, e.g, if nest could not be relocated.

Cols. 45-47
Date of Termination Known: For some nests the exact day of termination is known. For example, hatched young were observed in the nest; the nest was destroyed or abandoned because of investigator activity; the date the nest was abandoned was determined from the increase in number of eggs, advance in stage of incubation, or both of these; the date of a destructive event such as a storm or tillage operation is known. Do not confuse with estimated hatch date. Leave blank if unknown.


Computation of Nest Success

Data in columns 27-47 are sufficient for calculating nest success rates by computer. If a computer is not used, complete the following in the uncoded section of the Nest Record and calculate nest success as described on page 24.

Age When Found: Number of Eggs When Found plus Incubation Stage When Found. If nest is found when eggs are pipping (Cols. 30-31 = 44), use average incubation term minus 1 for incubation stage. A nest record is not used to calculate nest success if the age at discovery is unknown.

Estimated Initiation Date: Date Found minus Age When Found.

Estimated Hatch Date: Initiation Date plus Full Clutch plus Average Incubation Term (Table B-1). Use mean clutch size (Table B-1) if Full Clutch is not known. (Exception: If eggs are observed while pipping, the estimated hatch date is the following day.)


Table B-1

Exposure Days: Exclude abnormal nests (Col. 27 > 0) and those with Age When Found unknown; then determine exposure days as follows:

TERMINATION DATE KNOWN (Table B-2, nest 1): Exposure = Known Termination Date minus Date Found

TERMINATION DATE UNKNOWN:
* Nesting attempt successful (Table B-2, nest 2): Exposure = Estimated Hatch Date minus Date Found.
* Fate of nest is unknown (Table B-2, nest 3): Exposure = Last Visit Date When Clutch Was Viable minus Date Found .
* Nesting attempt unsuccessful (Table B-2, nests 4-7): Exposure = Known Exposure plus Probable Exposure where Known Exposure = Last Date Nest Viable minus Date Found and Probable Exposure = half the interval between Last Date Nest Viable and the lesser of Date Fate Determined and Estimated Hatch Date (Multiply by 0.4 instead of 0.5 if this interval is > 14 days; table B-2, nest 7).


Table B-2

Nest Success: Two methods for calculating nest success - the Mayfield method and a shortcut method - are described. Examples of calculations by both methods are presented in Table B-3.

MAYFIELD EXPOSURE METHOD.
Usable records. Nest survival rates are calculated from nests for which exposure days are available.
Number of nesting attempts that failed (Nu). Tally number of nesting attempts that failed (Col. 34 = 2, 3, or 4). Exception: Do not include losses due to investigator disturbance that occurred after the first visit, but do include exposure daya.
Total exposure days (E). Sum of exposure days for all usable records.
Mean age at hatching (h). From Table B-1.
Estimated nest success (P2). Calculate as in Table B-3.

SHORTCUT METHOD.
Usable records. Exclude abnormal nests (Col. 27 > O) and those with unknown fate and unknown age when found.
Apparent nest success (P1). Number of successful nests divided by all usable nests.
Mean age at hatching (h). From Table B-1.
Mean age of nests when found (f). Calculate mean age of nests when found from usable nest records.
Estimated nest success rate (P3). Calculate as in Table B-3.


Table B-3


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