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

Appendix D: Format for Quantifying Evidence
of Depredation of Duck Nests


We developed a format (Appendix D, Fig. 1) for quantifying evidence of depredation of duck nests in the Prairie Pothole Region. The nest depredation record was designed for use with nests being monitored for fate with information recorded on a nest record (Klett et. al 1986). Thus, for each completed nest depredation record, other information exists to help identify potential offending predators (e.g., history of missing eggs) or restrict analyses to nests meeting certain criteria (e.g., nest destroyed during incubation, clutch of ≥6 eggs when nest destroyed).

In developing the nest depredation record, we focused on 3 attributes of duck nests: (1) eggs and eggshells, (2) ground surface, and (3) nest material. We limited the area of collection of evidence to a 3-m radius around the nest. This helped insure that all reported data pertained to the subject nest, and kept the searched area small (necessary because most duck nests are in dense vegetation).

Critical factors in selection of variables for attributes and in development of descriptors for each variable were that no special expertise would be required of investigators to collect information, and prior visits by investigators to nests would not confound quality of the data. For these reasons, we avoided requesting information about tracks, hairs, odors, and tooth marks in eggs. Also, we abandoned attempts to quantify predator disturbance of vegetation (e.g., trails or matted areas). This decision was made because investigators had difficulty differentiating human disturbance of vegetation, trails made by attendant hens, and predator disturbance. Nevertheless, we encourage experienced investigators to record these and other supplementary evidence believed useful in identifying offending predators.

Before completing a nest depredation record, investigators must decide if any eggs hatched. No record is required if ≥1 egg hatched (Cowardin et al. 1985, Klett et al. 1986). An exception to this rule is for nests depredated when eggs were hatching and ≥1 dead duckling was found. Investigators may wish to fill out a record for such nests to insure that the mortality is documented.

Determination of whether an egg hatched requires careful examination of eggshells and shell fragments. Embryonic ducklings break the encircling shell by rotating their head in the air cell, and pecking a narrow slit around the blunt end of the egg (Terres 1980). This action separates the shell into a large intact piece with a round hole in the end, through which the duckling exits, and the end fragment (Appendix D, Fig. 2a, b). These weakened brittle eggshell pieces are often trampled by the hen and ducklings. Eggshell membranes may become separated from the shell and intermingled with shell fragments in the nest. Presence of a detached compressed membrane (Appendix D, Fig. 2c, d) is evidence of a hatched egg (Girard 1941, Klett et al. 1986).

The nest depredation record should be completed only for nests destroyed by predators, and nests abandoned by ducks because of predators (e.g., dead hen at nest but no eggs depredated). The record contains boxes for recording data control information, codes, counts, and measurements that describe depredation evidence found at nests.

Confusion can occur in recording values for some descriptors. Troublesome values are: (1) none (or zero), (2) not applicable, (3) undetermined, (4) no data, or (5) invalid data (extraneous conditions have influenced nest fate [e.g., nest site was cultivated, flooded, or trampled by cattle]). We recommend adhering to the following conventions when recording those values. If the value is none (or zero), record a zero. If the value is not applicable, undetermined, no data, or invalid data, draw a horizontal line through all boxes for the descriptor. Thus, there will be an entry in ≥1 box for every descriptor on completed records.

Instructions for Completing the Nest Depredation Record

Data Control and Merge

Variables comprising Boxes 1-16 identify data sets and link nest depredation records to nest records (Klett et al. 1986).

Box 1: Record a code to indicate this is a nest depredation record.

Boxes 2-16: Response codes recorded in these boxes must be the same as on the corresponding nest record for the final (termination) visit to the nest.

Boxes 17-20: Record the month and day when the nest was terminated by the investigator.

Boxes 21-22: Record the first and last initials of the observer.

Nest Material Displacement

Predators may displace nest material when destroying a nest. Material may be pulled from the nest and left on the ground and/or scattered aerially around the nest in tufts that become entangled on vegetation. The following questions pertain to percentages of total nest material that was pulled from the nest and left on the ground and total nest material scattered aerially around the nest. The percentage of nest material displaced on the ground, plus that displaced aerially, equals 100%. Nest material >5 cm from the original edge of the nest (before nest was destroyed) is classified as displaced, but it need not be disjunct from the nest. Nest material ≤5 cm from the edge of the nest or nest material rearranged or matted in the nest (e.g., from predator eating eggs in the nest) is not considered displaced.

For each variable, select the best response code from the 7 listed choices: 0 = none, 1 = trace, 2 = 1-5%, 3 = 6-10%, 4 = 11-25%, 5 = 26-50%, 6 = 51-75%, and 7 = 76-100%.

Box 23: Record the code that best describes the estimated percentage of the total nest material that was pulled, or otherwise displaced, from the nest and left on the ground, such as at the edge of the nest, in a trail, or where the predator ate eggs.

Boxes 24-26: Record the code that best describes the estimated percentage of the displaced nest material on the ground that is in each of 3 distance intervals: ≤20 cm from edge of nest, >20 cm - 1 m from edge of nest, and >1 m - 3 m from edge of nest.

Box 27: Record the code that best describes the estimated percentage of the total nest material that was scattered aerially around the nest (i.e., tossed in tufts or carried in feet, bills, or mouths) and became entangled on vegetation.

Boxes 28-30: Record the code that best describes the estimated percentage of aerially displaced nest material that is in each of 3 distance intervals: ≤20 cm from edge of nest, >20 cm - 1 m from edge of nest, and >1 m - 3 m from edge of nest.

Cached Eggs

Some predators cache eggs in soil or debris at nests, or cover eggs in nests with soil and/or debris. To find cached eggs, probe disturbed areas of soil or debris with your fingers. Predators may retrieve and eat cached eggs before investigators discover depredation. Treat eggshells at entrances to holes where eggs were cached and subsequently retrieved by predators as cached eggs. Treat holes that held a cached egg (e.g., impression of egg in bottom of hole), but for which no eggshell is found, as cached eggs.

Boxes 31-32: Record number of eggs in the nest that were covered with soil and/or debris by a predator (do not include eggs covered with nest material by a duck). Record a zero if there are none.

Boxes 33-34: Record number of cached eggs (including eggs cached but retrieved by predators) outside the nest. Record a zero if you find none.

Box 35: Record code that best describes depth (cm) of soil and/or debris covering top of the egg that was cached deepest outside the nest. If there is evidence that an egg was cached and subsequently exhumed by a predator (hole in soil with impression of egg in bottom), estimate depth of the soil and/or debris that covered top of the cached egg. Record zero if the predator left exposed any part of the deepest cached egg. Response codes are: 0 = none, 1 = ≤1 cm, 2 = >1 cm - 3 cm, and 3 = >3 cm.

Dug Areas

Some predators dig at nests. Dug areas can be holes that were dug but then filled with soil and/or debris (e.g., hole dug to cache egg), or areas where soil was piled.

Boxes 36-37: Record number of dug areas. Record zero if there are none. Record 99 if there is >1 dug area but you are uncertain of the number.

Box 38: Record code that best describes width (cm) of widest dug area. Response codes are: 1 = 1 cm - 5 cm, 2 = 6 cm - 10 cm, 3 = 11 cm - 20 cm, and 4 = >20 cm.

Whole Eggs

Predators may leave ≥1 whole egg at a nest with a destroyed clutch. Whole eggs include undamaged eggs, eggs with cracks and/or indentations but no visible contents, pipped or hatching eggs, and cached eggs. Record zero for each descriptor for which there are none.

Boxes 39-40: Record number of whole eggs in nest.

Boxes 41-42: Record number of whole eggs outside the nest and ≤53 m from the nest.

Instructions Based on Types of Shells Found

Amount of shell fragments and types of eggshells at nests with destroyed clutches can range from a single tiny fragment to many eggshells. Data recorded in Boxes 43-76 relate to eggshells or shell fragments.

Box 43: Record code of response that best describes amount of shell fragments and types of eggshell(s) found, and complete remainder of record according to the instructions. Response codes are: 0 = no eggshell or shell fragment is found (draw a horizontal line through Boxes 44-76 but complete Boxes 77-80), 1 = only fragments (includes 1 tiny chip) of shell are found (draw a horizontal line through Boxes 44-53 and Boxes 55-76 but complete Box 54 and Boxes 77-80), 2 = ≥1 eggshell of any type listed for Boxes 44-53 are found (complete all remaining boxes of the record).

Number of Shells by Type and Amount of Shell Fragments

Predators eat eggs at nests and leave eggshells with various types of damage as well as shell fragments. These variables pertain to number of eggshells of each specified type and amount of shell fragments found.

Boxes 44-45: Record number of eggshells with a small hole. Record a zero if there are none.

Boxes 46-47: Record number of eggshells with a large hole. Record a zero if there are none.

Boxes 48-49: Record number of fractured eggshells. Record a zero if there are none.

Boxes 50-51: Record number of trampled eggshells. Record a zero if there are none.

Boxes 52-53: Record number of crushed eggshells. Record a zero if there are none.

Box 54: Record code that best describes estimated total amount of shell fragments found. Response codes are: 0 = none, 1 = trace (1-5 tiny chips), 2 = total amount is less than that of 1 egg, 3 = total amount equals that of 1-3 eggs, and 4 = total amount exceeds that of 3 eggs.

Location of Openings In Eggshells

This variable pertains only to eggshells that have small or large holes. Predators generally open eggs by making small holes, then enlarge them to remove contents. Location of final opening (or openings) may be in the side and/or an end. All openings in eggshells are to be considered. For example, if an eggshell had an opening in the side and another in an end, recorded location would be side-end. Total number of eggshells recorded in Boxes 55-60 is to equal the total number of eggshells recorded in Boxes 44-47. Record a zero for each category when there are none.

Boxes 55-56: Record for combined eggshells with small or large holes the number that have the opening in the side. Record a zero if there are none.

Boxes 57-58: Record for combined eggshells with small or large holes the number that have the opening in an end. Record a zero if there are none.

Boxes 59-60: Record for combined eggshells with small or large holes the number that have the opening in a side-end. Record a zero if there are none.

Eggshells With Multiple Openings

This variable pertains only to eggshells that have a small or large hole. Predators may make ≥1 opening in an eggshell. Openings can range in size from a single small peck or canine puncture that breaks through the shell and underlying membrane to large holes that destroy much of the original shape of the egg. Openings must be separated by >1 cm of intact shell (intact shell can have cracks) to be considered distinct.

Boxes 61-62: Record for combined eggshells that have small or large holes the number that have ≥2 openings. Record a zero if no eggshell has multiple openings.

Eggshells With Egg Contents

This variable pertains only to eggshells that have a small or large hole. Predators may not completely eat all contents of an egg. Albumen left in eggs may dry and be inconspicuous, but yoke and/or embryos are usually conspicuous. Eggshells of eggs with contents completely eaten can be discolored on the inside but have no caked or "runny" residue. Record number found even if ≥1 eggshell with a small or large hole is not suitable for evaluation (e.g., eggshell is in water).

Boxes 63-64: Record for combined eggshells with small or large holes the number that are clean. Record a zero if there are none.

Boxes 65-66: Record for combined eggshells with small or large holes the number that have conspicuous yolk residue but <25% of original contents of the egg.

Boxes 67-68: Record for combined eggshells with small or large holes the number that have conspicuous yolk residue and ≥25% of original contents of the egg.

Eggshell Locations

This variable pertains to eggshells of all types but excludes shell fragments. Predators may leave eggshells in nests and/or at various distances from nests. The combined total of eggshells recorded in Boxes 69-76 is to equal the combined total of eggshells recorded in Boxes 44-53.

Boxes 69-70: Record number of eggshells in the nest. Record a zero if there are none.

Boxes 71-72: Record number of eggshells between the edge of the nest and ≤20 cm from the nest. Record a zero if there are none.

Boxes 73-74: Record number of eggshells >20 cm - 1 m from the nest. Record a zero if there are none.

Boxes 75-76: Record the number of eggshells >1 m - 3 m from the nest. Record a zero if there are none.

Dead Hens or Ducklings

Evidence may be found that indicates the hen and/or ducklings were killed by a predator or died from other causes. Evidence of a dead hen may range from a few feathers or blood to a whole carcass. A carcass is whole if no parts are missing, although it may be decomposed or largely consumed by insects. Evidence of a dead duckling consists of carcass parts or an entire duckling. A dead duckling must be detached from an eggshell to be considered a carcass; otherwise, it is egg contents. Assume that a dead hen or dead duckling(s) at a nest are from that nest unless the species differs from that of the eggs. Answer questions exclusively from evidence ≤3 m from the nest. Carcass or carcass parts found >3 m from the nest may warrant listing under comments in Box 80.

Box 77: Record code of the response that best describes fate of the hen. Response codes are: 0 = no physical evidence indicating dead hen, 1 = loose feathers or blood indicate dead hen, 2 = carcass part(s) with head attached to body are present, 3 = carcass parts with head missing or detached from body are present, and 4 = whole carcass is present.

Box 78: Record number of dead ducklings found using the following responses: 0 = none, 1-8 = record actual number if 1-8, and 9 = ≥9.

Predator Species Identification

Occasionally, predators that destroyed the nest are positively known. Describe under comments any details.

Box 79: Record a Y (Y = yes) if the predator species that destroyed the nest is positively known and write its name in the space provided.

General Comments

Sometimes it is desirable to provide written comments or even drawings to help explain evidence of depredation found at a nest.

Box 80: Record a 1 if comments are provided. Write comments on the record.


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