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Depredation Patterns of Predator Species

Black-billed Magpie

Black-billed magpies range widely over uplands and wetlands but are most likely to depredate duck nests near their own nests or perches (Williams and Marshall 1938, Jones and Hungerford 1972). Little is known about the vulnerability of duck nests in different habitats to this predator.

Response to Hens and Nests--We have no information about interactions between black-billed magpies and hens at nests, and found no evidence in literature of black-billed magpies attacking nesting hens.

We obtained information for 29 artificial duck nests visited by black-billed magpies and at which there was no evidence of interference from other potential predators. Nests were monitored for 1-4 days, after discovery by magpies. Film records revealed that ≥1 magpies visited nests at least 266 times during 45 exposure-days.

Depredation of the artificial nests usually was by individuals but occasionally 2 or 3 magpies were photographed together at a nest. Depredations customarily involved infrequent brief visits to the nests.

Black-billed magpies appeared only moderately interested in depredating duck eggs. There were numerous instances of a magpie visiting a nest without depredating an egg. Eggs in 5 (17%) of 29 artificial nests known discovered by magpies were not depredated. These nests were visited at least 22 times by magpies during 9 exposure-days. One artificial nest that was visited 23 times by ≥1 magpies during 4 exposure-days had only 3 of its 6 eggs depredated. Only 4 (14%) of 29 nests detected by magpies were destroyed on the day the nest was discovered.

Black-billed magpies occasionally covered whole eggs in artificial nests with nest material. These looked like nests at which hens had covered their eggs before departing for a recess. Magpies sometimes uncovered and then recovered eggs, often moving, eating, or removing eggs in the process.

Treatment of Eggs--Black-billed magpies depredated 122 (70%) of 174 eggs placed in 29 artificial nests visited exclusively by magpies. At each of 5 nests a depredated egg was left whole, but moved ≥1 m from the nest. Magpies removed most depredated eggs from the nest sites. We accounted for only 23 (19%) of 122 depredated eggs based on eggshells found at the nests. Eight eggshells each had a small hole pecked in the side and little contents removed. Magpies probably would have continued feeding on those eggs had exposure periods been longer. When exposure periods ended, ≥1 egg was missing from 21 (88%) of 24 nest sites with a depredated clutch. All eggs were missing from 12 (50%) sites. A few shell fragments were found at some nests.

We observed several instances of a black-billed magpie in flight and carrying a duck egg in its bill. Black-billed magpies customarily picked up a duck egg by first piercing it, and inserting a mandible to grasp the shell. However, we once observed a black-billed magpie fly and carry an intact duck egg (13 x 55 mm) 13 m and then hide the egg in vegetation. We recovered the egg and found it had no puncture marks or puncture holes. Jones (1958:68) reported that, "The magpie [black-billed magpie] carries eggs [of mallard and ring-necked pheasant (Phasianus colchicus)] a short distance before plugging [pecking open] them and eating the partially incubated chick and other foods inside the eggshell."

Appearance of Eggshells and Nest Sites--Blackbilled magpies customarily opened duck eggs by first pecking a small hole in the side or a side-end. They enlarged the hole to a narrow slit, then made a round opening (Appendix C, Fig. 9). Most of 23 eggshells examined for damage by magpies had small (39%) or large (57%) holes; only 1 (4%) was severely damaged (Appendix B, Table 3). There occasionally was a puncture hole near the main opening (Appendix C, Fig. 9c).

Most of 12 eggshells examined for location of openings made by black-billed magpies had the opening in the side (58%) or a side-end (33%); only 1 (8%) had the opening in an end (Appendix B, Table 4). There was little inward caving of shell pieces.

We found no evidence of black-billed magpies digging at nests. The proportion of nest material displaced by magpies from each of 24 artificial nests that were depredated ranged from 5 to 100%, and was >30% at 19 (79%) nests (Appendix B, Table 5). Magpies often dropped and/or scattered with their bill tufts of nest material, which became entangled on erect vegetation.

Most (65%) of 23 eggshells found at nests depredated by black-billed magpies were ≤20 cm from the nest (including in nest), but 7 (30%) were >1 m from the nest (Appendix B, Table 6). Conspicuous yolk residue usually was present in eggshells.

Conclusions--Black-billed magpies probably destroy completely few duck nests but may partially depredate numerous nests. Evidence indicating a nest was destroyed by black-billed magpies includes finding at the nest most or all of the following: (1) ≥1 whole egg, (2) eggshells of <50% of clutch or no eggshells, (3) trace of shell fragments or no shell fragments, (4) most eggshells have small or large holes, (5) conspicuous yolk residue in most eggshells, and (6) aerially displaced nest material (Appendix B, Table 2).

Evidence indicating a nest was not destroyed by black-billed magpies includes finding at the nest any of the following: (1) the hen was killed, (2) cached egg, (3) eggshells of ≥50% of clutch, (4) shell fragments that equal that of ≥1 egg, (5) dug area, and (6) no conspicuous egg content in any eggshell (Appendix B, Table 2).