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Age-related differences in diet and foraging behavior of the critically endangered Mariana Crow (Corvus kubaryi), with notes on the predation of Coenobita hermit crabs

  • Sarah K. FaegreEmail author
  • Lindsey Nietmann
  • Phillip Hannon
  • James C. Ha
  • Renee R. Ha
Original Article

Abstract

Knowledge of foraging behavior across life stages of endangered species is important for identifying potential drivers of age-dependent mortality. Juvenile mortality is a primary threat to the persistence of the single remaining Mariana Crow (Corvus kubaryi) population, which is found on Rota, Commonwealth of the Northern Mariana Islands. Therefore, variation in foraging behavior among fledglings, sub-adults and adults may highlight different needs or susceptibilities that could inform age-specific management strategies. During observations of 36 Mariana Crows, we found that of all food captures, 14% were plant-based, 56% were insects or their larvae and eggs and 30% were non-insect animal prey. Two food categories, fruits/seeds/plants and ants/termites/larvae, which were procured and processed with simple behaviors, were taken more frequently by fledglings. Crabs, which were processed using complex behaviors, were captured more frequently by adults. Adults acquired more food items from the ground than did fledglings and sub-adult birds, a result that was driven by the former’s high level of crab predation. We did not detect differences in foraging behavior between wet and dry seasons, suggesting that Mariana Crows maintain a similar diet year-round. Overall, our results highlight age-related differences in foraging behavior; however, future studies should identify whether these differences drive age-dependent variation in survivorship. Finally, we suggest that complex trophic interactions between non-native snails and Coenobita hermit crabs may have modified Mariana Crow foraging behavior, increasing their vulnerability to feral cat predation.

Keywords

Coenobita Anti-predator vigilance Endangered species Corvid Feral cat 

Zusammenfassung

Altersabhängige Unterschiede in der Ernährung und dem Nahrungssuchverhalten bei der stark gefährdeten Guamkrähe ( Corvus kubaryi ) unter Berücksichtigung der Prädation der Landeinsiedlerkrebs-Gattung Coenobita

Kenntnisse über das Nahrungssuchverhalten verschiedener Lebensstadien bei gefährdeten Arten sind notwendig, um potentielle Einflussfaktoren auf die altersabhängige Mortalität zu identifizieren. Die Sterblichkeit von Jungtieren ist eine der größten Bedrohungen für den Fortbestand der einzig verbliebenen Population an Guamkrähen (Corvus kubaryi) auf der Insel Rota, Commonwealth der Nördlichen Marianen. Unterschiede im Nahrungssuchverhalten zwischen flüggen Jungvögeln, subadulten und adulten Vögeln könnten verschiedene Ansprüche oder Gefährdungen hervorheben, die dann in altersabhängige Managementstrategien einfließen könnten. Während der Untersuchung von 36 Guamkrähen fanden wir heraus, dass 14% der aufgenommenen Nahrung pflanzlich war, 56% aus Insekten oder deren Larven und Eiern bestand und 30% aus anderer tierischer Beute. Die zwei Nahrungskategorien Früchte/Saaten/Pflanzen und Ameisen/Termiten/Larven, welche auf einfache Art und Weise zu beschaffen und bearbeiten sind, wurden häufiger von flüggen Jungvögeln aufgenommen. Krebse, dessen Handhabung eines komplexen Verhaltensmusters bedarf, wurden häufiger von adulten Vögeln gefangen. Adulte Vögel sammelten im Vergleich zu den anderen Altersklassen mehr Nahrung vom Boden, ein Ergebnis, dass vermutlich aufgrund des hohen Krebsanteils in der Beute zustande kam. Wir konnten keine Unterschiede im Nahrungssuchverhalten zwischen Regen- und Trockenzeiten entdecken, was auf eine ganzjährig gleichbleibende Ernährung der Guamkrähe schließen lässt. Zusammenfassend zeigen unsere Ergebnisse altersabhängige Unterschiede im Nahrungssuchverhalten auf. Jedoch sollten zukünftige Studien herausfinden, ob diese Unterschiede die altersabhängigen Variationen in der Überlebensrate verursachen. Schließlich gehen wir davon aus, dass komplexe trophische Interaktionen zwischen nicht einheimischen Schnecken und den Landeinsiedlerkrebsen der Gattung Coenobita das Nahrungssuchverhalten der Guamkrähe verändert haben, was zu einer Zunahme der Gefährdung der Krähen durch verwilderte Hauskatzen geführt hat.

Notes

Acknowledgements

We thank the numerous field biologists that assisted in radio-tracking crows for this study: Aaron Wuori, Andria Kroner, Brette Soucie, Calypso Gagorik, Colin Duncan, Cyrus Moqtaderi, Dacia Wiitala, Elizabeth Kain, Emily Cook, Evan Rehm, Gabrielle Robinson, Heather Brown, Henry Fandel, Hillary Henry, Jen Carpenter, Jen Wilcox, Jose Antonio Diaz, Kelle Urban, Kelsey McKune, Laura Bussolini, Lena Ware, Lydia Goy, Marissa Buschow, Matt Henschen, Mike Hitchcock, Rumaan Malhotra, Samantha Lantz, Scott Moore, Sean Jeffreys, Sarena Olson, Sinead Borchert and Steve Seibel. We thank Manny Pangelinan, Tony Benavente and Richard B. Seman for their support of our work in the Marianas and Julia Boland, Fred Amidon, Shelly Kremer, Megan Laut, Annie Marshal, Sheldon Plentovich, Paul Radley, and Lainie Berry for additional logistical and permitting support. We thank Michael Beecher, Aaron Wirsing and Aron Faegre for helpful comments on earlier drafts of this manuscript.

Funding

This work was supported by a National Science Foundation Graduate Research Fellowship to S.F.; it was also supported by the Commonwealth of the Northern Mariana Islands Department of Lands and Natural Resources-Division of Fish and Wildlife from U.S. Fish and Wildlife Section 6 Endangered Species funds, grants F14AF00120, F14AF01145, F15AF01118. Work was conducted in accordance with Federal Fish and Wildlife Permit TE09155B-0 and TE09155B-1, Federal Bird Marking and Salvage Permit 22802 and University of Washington Institutional Animal Care and Use Committee protocol number 2858-04.

Supplementary material

10336_2019_1705_MOESM1_ESM.mp4 (57.7 mb)
Supplementary material 1 (MP4 59,081 kb)
10336_2019_1705_MOESM2_ESM.mp4 (24.1 mb)
Supplementary material 2 (MP4 24,652 kb)

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Copyright information

© Deutsche Ornithologen-Gesellschaft e.V. 2019

Authors and Affiliations

  1. 1.Department of PsychologyUniversity of WashingtonSeattleUSA
  2. 2.San Diego Zoo Institute for Conservation ResearchEscondidoUSA
  3. 3.Rota Avian Behavioral Ecology ProgramRotaUSA

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