Factors associated with dusky Canada goose nesting and nest success on artificial nest islands of the western Copper River Delta Public Deposited

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  • The population of dusky Canada geese (Branta canadensis occidentalis; hereafter, dusky geese) nesting on the western Copper River Delta (CRD) in south- central Alaska has been in decline since the late 1970s. In an effort to alleviate mammalian predation, increase nest success, and avoid a listing under the U.S. Endangered Species Act, an artificial nest island (island) program was implemented on the western CRD in 1983. The installation of new islands on the CRD is the sole management action of the Pacific Flyway Council on the breeding grounds, but no comprehensive evaluation of the program has been published. I examined general trends in island use and nest success over time for three island types (donut islands, fiberglass floater islands, and sandbag islands) from 1984-2005. I used data from the island program to identify factors associated with dusky Canada goose nesting (hereafter, use) and nest success on islands from 1996-2005. I generated a series of candidate models and used logistic regression with model selection techniques to determine how variables representing pond characteristics, vegetative characteristics, interactions with conspecifics and larid species, the previous year’s island status, and the distance to predator corridors were associated with island use and nest success for each year. Use of islands by dusky geese nesting on the western CRD increased between 1987 and 2005 from a low of 10% in 1987 to 44% in 2005. There was annual variability in factors associated with island use; however, use of islands was most consistently and strongly associated with the previous year’s island status. The odds of a nest being placed on an island that contained a successful nest the previous year were 2.91 to 6.62 times greater than for islands not used the previous year. There was also evidence for an increased likelihood of island use further from shore and for islands with up to 55% aerial shrub cover and shrubs up to 1 m tall. Nest success was consistently high, indicating islands have long term potential for increasing dusky goose nest success on the Copper River Delta. Compared to island use, my analysis of nest success and habitat features resulted in many more competitive models (average 10 v. 2 per year) with fewer strong (95% confidence intervals for parameter estimates excluded zero) associations between explanatory variables and nest success. Explanatory variables representing vegetative characteristics, interactions with conspecifics and larid species, and the distance to predator corridors had a relatively low capacity to explain nest success on artificial islands. However, there was some evidence for an increased likelihood of nest success on islands further from shore with shorter shrubs. Lower variability in factors associated with island use is likely because factors associated with nest site selection are controlled by decisions of the nesting goose. The importance of previous year’s island status on island use likely reflects high site fidelity by breeding adult geese, with fidelity being higher for birds that have successful nests. Alternately, dusky geese use cues from the previous year’s nesting activity when prospecting for island nest sites. In contrast, nest success is dependent on a more complex combination of factors that vary within and among years outside the control of a nesting goose. Inter-annual variation in the timing and magnitude of eulachon (Thaleichthys pacificus) spawning runs on large sloughs of the CRD and prey-switching by important dusky goose nest predators may have considerable influence on dusky goose nest success on islands, similar to mainland- nesting dusky geese. The inability to control for eulachon presence and abundance may have confounded my analysis of habitat features important to nest success. Therefore, factors such as presence of alternate prey (and predator abundance) may be more important to dusky goose nest success on islands than habitat features. Current use of available nest islands is at a program high of 44% and nest success also remains high (63%); suggesting the artificial nest island program is valuable for dusky geese. However, the contribution of island-nesting dusky geese to the dusky goose population on the Copper River Delta is unclear. There is a need for a population model that incorporates all recent information on dusky goose reproductive ecology to determine if artificial nest islands can increase dusky goose population size, and if so, how many islands are needed.
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