Publication: Population Abundance and Habitat Requirements of the Marsh Wren (Cistothorus palustris) at Dyke Marsh Wildlife Preserve: An Urban Conservation Challenge
Date
2000-12
Authors
Spencer, Sandy C.
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Abstract
In 1998 and 1999, I estimated the abundance of a population of marsh wrens ( Cistothorus palustris) at Dyke Marsh National Wildlife Preserve based on number of territories. In addition, I estimated the reproductive success based on number of fledglings. For 1999 only, the percent of available habitat used; male and female habitat selection preferences; the variation in territory quality between successful and failed nests; causes of nest failure; and use of alternative vegetation for nest sites were investigated. In 1999, I also surveyed other marshes in the upper tidal zone of the Potomac to verify whether Dyke Marsh contains the only population of marsh wrens in this area. The differences between territories and non-territories for male selection preferences based on seven habitat variables. Nine habitat variables were used to measure differences between breeding and non-breeding territories and between successful and failed nests. Differences between groups were analyzed using non-parametric tests. Reproductive success was based upon actual number of fledglings.
Thirty-one territories were located in 1998, and 34 in 1999. The population estimate was 38 (31 territorial males and 7 breeding females) in 1998, and 48 (34 territorial males and 14 breeding females) in 1999. This number excludes roving bachelors, and second or third mates of polygynous males. The discrepancy in the female number between years may reflect insufficient manpower to discover all breeding nests in 1998. Reproductive success for 1998 was nine young fledged; for 1999, eleven young fledged. Only 30% percent of available habitat was used by marsh wrens in 1999.
Absolute density of tall, emergent vegetation (P = 0.003 ) and proximity to red-winged blackbirds (P = 0.011) were the most significant habitat variables driving male selection preferences. Percent Typha spp. was a marginally sigificant factor (P = 0.089) for female selection. No significant difference was detected between successful (n=S} and failed (n=11) nests in 1999 for any of the variables measured. The majority (79%) of nest failure is due to predation by conspecifics, or interspecific birds, snakes, small mammals. Only four territories used non-cattail vegetation for nest attachment but none of the nests attached to alternative species became breeding nests. A survey of similar tidal marshes in the upper Potomac tidal zone revealed that there are no other breeding populations of marsh wrens in this area.
Because the local population of marsh wrens has very narrow preferences for nesting territories, has experienced substantial habitat loss over the last few decades, are subject to high rates of predation, and do not appear to be very adaptable to alternative forms of vegetation, the population could be at risk over the long term.