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Reproductive ecology and population viability of Brewer’s Sparrows at the northern edge of the breeding range Mahony, Nancy Anne

Abstract

I examined the population dynamics of the provincially red-listed Brewer's Sparrow (Spizella breweri breweri) of British Columbia. This declining grassland bird reaches the northwestern limit of its breeding range in the province. My objectives were: 1) to explore spatial and temporal dynamics in fecundity and survival, 2) to model population viability based on these rates, 3) to examine the relative influences of vital rates on the population trend, and 4) to explore the relationship between the selection of nest site habitat characteristics and nest success. I found that seasonal fecundity was highest when the nest predation rate was low, the number of clutches/female was high and when breeding began later in the season. High seasonal fecundity alternated between the sites. The best site in 1998 became the least productive in 1999 and vice-versa, and a third site was most productive in 2000. The over-riding factor driving spatial and temporal variation in productivity was shifting rates of nest predation. However, elevation-related storm effects when breeding began early, and variation in the number of clutches laid, were partly responsible for this variation. Adult female survival varied between years, from a high of 66% in 1997-1998, an El Nino year, to a low of 26% from 1998-1999, a La Nina year. Survival did not vary between sites. A population viability model based on these demographic rates predicted that the population will decline to extinction within 100 years without immigration. For a bestcase scenario where mean adult survival was high and years of low survival occurred every 10 years, population growth rate (k) = 0.93. For a worst-case scenario where mean adult survival was low and years of low survival occur at random, X=0JS. Sensitivity analysis showed that the population growth rate was most sensitive to adult survival. Brewer's Sparrows selected nesting habitat that concealed nests. However, habitat variables related to concealment did not differ strongly between successful and depredated nests. Selected nest sites may not be the most successful because predation risk varies at larger spatial scales, and because the several generalist predators with differing search strategies cover all the possible safe havens.

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