Variation in Breeding Season and Clutch Size in Song Sparrows of the Pacific Coast

Richard F. Johnston

doi:10.2307/1364850

How do Brown-Headed Cowbirds (Molothrus Ater) CAUSE NEST FAILURES IN SONG SPARROWS (MELOSPIZA MELODIA)? A REMOVAL EXPERIMENT

The Auk ◽

10.1093/auk/120.3.772 ◽

2003 ◽

Vol 120 (3) ◽

pp. 772-783

Author(s):

James N. M. Smith ◽

Mary J. Taitt ◽

Liana Zanette ◽

Isla H. Myers-Smith

Keyword(s):

Clutch Size ◽

Breeding Season ◽

Molothrus Ater ◽

Melospiza Melodia ◽

Removal Experiment ◽

Song Sparrow ◽

Failure Rates ◽

Nest Failure ◽

Song Sparrows ◽

Egg Removal

Abstract A removal experiment was conducted to measure how much and by what mechanisms brood parasitic Brown-headed Cowbirds (Molothrus ater) cause nest failures in a commonly used host, the Song Sparrow (Melospiza melodia). When numbers of female cowbirds were reduced experimentally, nest failures fell from 65.0% (n = 663 nests) to 49.9% (n = 331). Cowbird reduction reduced the frequency of nest failure to one-third of control levels in Song Sparrows during the last 80 days of the sparrow's breeding season, the period when most parasitic laying took place. Cowbird reduction decreased nest failures strongly at the egg stage, and weakly at the nestling stage. Daily nest-failure rates were independent of whether or not a nest was parasitized by cowbirds. Two hypotheses were tested to explain how cowbirds cause host nests to fail: first, egg removal by female cowbirds lowers clutch size below a threshold where the host deserts; second, cowbirds cause host nests to fail by destroying entire clutches or broods. In support of the first hypothesis, desertion following parasitism and egg removal was less frequent when cowbird numbers were reduced (8.9% of n = 158 nests) than for unmanipulated controls (16.5% of n = 424 nests). In support of the second hypothesis, there were fewer cases where young were killed in the nest, or found dead near it, after cowbird numbers were reduced (2.5% of 158 nests) than in controls (4.7% of 424 control nests). In contrast, proportions of nests that failed after the disappearance of all eggs, young, or both, and after unparasitized clutches were deserted, increased when cowbird numbers were reduced. Although our study supports both hypotheses, cowbird-induced desertion had a greater effect on nest failure rates than did cowbird predation. Our study suggests that cowbird removal programs are likely to benefit commonly used and endangered hosts by reducing rates of nest failure.

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Dietary segregation between two cohabiting species of sparrows revealed with stable isotope analysis

Canadian Journal of Zoology ◽

10.1139/cjz-2012-0103 ◽

2013 ◽

Vol 91 (1) ◽

pp. 37-40

Author(s):

J.M. Hipfner ◽

B. Addison ◽

M.R. Charette

Keyword(s):

Stable Isotope ◽

Stable Isotope Analysis ◽

Pacific Coast ◽

Isotope Analysis ◽

Song Sparrow ◽

Habitat Types ◽

Plant Matter ◽

Song Sparrows ◽

The Pacific ◽

Passerella Iliaca

Fox Sparrows (Passerella iliaca (Merrem, 1786)) and Song Sparrows (Melospiza melodia (A. Wilson, 1810)) cohabit on many islands along the Pacific coast of North America, and previous studies suggest that they rely on similar prey types. We used δ13C and δ15N stable isotope analysis on blood collected from breeding adults of both species in each of two habitats on Triangle Island, British Columbia, Canada, to test the hypothesis that the two species exhibit a consistent pattern (direction) of dietary segregation in different habitat types. Both δ13C and especially δ15N values differed between habitats, indicating that the two habitats were isotopically distinct. As predicted, δ15N values differed consistently between the two species in the two habitats, averaging ∼1.5‰ higher in the smaller Song Sparrow than in the larger Fox Sparrow in both. We infer that Song Sparrows included more animal matter and less plant matter in their diets than Fox Sparrows, and suggest that fixed traits related to body size might underlie the dietary (trophic) differences. In contrast, δ13C values did not differ between species. We conclude that dietary segregation could help to facilitate the widespread cohabitation of these two species of sparrows.

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The Philological Association of The Pacific Coast

PMLA ◽

10.1017/s0030812900130597 ◽

1935 ◽

Vol 50 (4) ◽

pp. 1373-1374

Keyword(s):

Annual Meeting ◽

Pacific Coast ◽

Stanford University ◽

The Pacific

The thirty-seventh annual meeting of the Philological Association of the Pacific Coast was held at Stanford University, California, on November 29 and 30, 1935.

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