In recent decades, earlier and more frequent harvests of agricultural grasslands have been implicated as a major cause of population declines in grassland songbirds. From 2002 to 2005, in the Champlain Valley of Vermont and New York, USA, we studied the reproductive success of Savannah Sparrows (Passerculus sandwichensis) and Bobolinks (Dolichonyx oryzivorus) on four grassland treatments: (1) early-hayed fields cut before 11 June and again in early- to mid-July; (2) middle-hayed fields cut once between 21 June and 10 July; (3) late-hayed fields cut after 1 August; and (4) rotationally grazed pastures. Both the number of fledglings per female per year and nest success (logistic-exposure method) varied among treatments and between species. Although birds initiated nests earlier on early-hayed fields compared to others, haying caused 99% of active Savannah Sparrow and 100% of active Bobolink nests to fail. Both the initial cutting date and time between cuttings influenced renesting behavior. After haying, Savannah Sparrows generally remained on early-hayed fields and immediately renested (clutch completion 15.6 +/- 1.28 days post-haying; all values are reported as mean +/- SE), while Bobolinks abandoned the fields for at least two weeks (mean clutch completion 33 +/- 0.82 days post-haying). While female Savannah Sparrows fledged more offspring per year (1.28 +/- 0.16) than female Bobolinks (0.05 +/- 0.05), reproductive success on early-hayed fields was low. The number of fledglings per female per year was greater on middle-hayed fields (Savannah Sparrows, 3.47 +/- 0.42; Bobolinks, 2.22 +/- 0.26), and late-hayed fields (Savannah Sparrows, 3.29 +/- 0.30; Bobolinks, 2.79 +/- 0.18). Reproductive success was moderate on rotationally grazed pastures, where female Savannah Sparrows and female Bobolinks produced 2.32 +/- 0.25 and 1.79 +/- 0.33 fledgling per year, respectively. We simultaneously conducted cutting surveys throughout the Champlain Valley and found that 3-8% of hayfield habitat was cut by 1-4 June, 25-40% by 12-16 June, and 32-60% by 28 June-2 July. Thus, the majority of grassland habitat was cut during the breeding season; however, late-hayed fields served as high-quality reserves for late-nesting female Bobolinks that were displaced from previously hayed fields. For fields first cut in May, a 65-day interval between cuts could provide enough time for both species to successfully fledge young.

Summary1. Food availability has been considered one of the most important factors limiting bird populations, yet the eects of food abundance on non-breeding insectivores has rarely been investigated. We studied the eects of food abundance on the body condition of ovenbirds (Seiurus aurocapillus L.), a sexually monomorphic, groundforaging, Neotropical±Nearctic migrant warbler during two winters in three habitats on their wintering grounds in Jamaica. 2. Prey biomass decreased from early to late winter in all habitats. Concurrently, ovenbird body mass, corrected for dierences in structural size, decreased signi®-cantly in ®ve of six habitat±year combinations. Only in second growth scrub habitat in the 1995±96 winter did ovenbirds show no decrease in corrected body mass, and during this period there was no signi®cant decrease in ant biomass, the dominant prey item of ovenbirds in all habitats. 3. Signi®cant positive correlations were found between ovenbirds' rate of feather regrowth and the biomass of ants on their home ranges in early winter, and between overwinter change in ovenbird body mass and the biomass of ants on their home ranges in late winter. 4. The results of both the habitat-and home-range-based analyses suggest a similar threshold of ant biomass (2Á5±3Á0 mg m À2 ), below which ovenbirds did not maintain their body mass. 5. The results suggest that late winter rainfall mediates the biomass of prey abundance, which in turn aects the ovenbirds' overwinter body condition.

. Insectivorous birds have been shown to have direct effects on abundances of herbivorous arthropods, but few studies have tested the indirect effects of birds on plant performance through consumption of herbivorous insects. In a 3-year study at the Hubbard Brook Experimental Forest, New Hampshire, we tested whether bird predation indirectly affects leaf herbivory levels and leaf and shoot biomass production of understory sugar maple (Acer saccharum) saplings. Trees were randomly assigned to one of four treatments: an insecticide application to reduce herbivory levels, exclosures that prevented bird access, addition of Lepidoptera larvae, and controls. Trees sprayed with an insecticide supported significantly fewer Lepidoptera larvae than other treatments throughout the study. Also, trees in exclosures supported more Lepidoptera larvae than controls during one count each year, and pooled across all counts during the second year. As predicted, the mean proportion of leaf area consumed varied significantly among treatments and was least in the insecticide treatment, followed by controls, exclosures, and Lepidoptera additions. Significant differences among treatments in herbivory levels, however, did not lead to differences in leaf or shoot biomass production. Thus, bird predation decreased Lepidoptera abundances and decreased herbivory levels, but did not increase biomass production during the following year. Over 85% of the herbivores in our study were Homoptera nymphs that were not folivorous and are not important bird prey items, potentially dampening the indirect effects of bird predation on biomass production. A comparison of these results with previous studies suggests that the indirect effects of bird predation on plant biomass production may depend on the plant species, abundance and composition of the herbivore community, and primary productivity of the ecosystem.

Population growth and decline are particularly sensitive to changes in three key life-history parameters: annual productivity, juvenile survival, and adult survival. However, for many species these parameters remain unknown. For example, although grassland songbirds are imperiled throughout North America, for this guild, only a small number of studies have assessed these parameters. From 2002 to 2006, in the agricultural landscape of the Champlain Valley of Vermont and New York, USA, we studied Savannah Sparrow (Passerculus sandwichensis) and Bobolink (Dolichonyx oryzivorus) demography on four grassland treatments: (1) early-hayed fields cut before 11 June and again in early- to mid-July; (2) middle-hayed fields cut once between 21 June and 10 July; (3) late-hayed fields cut after 1 August; and (4) rotationally grazed pastures. We assessed whether these treatments affected adult apparent survival (phi) and recruitment (f), how sensitive these parameters were to the presence of nonbreeders and local dispersal, and the populations' ability to persist in these four habitats. On average, birds using late-hayed fields had > 25% higher apparent survival than those on the more intensively managed early-hayed, middle-hayed, and grazed fields. Overall male phi was 35% higher than female phi, and Savannah Sparrow phi was 44% higher than Bobolink phi. Across all analyses and treatments, apparent survival estimates were 0.58-0.85 for male and 0.48-0.71 for female Savannah Sparrows, and 0.52-0.70 for male and 0.19-0.55 for female Bobolinks. For males of both species, potential nonbreeders decreased the precision of and lowered apparent survival estimates by 25%; female estimates showed little variation with the inclusion of nonbreeders. Inclusion of local dispersal observations increased apparent survival estimates and, in many cases, increased precision, though the effect was stronger for Savannah Sparrows than for Bobolinks, and also stronger for males than for females. High Savannah Sparrow apparent survival rates resulted in stable or near stable populations (lambda approximately 1), particularly in late-hayed and grazed fields, while low Bobolink apparent survival rates resulted in strongly declining populations (lambda < 1) in all treatments.

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