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.

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No abstract

The information center hypothesis (ICH) suggests that birds breed in colonies because this behavior favors information exchange at the colony about the location of food patches. However, the complex suite of behaviors the ICH requires implies that information center following is more likely to evolve after colonial breeding has become established than to promote its initial development. A simpler hypothesis to explain the evolution of colonial breeding is that coloniality concentrates foragers in space, which leads to more rapid discovery of food patches and, by means of local enhancement, more efficient transfer of information about the location of patches than if foragers bred in a dispersed fashion. To assess the effects of breeding dispersion on foraging success, I simulated the foraging behavior of cliff-breeding seabirds (nesting either solitarily or colonially) searching for patchily distributed prey. Results show that colonial breeding is favored when food patches are sufficiently large or short-lived that competition for food is ameliorated. Conversely, dispersed nesting is favored when patches are small or long-lived. Individuals playing a colonial breeding strategy can invade a population of solitarily breeding birds, and once a colonial breeding strategy becomes established, it generally is resistant to invasion. These findings suggest that the spatial-concentration model is a plausible mechanism for the initial development of coloniality.