Allan M. Strong

Grassland Songbirds In A Dynamic Management Landscape: Behavioral Responses And Management Strategies

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.

Bird predation on herbivorous insects: indirect effects on sugar maple saplings

Abstract. 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.

Divergent foraging strategies of two neotropical migrant warblers : Implications for winter habitat use

Abstract To address whether foraging strategies affect habitat-use patterns of nonbreeding warblers, I quantified foraging behavior, bill dimensions, and diet (based on regurgitation samples) of Ovenbirds (Seiurus aurocapillus) and Swainsons Warblers (Limnothlypis swainsonii) wintering in three habitats in Jamaica. Ovenbirds primarily gleaned prey from the surface of the leaf litter (95% of foraging maneuvers), resulting in a diet comprised predominantly of ants (62% of all prey items), seeds (18%), and beetles (9%). Swainsons Warblers foraged by lifting leaves (80% of foraging maneuvers), resulting in a significantly different diet dominated by beetles (39%), spiders (22%), and ants (19%). More than 60% of the regurgitation samples from Swainsons Warblers contained orthopterans and/or gecko (Sphaerodactylus goniorhynchus) bones. Averaged across all habitat types, Ovenbirds consumed ants in direct proportion to their abundance based on visual counts of arthropods. Swainsons Warblers consumed beetles more than expected based on the abundance of beetles in visual counts and Berlese funnels. The use of a diversity of habitats by Ovenbirds may be related to their ability to feed opportunistically on ants, which are a widespread, abundant, and reliable resource. In contrast, based on their foraging behavior and diet, Swainsons Warblers may be restricted to habitats with a well-developed canopy and an abundant subsurface leaf-litter fauna.

Habitat Use Patterns of Bobolinks and Savannah Sparrows in the Northeastern United States

In the northeastern United States, grassland birds regularly use agricultural fields as nesting habitat. However, birds that nest in these fields regularly experience nest failure as a result of agricultural practices, such as mowing and grazing. Therefore, information on both spatial and temporal patterns of habitat use is needed to effectively manage these species. We addressed these complex habitat use patterns by conducting point counts during three time intervals between May 21, 2002 and July 2, 2002 in agricultural fields across the Champlain Valley in Vermont and New York. Early in the breeding season, Bobolinks (Dolichonyx oryzivorus) used fields in which the landscape within 2500 m was dominated by open habitats. As mowing began, suitable habitat within 500 m became more important. Savannah Sparrows (Passerculus sandwichensis) initially used fields that contained a high proportion of suitable habitat within 500 m. After mowing, features of the field (i.e., size and amount of woody edge) became more important. Each species responded differently to mowing: Savannah Sparrows were equally abundant in mowed and uncut fields, whereas Bobolinks were more abundant in uncut fields. In agricultural areas in the Northeast, large areas (2000 ha) that are mostly nonforested and undeveloped should be targeted for conservation. Within large open areas, smaller patches (80 ha) should be maintained as high-quality, late-cut grassland habitat. RÉSUMÉ. Dans le Nord-est des États-Unis, les oiseaux de prairie nichent régulièrement dans les champs agricoles. Cependant, la nidification de ces oiseaux échoue souvent en raison des activités agricoles, comme le fauchage et le broutement. Il s’avère donc nécessaire de connaître les caractéristiques spatio-temporelles relatives à l’utilisation de l’habitat pour gérer efficacement ces espèces. Nous avons examiné ces patrons complexes de l’utilisation de l’habitat au moyen de dénombrements par points d’écoute effectués à trois périodes entre le 21 mai et le 2 juillet 2002, dans des champs agricoles de la vallée de Champlain dans les États du Vermont et de New York. Tôt dans la saison de nidification, les Goglus des prés (Dolichonyx oryzivorus) ont utilisé des champs situés dans des paysages où les milieux ouverts dominaient dans un rayon de 2 500 m. Lorsque le fauchage a commencé, l’habitat propice dans un rayon de 500 m est devenu plus important. Les Bruants des prés (Passerculus sandwichensis) ont utilisé des champs qui offraient un habitat propice dans un rayon de 500 m dès le début de la saison. Une fois le fauchage terminé, les caractéristiques des champs (c.-à-d. la dimension et la quantité de lisières boisées) sont devenues davantage déterminantes. Les deux espèces ont agi différemment face au fauchage : les bruants ont utilisé les champs fauchés et les champs non fauchés de façon égale, tandis que les goglus étaient plus nombreux dans les champs n’ayant pas subi de fauchage. Dans les zones agricoles du Nord-est, les vastes régions (2 000 ha) University of Vermont, The Rubenstein School of Environment and Natural Resources, U.S. Geological Survey, Vermont Cooperative Fish and Wildlife Research Unit Avian Conservation and Ecology 5(2): 11 http://www.ace-eco.org/vol5/iss2/art11/ qui ne sont ni boisées ni développées devraient être considérées à des fins de conservation. Dans les grands milieux ouverts, des parcelles plus petites (80 ha) – pour lesquelles le fauchage serait retardé – devraient être conservées en tant que milieux de prairie de qualité supérieure.

Effects of Hay Cropping on Invertebrate Biomass and the Breeding Ecology of Savannah Sparrows (Passerculus Sandwichensis)

Abstract In hay fields in the northeastern United States, Savannah Sparrows (Passerculus sandwichensis) whose first nests fail as a result of the harvesting of hay renest in the same field in a dramatically altered habitat. We evaluated availability of invertebrate prey in two treatments (harvested and unharvested hay fields) in Vermonts Champlain Valley and assessed potential effects of food resources on clutch size, food provisioning by adults, and growth of nestling Savannah Sparrows. A relative measure of invertebrate biomass (sweep-net samples) showed a 36–82% decline after harvesting, compared with continual increases throughout the nesting season on unharvested fields. Clutch sizes were not significantly different between treatments. Nestling provisioning rates obtained through video observations differed between treatments, with birds on unharvested fields delivering 73% more food than those on harvested fields (P = 0.02). However, food provisioning differences did not translate into differences between treatments in either average nestling mass or the mass of the lightest nestlings within nests. A measure of total biomass (vacuum samples) on harvested fields showed a 28–56% decline after harvesting; this reduction was insufficient to induce food limitation based on the energetic requirements of Savannah Sparrows. Our results suggest that adult Savannah Sparrows must compensate for reduced food availability on harvested hay fields, possibly by increasing the total time spent foraging.

A Model for Integrating Wildlife Science and Agri-Environmental Policy in the Conservation of Declining Species

ABSTRACT We examined a case study where a successful wildlife-friendly model for intensively managed hayland was developed from field data and implemented locally as policy by a federal agency. Farmers were ensured a first hay-harvest with high protein content; after a 65-day delay (compared to the normal 35–40-day cutting cycle) farmers took a second harvest of greater quantity but decreased quality. Farmers were paid

Natal and breeding dispersal of Bobolinks (Dolichonyx oryzivorus) and Savannah Sparrows (Passerculus sandwichensis) in an agricultural landscape.

247–333/ha in 2008–2010 to offset costs associated with the decreased nutritional content caused by the approximately 25-day second harvest delay. Bobolink (Dolichonyx oryzivorus) reproductive rates improved from 0.0 to 2.8 fledglings per female per year. Creation and implementation of this policy required communication among scientists, federal agricultural agencies, farmers, and state and federal fish and wildlife departments. Data collection, analyses, and communication processes served as an effective global model for practitioners to apply to other agricultural products and taxa.

Agricultural management affects evolutionary processes in a migratory songbird

ABSTRACT. Dispersal is a key process in the metapopulation dynamics and genetic structure of spatially segregated populations. However, our knowledge of avian dispersal, particularly in migratory passerines, remains limited. We studied dispersal of Bobolinks (Dolichonyx oryzivorus) and Savannah Sparrows (Passerculus sandwichensis) to determine whether agricultural management practices affected dispersal patterns and habitat selection. From 2002 to 2006, we banded adults and nestlings on six focal hay fields and two pastures in the Champlain Valley of Vermont and New York and searched for banded birds within 1.5 km of Vermont field sites during two years. Natal dispersal distances were greater than breeding dispersal in both species, and breeding dispersal distances of Bobolinks were greater than those of Savannah Sparrows. Site fidelity was high in both species, with >80% of detected adults and ∼30% of detected natal dispersers returning to the same field in subsequent years. During natal dispersal, movement was random with respect to habitat quality. Adult Bobolinks dispersed to fields with annual reproductive rates greater than or equal to those of their original field; by contrast, adult Savannah Sparrows were more likely to move to or remain in low-quality habitats. During breeding dispersal, strong site fidelity took precedence over the effect of the previous years nest success on the probability of dispersal, particularly for Savannah Sparrows. Site fidelity has implications for management of agricultural fields because consistency of cropping patterns and cutting dates are important for maintaining populations of these species. RESUMEN. La dispersión es un proceso clave que afecta la dinámica de metapoblaciones y la estructura genética de poblaciones segregadas espacialmente. Sin embargo, nuestro conocimiento de la dispersión de aves es limitado, particularmente para pájaros migratorios paserinos. Estudiamos la dispersión de Dolichonyx oryzivorus y Passerculus sandwichensis para determinar si las prácticas de manejo agrícola influyen sobre los patrones de dispersión y selección de hàbitat. De 2002 a 2006, marcamos adultos y polluelos en seis campos de heno y dos potreros en el valle Champlain de Vermont y Nueva York, EE.UU., y buscamos pájaros marcados en un radio de 1.5 km alrededor de los campos de Vermont durante dos años. Las distancias de dispersión natal fueron mayores que las de dispersión reproductiva en las dos especies, y las distancias de dispersión reproductiva de D. oryzivorus fueron mayores que las de P. sandwichensis. La fidelidad al territorio fue alta en las dos especies, debido a que más del 80% de adultos encontrados y aproximadamente el 30% de los dispersores natales volvieron al mismo campo en años posteriores. El movimiento en la dispersión natal fue aleatorio con respecto a la calidad del h´bitat. Los adultos D. oryzivorus se dispersaron a otros campos con tasas reproductivas anuales mayores o iguales a las de su campo original; en contraste, fue más probable que los adultos P. sandwichensis se movieran o se quedaran en hàbitats de baja calidad. En la dispersión reproductiva, el efecto que el éxito reproductivo en años anteriores tuvo sobre la probabilidad de dispersión fue menor que la firme fidelidad al territorio, especialmente en P. sandwichensis. La fidelidad al territorio tiene implicaciones para el manejo de los campos agrícolas, porque la consistencia de los patrones de cultivo y las fechas de corte son importantes para mantener poblaciones de estas especies.

Correlates and Consequences of Breeding Dispersal in a Migratory Songbird

Hay harvests have detrimental ecological effects on breeding songbirds, as harvesting results in nest failure. Importantly, whether harvesting also affects evolutionary processes is not known. We explored how hay harvest affected social and genetic mating patterns, and thus, the overall opportunity for sexual selection and evolutionary processes for a ground‐nesting songbird, the Savannah sparrow (Passerculus sandwichensis). On an unharvested field, 55% of females were in polygynous associations, and social polygyny was associated with greater rates of extra‐pair paternity (EPP). In this treatment, synchrony explained variation in EPP rates, as broods by more synchronous females had more EPP than broods by asynchronous females. In contrast, on a harvested field, simultaneous nest failure caused by haying dramatically decreased the overall incidence of EPP by increasing the occurrence of social monogamy and, apparently, the ability of polygynous males to maintain paternity in their own nests. Despite increased social and genetic monogamy, these haying‐mediated changes in mating systems resulted in greater than twofold increase in the opportunity for sexual selection. This effect arose, in part, from a 30% increase in the variance associated with within‐pair fertilization success, relative to the unharvested field. This effect was caused by a notable increase (+110%) in variance associated with the quality of social mates following simultaneous nest failure. Because up to 40% of regional habitat is harvested by early June, these data may demonstrate a strong population‐level effect on mating systems, sexual selection, and consequently, evolutionary processes.

Evaluating the Roles of Visual Openness and Edge Effects on Nest-Site Selection and Reproductive Success in Grassland Birds

Abstract. Knowledge of breeding dispersal, defined as shifts in territory location between two successive breeding seasons, remains limited for migratory passerines. We investigated the relationship between two ecological factors, habitat structure and reproductive success, and 499 breeding dispersal events in a Nearctic—Neotropic migratory songbird, the Black-throated Blue Warbler (Setophaga caerulescens) breeding at the Hubbard Brook Experimental Forest, New Hampshire, from 1998 to 2008. Male dispersal distance was correlated with both individual age and habitat structure, with older males moving shorter distances than younger males, and males on the high-shrub-density plot (i.e., higher quality) moving shorter distances than males on the plot with lower shrub density. Female dispersal distance was also correlated with habitat structure; individuals on the higher-quality plot moved shorter distances than those on the lower-quality plot. In contrast to that of males, female dispersal distance was independent of age, but correlated with reproductive success: females that fledged relatively few offspring in a year subsequently dispersed farther than those that experienced high reproductive success. Mean (± SE) breeding dispersal distance for females (245 ± 20 m) was greater than that of males (163 ± 11 m). We also examined reproductive consequences of breeding dispersal and found that males that moved shorter distances fledged more offspring after dispersal than those that moved longer distances; no trend was found for females. These differences in dispersal patterns and outcomes suggest sexspecific selective pressures and life-history strategies.