Lowell C. McEwen

Impacts of a Severe Drought on Grassland Birds in Western North Dakota

We studied the effect of a severe drought on the population dynamics and community structure of grassland birds in western North Dakota. During the spring and summer of 1988 the northern Great Plains suffered one of the warmest, driest periods in its recorded history. We compared the changes in bird populations and nesting productivity over a 3-yr period before, during, and after the drought. Total grassland bird density declined 61% (P < .05) between June 1987 and June 1988. Densities of six of eight common species declined significantly during the drought. Populations of all but two species recovered in 1989 and total bird density in June 1989 did not differ significantly from June 1987. Species richness and species diversity both declined significantly during the drought and recovered to predrought levels in 1989. Species richness declined more on fair condition than on good condition range during the drought. Vesper Sparrow (Pooecetes gramineus) hatching success, number of young fledged per successful nest, and nesting success were significantly lower in 1988 than either 1987 or 1989. Clutch size did not differ among the three years. The decline in nesting success in 1988 was primarily due to nest abandonment during incubation. Nesting of Vesper Sparrows, Horned Larks (Eremophila alpestris), and Western Meadowlarks (Sturnella neglecta) ended abruptly in mid-June 1988 during a period of extremely hot weather. In 1987 and 1989, nesting continued into July. Despite substantial reductions in bird density and productivity during the drought, many species recovered to predrought levels 1 yr following the drought. This suggests that year-to-year fluctuations in densities of some of these species may not be tightly linked to short-term changes in local productivity. However, sequential years of low productivity may have more substantial effects on these short-lived species. Thus, if drought conditions in North American grasslands become more frequent, as some climate models predict, there could be related changes in the avifauna of the region.

EFFECTS OF AVIAN PREDATION ON GRASSHOPPER POPULATIONS IN NORTH DAKOTA GRASSLANDS

We experimentally studied the effects of avian predation on grasshopper abundance in western North Dakota during the summers of 1988 and 1989. Grasshopper densities in 15 x 15 m plots from which birds were excluded (NO BIRDS) were compared with similar-sized plots where birds were allowed to forage (BIRDS). Plots were established in early June at 16 sites (8 per year), and grasshopper densities were estimated from hoop counts in NO BIRDS and BIRDS plots at 2-wk intervals until the end of July. There were significantly more grasshoppers in NO BIRDS plots than in BIRDS plots in 1989 (P .388). After the final hoop count each year, sweep-net sampling was also used to estimate grasshopper densities. By this method, average grasshopper density in late July was 26% and 37% lower in BIRDS plots than in NO BIRDS plots in 1988 and 1989, respectively. Average length, total biomass of grasshoppers, species richness, and species diversity, however, did not differ between the treatments. In 1988, 2 of 15 grasshopper species were significantly more abundant in the NO BIRDS plots. There was no difference between the treatments among 16 species identified in the 1989 samples, but the power of our tests to detect differences for individual species was low. Our results support the hypothesis that avian predation reduces insect populations at low and moderate densities.

EFFECTS OF SELECTIVE SEED PREDATION BY RODENTS ON SHORTGRASS ESTABLISHMENT

Our experiment tested whether selective seed predation by rodents affects the establishment of blue grama (Bouteloua gracilis) and buffalo grass (Buchloi dactylo- ides) at a disturbed site in the Colorado shortgrass steppe. We hypothesized that foraging rodent granivores respond positively to large seed sizes and to high seed densities, and we predicted that intensive foraging would constrain grass establishment. Capture-recapture studies confirmed that heteromyid granivores, kangaroo rats (Dipodomys ordii) and hispid pocket mice (Perognathus hispidus), comprised 48% of the local rodent community. Seeds of blue grama and buffalo grass were shallowly planted at three densities in replicated plots within and outside rodent exclosures. The frequency of digging in seeded rows (seed predation) and the frequency of herbivory on seedlings were calculated. We assessed grass establishment using seedling frequency and harvested aboveground biomass. Seed foraging was related positively to seed size, but foraging responses to seed densities were not confirmed. Herbivory on seedlings was not intensive and was similar between the two grasses. Seedling frequency and harvested biomass numbers for the species with the larger seeds, buffalo grass, were reduced more in rodent-accessible plots than were values for blue grama. Rodent seed predation can constrain the establishment of large-seeded species, but results should be interpreted to reflect site-specific conditions and the characteristics of seeded species.

Survival and Growth of Nestling Vesper Sparrows Exposed to Experimental Food Reductions

We examined the effects of experimental food reductions on the reproductive biology of nestling Vesper Sparrows (Pooecetes gramineus) during 1991 in the Little Missouri National Grasslands, North Dakota. Grasshopper densities on territories around 24 individual nests were experimentally reduced while 31 nests served as controls. There were no significant differences between treatment and control nests in number of young fledged per nest, 2-8 day nestling growth rate, or final 8 day nestling mass. Nest failure was due to predation (83.3% of nests that failed), and abandonment or parental death (16.7% of nests that failed). Grasshoppers were the principal food, comprising 67.7% of all identified food items brought to all nests when nestlings were 7-8 days old. Breeding adults on treated territories foraged significantly further from the nest than control birds suggesting that birds on treated sites compensated for the reduction in food by altering foraging behavior.

Effects of grasshopper control programs on rangeland breeding bird populations.

We investigated the effects of grasshopper control methods on breeding bird populations in western rangelands. We estimated bird densities on 13 treated and 11 untreated sites before and after grasshopper control operations. Four different treatments were used in these applications: malathion, sevin-4-oil, carbaryl bait and Nosema locustae bait. There were few differences among the 4 treatments in their effects on bird community parameters (total bird density, and species richness, diversity, or evenness). Bird community parameters did not differ between spray and bait applications but sample sizes were small for bait treatments. When data from all treatments were combined for analysis, there was no difference in any of the bird community parameters between pre- and post-treatment samples. Densities of western meadowlarks (Sturnella neglecta), however, were significantly lower on treated than untreated sites 10 and 21 days after treatment. We found no relationship between changes in bird numbers 21 days post-treatment and either the size of the area treated or the date of treatment (i.e., early or late in nesting season). Malathion is an acetylcholinesterase (AChE) inhibitor, but brain AChE levels in birds collected on sites treated with malathion showed no significant inhibition. These results suggest that grasshopper integrated pest management treatments generally have little effect on breeding bird communities but some insectivorous bird species may decline on rangeland treated with broad-spectrum insecticides because of reduction in food base. Pesticide applications that have adverse impacts on birds and other nontarget wildlife that prey on grasshoppers may be counterproductive to long-term integrated pest management goals.

RESPONSES OF NESTING SAVANNAH SPARROWS TO FLUCTUATIONS IN GRASSHOPPER DENSITIES IN INTERIOR ALASKA

ABSTRAcr.-Grasshopper populations on the Delta Agricultural Project area in interior Alaska exhibit a strong biennial periodicity. This phenomenon allowed us to study dietary, reproductive, and numerical responses of nesting Savannah Sparrows (Passerculus sandwichensis) to fluctuations in prey abundance. Grasshopper densities exceeded 25/M2 in 1990 but were less than 1/M2 in 1991. In 1990, 61% of the identifiable food items brought to the nest were grasshoppers. In 1991, Lepidoptera larvae and Diptera were the most common items brought to the nests and no grasshopper deliveries were observed. Fledging success was 97% in 1990 and 96% in 1991. At least one nestling per nest successfully fledged each year. Mean clutch size was significantly higher during the high grasshopper densities in 1990. Reduced growth measurements of nestlings were obtained in 1991 compared to 1990. Effects on growth appeared to be greater early in development, rather than prior to fledging. Savannah Sparrow densities did not differ between the two years. Although a positive response to high grasshopper densities was observed, the magnitude of the response was diminutive compared with the large difference in prey abundance between years. This suggests that factors other than food limit reproductive output in this population of Savannah Sparrows. Received 13 May 1993, accepted 2 July 1993.

Shade Effects on Chemical Composition of Herbage in the Black Hills.

Discussion Clear-cut results as to what is the best level of utilization were not expected. October clipping once a year is far different from natural browsing. Deer use throughout the year could hardly be simulated by a clipping study, but it probably would have more adverse effect on plant vigor and future production than the October clipping. The plant species and its usual place in succession has an important bearing, as shown by the results with sassafras. The fact that all study plants were under a forest canopy is essential. A given degree of utilization on a plant in heavy shade may eliminate all lower limbs or cause mortality; and the same degree of utilization on a similar plant in an open situation might keep LAY

Relationships Between Bird Density, Vegetation Characteristics, and Grasshopper Density in Mixed-Grass Prairie of Western North Dakota

We monitored the abundance of grassland birds and the density of then-principal prey (grasshoppers, Acrididae) on 20 transects in mixed-grass prairie of western North Dakota from 1987-1990. Bird and grasshopper densities were estimated on 10 occasions during this period, 4 times in 1987 and once in late May and June every year thereafter. Twelve transects were treated for grasshopper control in late June 1987. During all of the census periods except July 1987 and June 1990, there was a negative relationship between grasshopper density and total bird density; the relationship was significant (P>0.05) during June 1987 and May 1989. Of the four most common bird species recorded on the transects, 2 species, western meadowlarks (Sturnella neglecta) and grasshopper sparrows (Ammodramus savannarum) ,were negatively correlated with grasshopper densities during most of the census periods. The relationship was significant for western meadowlarks in June 1987, May 1989, and June 1990. However, density of the other 2 species, horned larks (Eremophila alpestris) and vesper sparrows (Pooecetes gramineus) ,were not correlated with grasshopper density. We explore two possible explanations for the negative relationship between bird abundance and grasshopper abundance: habitat association and bird predation. Available evidence supports both of these nonexclusive hypotheses. However, it is unlikely that bird predation has a significant impact when grasshopper densities are high. Our results suggest that habitat manipulation may be effective in decreasing grasshopper numbers and increasing bird populations in these grasslands.