Management Implications Derived from Bird Damage Assessments in North Dakota Sunflower

The most recent estimate of blackbird damage to ripening sunflower (Helianthus annuus) in North Dakota in 1980 exceeded $6 million. Although less than 5% of the sunflower fields in the state received greater than 10% damage in each of four past survey years, as much as 50% of the total state-wide damage occurred in these fields. This loss can be combated with cost-effective controlSuccessful control requires timely action when blackbirds are in large concentrations. Frightening blackbirds from fields early in the damage season disperses damage and this can result in compensatory growth by sunflower, whereby yields at harvest are largely unaffected. INTRODUCTION North Dakota produced 72 percent of the United States sunflower crop on about 3.5 million acres of land in 1982 (Statistical Reporting Service 1983). Production has increased rapidly since 1959 when only about 13,500 acres were harvested (Thomason 1974). With the expansion of sunflower into every county in North Dakota came an increasing number of complaints of seed losses caused by blackbirds. Red-winged Blackbirds (Agelaius phoeniceus) are primarily responsible for the damage, although Common Grackles (Quiscalus quiscula) and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) are becoming increasingly important. The oil varieties of seed are preferred by Red-winged Blackbirds and bear the brunt of the damage (Besser 1978). State-wide losses to blackbirds are significant and warrant our attention. Damage is not uniformly Yo U. S. Fish and Wildlife Service, Denver Wildlife Research Center9 Bldg. 169 Denver Federal Center9 P. 0. Box 25266, Denver, Colorado 80225-0266 distributed, rather it is localized and at each foci proportional to the size of nearby marsh roosts. The patterns of damage (how muchs where, and when) to ripening sunflower are highly variable and are influenced by the numbers and species of birds together with the sex and age composition and origin (migratory or resident birds) of the depredating flocks. Weather, planting and harvest dates, crop maturity, and field location in relation to marsh roosts also contribute to variability in damage patterns. This paper briefly summarizes findings from several recent blackbird damage assessment studies conducted on ripening sunflower in North Dakota by personnel of the Denver Wildlife Research Center (DWRC), U. S. Fish and Wildlife Service (FWS). Data from which blackbird damage patterns were established were collected by DWRC personnel during state-wide9 county-wide, local, and individual sunflower field objective surveys. Our purpose is to summarize the characteristics of these damage patterns in order to develop a practical management strategy. EXTENT AND MAGNITUDE OF DAMAGE North Dakota produced 183,150 tons of sunflower seed on 407,000 acres in 1972 and 1,252,200 tons on 1,880,000 acres in 1978 (Lilleboe 1979). In 19729 blackbirds consumed 1.4% of the North Dakota sunflower crop in 11 eastern counties — the principal areas for the crop (Henne et al. 1979); the damage amounted to $3009000. In 1978, sunflower was grown in all North Dakota counties and the loss to birds was estimated at 1.2% of the crop or $3.2 million (Besser and DeHaven 1980). Thus, blackbirds destroyed 2,564 tons (12.6 Ib/acre) and 15,026 tons (16.0 lb/acre) in 1972 and 1978, respectively (Table 1).