A Method for Assessing Blackbird Damage to Ripening Rice

A method for measuring and mapping the location of blackbird (Icterinae) damage to ripening rice over time was developed and employed in 7 commercial rice fields ranging from 20.6 to 47.4 ha in Matagorda County, Texas, during the 1985 and 1986 growing seasons. Ten evenly-spaced transects were established perpendicular to the longest side of the field and each transect was sampled at pre-determined distances. Transects were subdivided into "edge" (<= 60 m from field border) and "middle" (> 60 m from the field border) strata. The measured percent damage was compared to visual estimates for 3 samplings to assess the accuracy and precision of the l a t t e r . Measuring the percent damage to individual panicles appears to be valid, but damage may be underestimated. This sampling method may be useful for assessing the efficacy of current and proposed damage control techniques. Visual estimates were too high at low (< 5%) damage levels and too low at higher (> 5%) levels compared to measured damage. INTRODUCTION Blackbirds ( I c t e r inae ) have been damaging r ipening domestic r i c e since colonial times (Meanly 1971). Although t h i s damage i s i n s i g n i f i c a n t on a regional basis, i t is not evenly distributed, and individual producers can suffer catastrophic losses (DeHaven 1971, Meanly 1971). To combat this damage most producers employ a variety of pyrotechnic and auditory scare devices. The efficacy of these control measures is unknown. In order to test the efficacy of current damage control techniques, and to develop new techniques, damage must be accurately as2/Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843. 2/Texas Agricultural Research and Extension Center, Route 7, Box 999, Beaumont, TX 77706. sessed. Many millions of dollars are believed to be lost annually from bird depredations on ripening rice worldwide (DeHaven 1971, Meanly 1971, Elliott 1979, Bruggers and Ruelle 1981), yet objective damage assessment methods have only recently been developed (Manikowski 1985). The problem is twofold. First , damage tends to be sporadic and unpredictable. Quantitative assessment is also difficult due to the the large and highly variable number of grains per panicle, the dense plant stand, and the often very large fields (Lefebvre et a l . 1983). Bird damage assessment techniques for row crops employ random selection of rows and distances along each row (DeGrazio et a l . 1969, Stickley et a l . 1979, Avery and DeHaven 1982, Conover 1984). This is impractical for rice, which is broadcast seeded or dr i l l planted in very close rows. Holler et a l . (1982) solved this problem by employing randomly-located transects and sample points to estimate blackbird damage to sprouting rice in Louisiana. However, this method is inadequate for mapping damage within the field because damage is often unpredictable (Meanly 1971, Lefebvre et a l . 1983, Manikowski 1985). The most vulnerable stage of ripening rice also is unknown and may be important. In this paper we report on a technique developed for documenting the amount and location of blackbird damage to ripening rice over time. We are grateful to R. Corporon, B. Ottis, and M. Ottis -for permission to survey their rice fields. J. Engbrock made the ini t ia l producer contacts and provided logistical support throughout the study. We thank C. E. Gates for his s ta t is t ica l expertise and T. B. Doerr, H. E. Hunt, and R. D. Slack for reviewing earlier drafts of the manuscript . Funding was provided by the Texas Rice Research Foundation, Texas Agricultural Experiment Station, Texas Ornithological Society, and the Wilson Ornithological Society.