Evaluation of Field Sampling Techniques for Estimation of Bird Damage in Pistachio Orchards

Pistachio orchards were selected and evaluated for damage caused by either scrub jays (Aphelocoma coerulescens) or American crows (Corvus brachyrhynchos). The distribution of damage caused by each species was evaluated and quantified. The percentage of trees damaged by scrub jays ranged from 58% to 99% and tended to be distributed randomly throughout the. orchard. In orchards with crow damage, the percentage of trees damaged ranged from 18% to 46% and damage tended to be aggregated. Data from orchards were used to evaluate the relative accuracy and precision of various sampling strategies. Randomly distributed bird damage could be sampled with relatively simple strategies such as walking diagonally across the orchard. Aggregated bird damage was most effectively sampled using stratified random sampling. If action thresholds are going to be used to determine when bird control programs should be initiated, an understanding of the distribution of bird damage in a crop must be understood so reasonable sampling techniques can be developed. Proc. 16th Vertebr. Pest Conf. (W.S. Halverson& A.C. Crabb, Eds.) Published at Univ. of Calif., Davis. 1994. INTRODUCTION There are a number of birds species known to damage various crops (Besser 1985). The crops damaged (as well as the type, extent and distribution of damage) vary with the bird species. For any given bird species, the preferred diet, availability of alternate food sources, individual and flock behavioral patterns, and type of crop will determine the potential for the bird to cause crop damage. One question that arises is what impact the factors that determine the extent and distribution of damage in a crop have on our ability to accurately assess crop losses? A great deal of effort has been put into studying the in-crop distribution of non-vertebrate agricultural pests, including insects, nematodes, and plant pathogens. With the distribution of pests in a crop known, there has been significant refinement in techniques used to assess crop damage by various pests. Examples of studies into the distribution of insect pests in crops include work by Burts & Brunner (1981), Wilson & Room (1983), Wilson, Room & Bourne (1983), and Pickett & Gilstrap (1986). Ferris (1984) and Wheeler, Keneerley, Jeger, & Starr (1987) assessed the distribution patterns of nematodes. Perhaps the greatest effort in studying the distribution of plant pests has been done in the field of plant pathology. Examples where the distribution of disease organisms in the field were studied include work done by Marois & Adams (1985), Schuh, Frederiksen & Jeger (1986), Sylvia (1986), Thai & Campbell (1986), and Madden, Pirone, & Raccah (1987). In contrast, there has been little effort to conduct similar studies with vertebrate pest species. Information concerning the distribution of plant pests has been used to investigate various sampling strategies to enhance the detection of crop damaging organisms. Lin, Pousheinsky & Mauer (1979), Nicot, Rouse & Yandell (1984), and Delp, Stowell & Marois (1986) evaluated different sampling strategies for estimating disease incidence. Barker, Schmitt & Noe (1985) reviewed the role of sampling for assessment of crop loss due to nematodes. Burts & Brunner (1981), Mollet & Sevacherian (1984), Pickett & Gilstrap (1986), and Schotzko & O'Keefe (1986) evaluated various sampling schemes for monitoring pest insects. Methods to estimate wildlife populations are more developed. Davis (1982), Verner & Ritter (1985) and Wywialowski & Stoddart (1988) discussed various methods of conducting population censuses generally dealing with non-pest vertebrate species. Kaukeinen (1984) looked at various activity indices to determine relative changes in vertebrate pest populations. There has been relatively little research to assess the distribution of vertebrate pest damage. A greater effort has been made to evaluated sampling methods for use in assessing efficacy of various vertebrate pest management strategies [Granett, Trout, Messersmith, & Stockdale (1974), Moulton (1979), and Manikowski (1985)]. Papers reviewing methods of assessing crop loss by vertebrate pests as a means to determine the need to implement control programs are rare. Dolbeer (1981) and Ho & Heong (1984) considered strategies that could be used to make decisions about the need to institute some type of vertebrate pest control program. The results of a survey of California pistachio growers suggested that most pistachio growers lack reasonably accurate information about the extent of damage caused to the pistachio crop by birds (Crabb, Salmon & Marsh 1987). Loss estimates are based on subjective evaluation or on the difference between the actual yields and what they thought the yield should be. The focus of this study was two fold. First, we set out to