Studies in Avian Biology No. 13:29-37, 1990. ARTHROPOD SAMPLING METHODS IN ORNITHOLOGY ROBERTJ.COOPER AND ROBERT C. WHITMORE Abstract. We review common methods used by entomologistsand ornithologists for sampling ter- restrial arthropods. Entomologists are often interested in one speciesor family of insects and use a trapping method that efficiently samplesthe target organism(s).Ornithologists may use those methods to sample a single type of insect or to compare arthropod abundancebetween locations or over time, but they are often interested in comparing abundancesof different types of arthropods available to birds as prey. Many studies also seek to examine use of prey through simultaneousanalyses of diets or foraging behavior. This presents a sampling problem in that different types of prey (e.g., flying, foliage dwelling) must be sampled so that their abundances can be compared directly. Sampling methods involving direct observation and pesticide knockdown overcome at least some of these problems. Trapping methods that give biased estimates of arthropod abundance can sometimes be related to other methods that are lessbiased (but usuallymore expensive)by means of a ratio estimator or estimation of the biased selection function. Key Words: Arthropods; insects;prey abundance;prey availability; sampling. Numerous techniques exist for sampling in- area will be eaten by the bird. The size, life stage, sects (e.g., Southwood 1978); many have been palatability, coloration, activity patterns, and used in ornithology. Most ornithological studies other characteristics of arthropods influence the use insect sampling to determine types, numbers, degree to which they are located, captured, and and distribution of insects available to birds as eaten. These are the “translators” (Wiens 1984b) prey; many also are designed to examine the use or factors that translate simple arthropod abun- of those prey through simultaneous studies of dance into availability (e.g., see Hutto, this vol- diet and foraging behavior. Most techniques, ume; Wolda, this volume). The problem is one however, effectively sample only a portion of the of perception; the researcher must assess avail- total insect fauna available to birds, and esti- ability as the bird does. This, of course, is im- mates of total arthropod abundance using these possible. One approach is to use dietary data to techniques will be biased accordingly. determine availability (Sherry 1984, Wolda, this Our objectives are (1) to review a portion of volume). Using this approach, the prey available the literature on sampling techniques commonly to the bird are those it commonly eats. This ap- used in entomological field studies, (2) to de- proach is useful in some types of investigations scribe the advantages and disadvantages of those (e.g., foraging behavior), but it is nonsensical in techniques, (3) to review their use and misuse in others (e.g., dietary preference). ornithological studies, and (4) to make recom- Usually, as in this paper, when arthropods are mendations concerning arthropod sampling sampled to determine availability, the sampling methods in light of the objectives of field omi- is designed to estimate the types, numbers, or thologists. Our review of the entomological lit- distribution of some or all arthropods in the for- erature is largely limited to sources in which ar- aging area of one or several species of birds. Or- thropod sampling is a focal point of the paper; nithologists are often interested in locations (e.g., it excludes techniques that sample arthropods tree species, heights) where birds forage in rela- normally unavailable to passerines, such as light tion to arthropod availability. In this case, avail- trapping, and techniques that focus on a single ability would be defined only in terms of the species, such as capture-recapture sampling. An specific prey types in the location(s) of interest. excellent review of these techniques is contained Dietary data can be used to narrow the focus of in Southwood (1978). Other reviews of interest the sampling effort. If a species eats large per- include Morris (1960) and Strickland (196 1). centages of caterpillars a sampling technique spe- cific to caterpillars should be used. Most methods DEFINING ARTHROPOD AVAILABILITY described here are designed to sample one type of The usual objective of most ornithological arthropod (e.g., flying, foliage dwelling). Such an studies that sample arthropods is to relate some objective is generally easier to achieve than sam- aspect of bird behavior or ecology (e.g., diet, for- pling different types of arthropods for compari- aging behavior, territory size, productivity) to son (see Sampling Problem). arthropod abundance and distribution (avail- If a study seeks to assess “preference,” one ability). Simple arthropod abundance, however, must estimate the numbers and types of all ar- may not reflect the prey actually available to birds, thropod prey in the foraging area and compare because not all arthropods in a birds’ foraging prey eaten and not eaten by the birds. Reasons 29 30 STUDIES IN AVIAN BIOLOGY NO. 13 for their choices may be found in the ecologies of predator and prey. For example, Robinson and Holmes (1982) and Cooper (1988) found that prey types eaten by different speciesreflected where R, is the estimate of mean abundance from differences in searching and attack strategies. sampling technique 1, p, is the estimate of mean Aviary studies involving feeding trials (Whelan abundance from sampling technique 2 when ap- et al., 1989) or simulated ecosystems (Hein- plied to the same study site, and K2 is the estimate rich and Collins 1983) can also provide infor- of mean abundance from sampling technique 1 mation about why certain prey are eaten or when applied by itself in a second study site. avoided. Usually x is less costly and less accurate than y. Both x and y are measured on several sampling THE SAMPLING PROBLEM units and the relationship between them is ex- Estimates of arthropod abundance are either pressed as a ratio QJZ,). Then a larger number relative or absolute. Relative measures provide of samples is taken measuring only x. An esti- only indices of abundance, such as numbers per mate of y for the entire study area can then be surface area of sticky trap in a given time period. obtained by using the general formula above. They have limited utility in studies of arthropod This method is mentioned frequently in the fol- abundance and availability. Absolute measures, lowing sections. McDonald and Manly (1989) on the other hand, permit estimates of arthropod consider an alternative to ratio (regression) es- density that can be used for interspecific com- timation in which an attempt is made to calibrate parisons and comparisons among different hab- a biased sampling procedure by estimating a se- itats and seasons. Absolute measures usually re- lection function. quire an intermediate sampling effort to quantify the density of the unit (plant, leaf, branch, and ESTIMATING RELATIVE ABUNDANCE so on) used to assess arthropod numbers. Ar- (INDICES) thropod sampling, as used in this paper, covers The following techniques are designed to give both types of measures. indices of relative abundance, not to estimate An obvious problem is comparing results from absolute abundance or density of arthropods. one or more methods involving different sam- Most of the methods use a trapping device, such pling units. This is especially the case if one wish- that the sampling unit is insects per trap or time es to compare arthropods eaten by birds with period. those available. Because various foraging sites and maneuvers differ among bird species, many Sticky traps sorts of arthropods may have to be sampled si- Sticky traps of various design (see Southwood multaneously in a given habitat, including flying 1978:250-252) are commonly used to sample insects, foliage-dwelling forms (e.g., spiders, Ho- flying insects inexpensively; an insect settles on moptera, some beetles), caterpillars, and litter- or strikes an adhesive surface and is trapped. dwelling insects (e.g., ground beetles, ants). Trap size and shape (Heathcote 1957a, b; You- Because most of these groups require a unique nan and Hain 1982) and color of the trap surface sampling technique, one cannot easily relate re- (Purcell and Elkinton 1980; Weseloh 1972, 198 1) sults from one group to another. For example, are important. The traps are messy. Temperature how might we compare a frequency of two cat- can affect the consistency and effectiveness of the erpillars/ 100 leaves with a density of 10 ichneu- adhesive, and large insects tend to bounce off or monid wasps/400 cm2 of sticky-trap surface per escape. week? The problem requires either (1) a tech- Sticky traps have been compared with other nique that equivalently samples all types of ar- flying insect traps or sampling techniques mostly thropods in a given habitat (defined as unbiased with unfavorable results, in that certain insect measurement of the abundance of all arthropod taxa, or sizes, or both, were underrepresented taxa on experimental units that completely cover (Trumble et al. 1982, Younan and Hain 1982). the habitat of those arthropods), or (2) a method Because trap color alters the effectiveness of the that permits unbiased comparison of results from traps for many insects, between-species compar- one technique with those of another. The first isons of abundance may be biased. However, solution can sometimes be achieved in arthropod strong correlations were documented