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Fourth of July Butterfly Count


by
Ann B. Swengel
International Count Co-editor
North American Butterfly Association
Painted lady (Vanessa cardui) nectaring on showy milkweed (Asclepias speciosa). Courtesy A.B. Swengel
The Xerces Society started the Fourth of July Butterfly Count (FJC) in 1975, sponsoring it annually until 1993, when the North American Butterfly Association (NABA) assumed administration. The general methods of the butterfly count are patterned after the highly successful Christmas Bird Count (CBC), founded in 1900 and sponsored by the National Audubon Society (Swengel 1990).
The results of the FJC, including butterfly data, count-site descriptions, and weather information on count day, are published annually. The count was designed as an informal program for butterfly enthusiasts and the general public. These counts can never substitute for more formal scientific censusing because data sets from the counts have flaws that impair scientific analysis. Nevertheless, the FJC program does provide data that, with considerable caution, can be useful for science and conservation (Swengel 1990). FJC data have been used to study the biology, status, and trends of both rare and widely distributed species (Swengel 1990; Nagel et al. 1991; Nagel 1992; Swengel, unpublished data).

Analysis and Application
I reviewed FJC count reports and other publications for applications of FJC data to monitor the status and trends of North American butterfly species. These studies varied considerably in sample size, amount of data manipulation and statistical analysis, and degree of variable control. Different methods of using FJC data include, in order of ascending statistical refinement: presence or absence of a species in a subset of counts; highest observed number of a species on a single count; individuals of a species per count for a subset of counts in a given year; and individuals of a species per count hours or per count miles. The subset of counts used to supply data for analysis also varied from a single count to all counts in a certain region or all counts ever reporting a given species during the study period. The sample subset and statistical approach are best determined by the nature and extent of available data.
Monarch (Danaus plexippus) nectaring on dwarf blazingstar (Liatris cylindracea). Courtesy A.B. Swengel
The rationales, methodologies, shortcomings, and validity of analyzing FJC data have been detailed elsewhere (Swengel 1990), but are based on the substantial ornithological literature regarding the scientific use of CBC and other types of survey data. As ornithologists have clearly indicated, these kinds of data sets must be used with great care because (1) the sample sites and dates depend on when and where volunteer observers choose to conduct a count; (2) the quality of sampling and accuracy of data vary among counts; (3) only certain species are sampled adequately enough to allow data interpretation; and (4) the species complex can vary somewhat from year to year. Even with such constraints, these data sets are valuable because of the numerous sites surveyed, their wide geographic scope, and the relatively low cost of data acquisition.

Interpreting Count Data
Karner blue (Lycaeides melissa samuelis) male basking on grass. Courtesy A.B. Swengel
For the first 11 years of the count program (1975-85), only a few dozen counts were held annually, but since then the number of annual counts has increased steadily to 209 in North America in 1993. Each FJC annual report since 1982 has provided a table that details how many counts reported each species and which single count found the most individuals of each species. Although informal, this table indicates the frequency and abundance of butterfly species as observed in the counts.
Several rare species with federal status under the Endangered Species Act have been sampled in the counts, as reviewed in the introduction to the 1993 FJC annual report (Opler and Swengel 1994). A researcher using FJC to study rare butterflies must be careful in interpreting the data, however. Unless a number of FJC counts are specifically designed to sample rare species well, it is unlikely that rare species will be sampled adequately enough to allow scientific analysis of status and trends. Even in these cases, however, site data for rare species reported in FJC remain useful as leads to follow in status surveys of extant populations for these species (Opler and Swengel 1994). Most likely, the data should be considered as augmenting additional, more formal scientific study and should be confirmed, either by alternative survey means or by contacting the counters for documentation.
Because of the larger sample size, FJC data may better demonstrate the population trends of more abundant and widespread species. For example, the painted lady (Vanessa cardui) is a subtropical species with a tendency to wander (immigrate) outside its residential range into temperate regions, with periodic years of massive invasions. FJC data clearly reflect this aspect of the species' natural history by showing dramatic fluctuations in painted lady frequency and abundance in the counts in 1979, 1983, and 1992 (Swengel 1993; Fig. 1). These outbreaks may correlate with weather perturbations in the species' residential range (Myres 1985; Swengel 1993).

Fig. 1. Number of painted ladies (Vanessa cardui) per count and percentage of counts reporting this species, for all counts in North America north of Mexico, 1977-93.
FJC data have also been used to document fluctuations in other immigrant species (Swengel 1990), but especially to monitor population trends of the migrant monarch butterfly (Danaus plexippus; Swengel 1990 and unpublished data) that breeds in temperate North America and overwinters in Mexico and coastal California. The number of these butterflies fluctuates considerably (Fig. 2); fluctuations tend to correlate with major climatic perturbations such as the El Niño Southern Oscillation and major volcanic eruptions (Swengel, unpublished data). Monarchs and painted ladies often show dramatic fluctuations in the same years (e.g., 1978-79, 1982-83, 1991-92), but usually they vary in opposite directions (Figs. 1 and 2), suggesting that the same widespread climatic phenomena tend to affect both species in different ways. Because conservationists are concerned about threats to the overwintering habitat of monarchs, long-term data sets such as FJC are valuable to check for persistent downward trends.

Fig. 2. Mean number of monarchs (Danaus plexippus) per party-hour for counts reporting the species east of the Rocky Mountains, 1977-93, and west of the Rocky Mountains, 1987-93.
While FJC cannot replace more formal and intensive scientific surveying, it does offer a readily available and ever-enlarging data set that, with caution, is useful for science and conservation because of its relative continuity, inexpensiveness, large size, and widespread sampling.
For further information:
Ann B. Swengel
International Count Co-editor
North American Butterfly Association
909 Birch St.
Baraboo, WI 53913
References
Myres, M.T. 1985. A southward return migration of painted lady butterflies, Vanessa cardui, over southern Alberta in the fall of 1983, and biometeorological aspects of their outbreaks into North America and Europe. The Canadian Field-Naturalist 99:147-155.

Nagel, H. 1992. The link between Platte River flows and the regal fritillary butterfly. The Braided River 4:10-11.

Nagel, H.G., T. Nightengale, and N. Dankert. 1991. Regal fritillary butterfly population estimation and natural history on Rowe Sanctuary, Nebraska. Prairie Naturalist 23:145-152.

 Opler, P.A., and A.B. Swengel. 1994. NABA-Xerces Fourth of July butterfly counts 1993 report. North American Butterfly Association, Morristown, NJ. 72 pp.

Swengel, A.B. 1990. Monitoring butterfly populations using the 4th of July butterfly count. American Midland Naturalist 124:395-406.

Swengel, A. 1993. Permutations of painted ladies. American Butterflies1(2):34.


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