In this section we present a listing of scientific publications that are based on monarch migration monitoring data. The abstract from each publication is also given below.

(In descending chronological order)

Gibbs, D., D. Walton, L.P. Brower and A.K. Davis. 2006. Monarch butterfly (Lepidoptera, Nymphalidae) migration monitoring at Chincoteague, VA and Cape May, NJ: a comparison of long-term trends. Journal of the Kansas Entomological Society 79(2): 156-164.

ABSTRACT. The recent increase in illegal logging of the eastern North American monarch butterfly population’s overwintering site in central Mexico has emphasized the need for accurate long-term population monitoring tools, especially for this population. One way to obtain indices of annual population size is to record the numbers of monarchs passing by fixed points along the migration route each year, using standardized censusing methods. This methodology has been employed at several sites in eastern North America; however, long-term trends obtained from these sites have never been compared. Since 1997, the numbers of migrating monarchs have been censused at the Chincoteague National Wildlife Refuge, VA, and 100 km directly north at Cape May, NJ, every day each fall using similar methodologies. We compared the daily census counts from these sites to determine if each site recorded similar numbers overall, and if the year-to-year trends obtained from each site reflected the same patterns over 8 years (1997-2004). From comparisons of 210 days where both sites were operated simultaneously, we found that consistently more monarchs were counted at the Cape May than at the Chincoteague site. This may point to a higher preference for Cape May as a stopover site or to the proximity of the Cape May census route to the tip of the peninsula where grounded monarchs accumulate. However, regardless of abundance, the year-to-year trends were the same at both sites, with each site showing that 2004 was the lowest of the 8 years of data. These results point to the strengths of migration monitoring as an accurate population monitoring tool, but also suggest that future monitoring sites should be widely separated to minimize redundancy in data collection.

Walton, D., L.P. Brower and A.K. Davis. 2005. Long-term monitoring and fall migration patterns of the monarch butterfly in Cape May, NJ. Annals of the Entomological Society of America 98(5): 682-689.

ABSTRACT. Each year in eastern North America, monarch butterflies, Danaus plexippus (L.) (Lepidoptera: Nymphalidae), undergo an annual migration to wintering sites in central Mexico. We used monarch migration census data from Cape May, NJ, over a 13-yr period (1992-2004) to test for annual and within-season variation in the numbers of monarchs seen during daily censuses and to examine the timing and patterns of migration waves. Across all years, the total number of monarchs counted over the 60-d season ranged from 452 to 15,751 with a 13-yr average of 3,490 monarchs per yr. There was significant annual, diurnal, and within-season temporal variation in the census counts. Within seasons, monarch numbers increased during September up until early October, and gradually declined thereafter. Comparison of season averages across years, which we consider indices of population size in the northeastern United States, indicated a highly ßuctuating population size with the lowest year on record in 2004. We also found that greater than average daily counts, which we termed “notable migration days,” were reported for an average of 19 d per season. On average, seven “migration waves” occurred each year, defined as a period of one or more notable migration days separated by below average days. Waves lasted an average of 3 d and were separated from others by 6 d. There was no significant interannual variation in wave duration or time between waves. Our 13-yr study is the longest standardized census of the monarch's fall migration, and we believe its continued operation can provide insights into the population trends of monarchs in the northeastern United States, which may reßect long-term trends from other populations in North America.

Meitner, C.J., L.P. Brower and A.K. Davis. 2004. Migration patterns and environmental effects on stopover of monarch butterflies (Lepidoptera, Nymphalidae) at Peninsula Point, MI. Environmental Entomology 33(2): 249-257.

ABSTRACT. Since 1996, the numbers of migrating monarch butterflies stopping over at Peninsula Point, MI have been monitored by volunteers during the fall migration with standardized daily counts. Here we describe this project and examine 1) general patterns of migration and stopover of monarchs at this site, and 2) how environmental conditions influence monarch stopover frequency. We tested for yearly, seasonal and diurnal variation in monarch counts within each season. We further combined these data with basic weather information recorded at the time of each count to explore the effects of wind direction and speed, temperature and cloud cover on monarch stopover abundance. A total of 22,539 monarchs were counted over 7 years, with yearly totals ranging from 757 in 1998 to 6638 in 1997. Over the 7 year period, an average of 29 monarchs were recorded per count at Peninsula Point. Interestingly, in the migration season immediately following a major population decline at overwintering sites in Mexico, the total number of monarchs counted at Peninsula Point was not significantly different from long-term average counts. The timing of the peak of migration was not consistent from year to year, and there were few consistent temporal trends within seasons. More monarchs were counted with walking transects during the day than with a roost count in the early morning. Further, more monarchs were counted earlier in the season than later. Of the environmental variables we examined, wind direction had a significant influence on the number of monarchs recorded on each count with higher counts during north winds. Cloud cover also influenced monarch counts, so that the number of monarchs observed increased with temperature and decreased with cloud cover. Based on the large numbers of monarchs that stop there each fall, we suggest that Peninsula Point represents an important monarch stopover site and thus has the potential to increase our knowledge of monarch migration and stopover ecology greatly.

Davis, A.K. and M.S. Garland. 2002. An evaluation of three methods of counting migrating monarch butterflies in varying wind conditions. Southeastern Naturalist 1(1): 55-68.

ABSTRACT. Migrating monarch butterflies can be censused several ways, but studies that compare censusing methods are lacking. Furthermore, although it is known that monarch butterflies alter their flight strategies in varying wind conditions, it is not known if and how counts of monarchs made during migration are affected by wind conditions. We assessed the abundance of migrating monarchs in the fall of 2000 using 3 techniques, which each differ in the flight strategy they target. We tested for differences between the resulting counts, and examined the influence of wind conditions on each method. We (1) censused monarchs on a five mile driving route, (2) counted monarchs from a hawkwatching platform, and (3) counted monarchs at a roost site. The hawkwatch and driving census produced similar overall indices of abundance. Furthermore, wind conditions affected each count. As we predicted, the driving census detected the most monarchs when wind speeds and directions were unfavorable for migration, and the most were counted with the hawkwatch census during light favorable wind directions. Unexpectedly, counts of roosting monarchs were highest during strong winds in both favorable and unfavorable directions. We recommend that long-term monarch monitoring stations employ a combination of techniques to obtain comparable counts of monarchs.

Walton, R. K. and L.P. Brower. 1996. Monitoring the Fall Migration of the monarch butterfly Danaus plexippus L. (Nymphalidae: Dananinae) in eastern North America: 1991-1994. Journal of the Lepidopterists' Society 50: 1-20.

ABSTRACT. The results of two different sampling methods are presented for summer resident and migrant monarch butterflies, Danaus plexippus L. (Nymphalidae: Danainae), in the northeastern region of North America during the period 1991-1994. Estimates of the relative numbers of butterflies obtained by the Xerces/NABA Fourth of July Butterfly Counts and by a newly instituted fall census in Cape May, New Jersey are correlated. Monitoring of the relative annual abundance of monarchs of the eastern population and its predicted decline therefore appears feasible. Our Cape May data, as well as historical records, indicate that the monarch's annual fall migration along the Atlantic coast is a normal, and not an "aberrant" phenomenon. An alternative migration model, based on avian research, is presented as a possible adaptive explanation for the fall migratory movements of the monarch butterfly along the Atlantic coast.