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Migration Information - When to look for migrating Monarchs?

The Monarch migration begins as the length of daylight shortens in mid August and September. At this time, Monarchs in northern latitudes, i.e. near the Canadian border, begin to migrate. Monarchs farther south will begin their journey a few weeks later. Tagging and monitoring should begin in late August in all regions, with a concentrated effort made in September and early October.

A GOOD RULE: when the wild asters, especially A. novae-angliae, goldenrod and Joe Pye weed are in bloom, the Monarchs are migrating. In much of the lower midwest, migrating Monarchs are attracted in large numbers to a tall late blooming thistle (Cirsium altissimum) several species of sunflowers and other species of Asteraceae.

The table below gives the latitude, the midpoint of the migration and the period of peak abundance. These predictions are derived from reports to our list serve, communications directly to Monarch Watch, personal observations and the thousands of tagged butterflies that have been recovered over the years. Notice that I have used midpoint as a predicted date rather than a mean. We don't have enough information on the flow of the migration to generate a mean. Further, the distribution of the migrants appears not to be a normal bell-shaped curve but a curve that is shifted strongly to the left. Hence, when estimating the time of peak abundance below I have used a 12-day interval with 7 days before the midpoint and 4 days after the midpoint.

    Latitude / Midpoint / Peak in monarch abundance
    49 degrees / 26 August / 18-30 August
    47 degrees / 1 September / 24 August -5 September
    45 degrees / 6 September / 29 August - 10 September
    43 degrees / 11 September / 3 - 15 September
    41 degrees / 16 September / 8 - 20 September
    39 degrees / 22 September / 14-26 September
    37 degrees / 27 September / 19 September - 1 October
    35 degrees / 2 October / 24 September - 6 October
    33 degrees / 7 October / 29 September - 11 October
    31 degrees / 12 October / 4-16 October
    29 degrees / 18 October / 10-22 October
    27 degrees / 23 October / 15-27 October
    25 degrees / 28 October / 20 October - 1 November
    23 degrees / 4 November / 27 October -8 November
    21 degrees / 11 November / 3-15 November
    19.4 degrees * / 18 November / 10-22 November
*This latitude represents the general vicinity of the overwintering colonies. The monarch colony at El Rosario is usually opened to the public around the 18th of November.

Keep a journal! Some of our most useful information has been obtained from individuals who simply maintain a Monarch journal, diary or calendar. In these journals, records are kept of the first appearance of migratory Monarchs (those showing strong directional rather than local flight), the numbers seen each day, particularly at a roost or roosts, or on flowers in a garden, etc. In the past, Monarch Watchers have obtained very good quantitative data by counting the number of Monarchs passing a given observation site each hour. It would also be useful to record the number of mating pairs seen along with the dates and circumstances of these observations.

Monarch Populations
Each year the Monarch Watch sponsers a Monarch Tagging project and sends out a Season Summary and PreMigration Newsletter. They also provide in written form a qualitative assessment of the condition of the Monarch population in eastern North America. These assessments are based on reports received from the Email Discussion List, emails, faxes, phone calls, letters, and personal experience. They emphasize that the assessments are qualitative. More quantitative information on population sizes and dynamics is needed to understand Monarch biology and to develop sound environmental policy if we wish to sustain Monarch populations.
--Dr. Chip Taylor, University of Kansas.

Additional Monarch Information from the Monarch Watch!

Monarch Biology

Milkweed Guide

Tagging project run by the Monarch Watch!

Monarch Information from Past Seasons by Dr. Chip Taylor

Report on the Fall of 1998
The fall migration was unremarkable, especially in contrast to the migrations of the previous two years. With the exception of scattered reports from Nebraska and Kansas, few concentrations of roosting Monarchs were sighted during the fall. Monarchs were present in most locations but the numbers seemed to be down everywhere and tagging in most areas was only moderately successful. The weather during the fall migration was warmer than usual and fewer weather fronts with northwesterly winds passed through the Midwest in September and October. Storm fronts appear to concentrate the Monarchs. In some cases the Monarchs seem to ride the fronts and occasionally tagged Monarchs are found southeast of their origin following the passage of fronts with strong northwesterly winds. The best quantitative assessment of Monarch numbers comes from Dick Walton's monitoring program in Cape May. Dick and his volunteers recorded an average of 47 Monarchs per hour on their transects. This contrasts with a low of 10 seen per hour in 1992 and highs of 85/hr in 1994 and 107/hr in 1997. The average for all 7 years of Dick's monitoring program is 47.6 Monarchs per hour. Thus, in New Jersey, the 1998 population appeared to be close to the long term average. It's still unclear whether the numbers of Monarchs recorded at Cape May are representative of the entire eastern Monarch population. However, the highs and lows seem to correspond to the qualitative assessments we've made of Monarch numbers over the same period.

Winter 1998-1999
There were a number of alarming newspaper accounts of low Monarch numbers at the overwintering sites in Mexico this past winter. The numbers of Monarchs were certainly lower than in 1996 and 1997 but these were years with high population numbers. Were the numbers really down or were they average? We don't know. Eligio García counts the number of trees and measures the areas occupied by Monarchs at each of the known overwintering sites every winter. However, Eligio has only been conducting these surveys for the last few years so there is no clear sense of the long term average for the number of overwintering Monarchs. This past winter Eligio estimated the total area occupied by Monarchs for all the overwintering sites to be 5.55 hectares. If we use the Brower estimate of 10 million Monarchs per hectare, the number of overwintering Monarchs in 1998 was close to 55.5 million. If we use the Calvert estimate (pers. com.) of 13 million Monarchs per hectare, the number of overwintering Monarchs was 72.15 million. Both estimates assume that all the overwintering colonies were found and measured. Estimates of the total overwintering population will be needed for many more winters to establish the pattern of population fluctuations. These data are critical. Changes in weather patterns due to global warming, new agricultural practices in the United States and loss of habitat at the overwintering sites could all have a significant negative impact on Monarch numbers in the future. This information is required to save the eastern Monarch population. Without data on the population trends, it will be difficult to convince the governments of Canada, the United States and Mexico to adopt effective conservation policies to protect Monarchs.

Spring and Summer 1999
In spite of the many indications of a low Monarch population at the overwintering sites in Mexico, the reports of Monarchs seen this spring and the number of eggs and larvae found in many areas early in the summer suggest a return to normal Monarch numbers this fall. The number of Monarch sightings reported to Journey North in spring through mid-May in 1998 were surprisingly low considering the size of the overwintering population the during the 1997-98 winter. The low number may have been the result of a prolonged El Niño related drought that reduced the survival of the overwintering Monarchs and the returning migrants. The number of spring sightings was only about 60% of those reported for 1997 and signaled a generally poor summer and fall for the Monarch population. The weather conditions during the 1998 fall migration were extremely mild with few severe weather fronts. This may have aided the survival of the migrants particularly the stragglers since there were a number of recoveries of Monarchs that departed from northern areas as late as early October. There were no reports of weather related mortality during the winter in the colonies and the butterflies seemed to have access to water throughout the season. In March, it was not as dry in northern Mexico and southern Texas as it had been the previous year and these conditions may have been more favorable for the returning reproductive Monarchs. In any case, even though the number of overwintering Monarchs was lower in 1998-99, the number of spring sightings was similar to that of 1998. The difference between the two years may have been survival of the first generation Monarchs. The last of the first sweep Monarchs (i.e., the overwintering Monarchs) usually complete their migration and die by the first week of May in the Midwest. These few remaining stragglers from the previous fall are actually overlapped by first generation offspring from the southern states, especially Texas, which begin moving north in mid April. These butterflies begin moving into the northern states in the first week of May and this year they appear to have arrived in unusual numbers all across the northern tier of states. Karen Oberhauser reported higher numbers of Monarchs than usual in Minnesota and Wisconsin from May through mid July. There are similar reports of good early season numbers of Monarchs in Michigan, Maine and Ontario. In Kansas, the number of Monarchs appears to be average for mid July. On the other hand, Monarchs appear to be down along the mid Atlantic coast which has been experiencing a significant dry spell through most of the summer.

Fall 1999
What are the prospects for the fall migration? At this writing (19 July), they appear to be quite good. Large numbers of new adults are beginning to emerge in the northern states and the females will lay the eggs (mostly from 20 July - 5 August) that give rise to the migratory generation. Overall, the migration in 1999 should be better than in 1998 but the migrants will probably be less abundant than in 1996 and 1997 both of which were extraordinary years for Monarchs.

Winter 1999-2000
Monarchs began to arrive at the overwintering sites in Mexico during the last few days of October - right on time for the 1 November “Day of the Dead” celebrations in Mexico. By all accounts the winter was mild in Mexico and there were no reports of winter kill due to severe winter storms. It was a dry winter but not as dry as the 1997-1998 El Niño winter. Although water sources dried up and dust levels increased as the season advanced, creating concerns about the impact of tourism on the Sanctuaries and the monarchs, the population seemed to get through the winter in relatively good condition. Curiously, there was a shift in the proportion of the monarchs at the main colonies this past winter. Eligio García, who measures the size of the colonies, reported that the colony at El Rosario measured 3.78 hectares or 42% of the population of all colonies combined. This was an increase from 2.12 hectares (33%) in 1998. While El Rosario increased, the size of the colony at Chincua decreased to 0.92 hectares from 1.96, leading to speculation about the impact of tourism on the quality of this site. The two colonies are usually similar in size so the 4:1 ratio (3.78/0.92) this year was unusual. Generally, the colonies at El Rosario and Chincua represent 60-70% of the total overwintering population but this year only 52% of the monarchs overwintered at these two sites. How monarchs select overwintering sites is not known. The importance of characteristics of the forest habitat and the influence of proximate factors, such as weather or disturbance, is unclear. Continued monitoring of the overwintering populations is needed to establish the factors that determine yearly differences in the use of the overwintering locations.

Spring 2000
Monarchs were off to the best start since the spring of 1997. Modest numbers of monarchs were reported in Texas in late February. Despite the extremely dry conditions in much of the state and an abundance of fire ants in many locations, monarchs appeared to have reproduced with sufficient success to produce the wave of first generation adults which swept northward to colonize the northern part of the milkweed habitat. However, there is an ominous sign of drought in the Corn Belt, the heartland of monarch reproduction. As of mid-May the drought covered most of the Corn Belt; according to the isotope work of Wassenaar and Hobson (1998), this area accounts for 50% of the monarchs that reach the overwintering sites in Mexico. Weekly updates on soil moisture conditions throughout the United States are available online at enso.unl.edu/monitor. A few years ago it was easy to dismiss long-range weather predictions, they simply weren’t very reliable. The present weather models are much more accurate, so these forecasts are of real concern.

Fall 2000
As anticipated, the fall population was smaller than in 1999. Large numbers of fall migrants were reported only from the eastern Dakotas, Minnesota, western Wisconsin, and northern Iowa. Throughout the remainder of the range the number of fall migrants was low and in large areas of the northeast monarch numbers were extremely low. At Cape May, the census managed by Dick Walton produced 30.58 monarchs per hour for the eight-week migration season. This was the third lowest number of monarchs recorded since this program began in 1991. For detailed information, visit the program’s site at: www.concord.org/~dick/mon.html In spite of the lower number of migrating butterflies, approximately 70,000 butterflies were tagged by Monarch Watch participants. Again, this suggests that the number tagged is related to the weather conditions. Tagging seems to be most effective when large numbers of monarchs are kept from migrating by inclement weather. Under these conditions clustered or nectar feeding monarchs are available to taggers for longer periods. In most parts of the country the migration proceeded at the normal pace and was generally quite predictable. The exception was Texas where large numbers (100’s of thousands, perhaps millions) of monarchs were delayed late into October in south Texas by cool, wet conditions caused by a large Pacific frontal system that lingered over the state for at least two weeks. These butterflies were roughly 650-750 miles and three weeks away from the overwintering sites on 27-30 October 2000. If they made it, they were among the late arrivals.

Winter 2000-2001
Late in the fall, I predicted that the overwintering population would be 60- 70 million butterflies. This prediction was based on reports from numerous observers and many taggers. The previous year my estimate had been fairly close but I was wrong this year. The population was much smaller than I expected. Each year Eligio García counts the number of trees and measures the acreage occupied by monarchs at all the known overwintering colonies. The monarch colonies this year were smaller than any previous year, measuring only 2.83 hectares. At roughly 10 million butterflies per hectare, this translates to 28.3 million butterflies in the overwintering population. This is a significant drop from the 96-108 million in 1999 or even the 55.5+ million of the previous low year (1998). Although the number of butterflies was low, the condition of the butterflies seemed to be good when we visited El Rosario and Chincua in January. The weather and moisture conditions had been favorable and Eligio indicated that mortality had been quite low in the early part of the season. By early February, cold weather and exposure at San Andres, a monarch colony outside of the reserves, had evidently killed a large portion of the monarchs. The forest on this mountain has been illegally logged and partially burned. The area occupied by the monarchs in San Andres was described as significantly degraded. In November an estimated one million butterflies had taken up residence at this degraded site but by January the monitoring team estimated that 300,000 of the butterflies had died. A month later no living butterflies were found at this location; all that remained were dead butterflies that littered an area of 10,000 square yards. Whether the butterflies remaining in January survived by moving to another portion of the forest is unknown. A second catastrophic event killed large numbers of monarchs in several of the easternmost colonies. On the evening of 2 March, a severe rain, snow, and ice storm accompanied by high winds, took down trees, destroyed houses, and killed large numbers of monarchs at Cerro Pelon, Herrada (Los Saucos) and Palomas. Again, it is not clear how many monarchs were lost as the result of this storm. These colonies were small this year and according to Bill Calvert only twenty trees were occupied by monarchs at Herrada. Even though these colonies usually constitute less than 10% of the total population, the loss at these sites is significant at a time when the overall population is low. The same storm deposited a substantial amount of snow at El Rosario and Chincua but the butterflies survived well and only 7% of the population died as a result of this storm. Rumors that the monarch deaths at San Andres were due to intentional spraying by loggers have been refuted and similar accusations by local residents at another colony have not been confirmed.

Spring 2001
A critical factor in the build-up of the monarch population each year is the number of female monarchs returning from Mexico each spring. These females lay eggs on milkweed as they move northward and the success of this reproductive effort determines the number of monarchs that move from the southern states in May to colonize the northern portion of the breeding range in May and June. The number of female monarchs returning north this spring (4.9 million, see box page 42) was lower than any year since at least 1992. Approximately five million females seems like a good number but it represents only one-third to one-half the number of females that normally move Amonarch tagged during the University of Kansas Audubon Society’s tagging day September, 2000. In March and April the population did not appear to be off to a good start. This was an unusually cold, wet spring in Texas and monarchs were 10-14 days behind their average arrival dates as they moved north. They were a month behind in eastern Kansas. Fortunately, in spite of the delays, the rains of the previous fall, winter, and early spring in Texas provided excellent conditions for milkweed and nectar resources for the breeding monarchs. Each year the key to the rest of the season often seems to be the reproductive success of the first generation. This year the reproduction seems to have been better than usual. Good numbers of monarchs had been seen moving northward through eastern Kansas in mid- May and this movement continued into June. More importantly, monarchs arrived in the northern states in mid May and substantial numbers of eggs were reported in a number of locations.

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