October 17th 2005: Common Cranes (Grus Grus) exceptional migrating day in the Nièvre county (France)
Thanks to its excellent location on the migratory path of Common Cranes, the Nièvre (France) sees an average of about 60% of the total Common Crane population migrating through France. The Loire river, along which most of the Nièvre population lives, is located almost perpendicular to the migratory axis.
On October 17th 2005, there was a massive Common Crane migration
over Nièvre county which was exceptionally big and early in the season.
This report aims at describing the event and placing it in a broader context.
The 'record' flight in figures
|297 data were sent over by 49 different observers; our
first task was to suppress double data, particularly those from Clamecy and
Châteauneuf-Val-de-Bargis, two towns situated well above the imaginary Loire
frontier on the main migratory path, because the birds were most probably
seen again later (although that is a moot point too –see Discussion). The
remaining data were then more accurately analysed, and sorted according to
several criteria. Reliable and accurate data (such as precise time to the
minute, exact location, number of birds for each separate flock, etc.)
resulted in a minimum total number to which was added the less accurate data
(locations that were outside the main axis, a suspicion of double data that
remained, etc.) in order to find a "maximum" total number. That number
obviously cannot be considered as a true maximum, for that would imply that
all flocks were seen! However, it represents a fairly good maximum indicator,
given the error margin of counting.
That is an impressive number if one considers that between 44,000 and 51,000 Common Cranes flew over Nièvre in a single day ! That is, 47,000 ( plus or minus 4,000 )birds, with a reasonable error margin.
Timing and locating the 'record' flight
|The first flocks were reported early in the morning:
between 9:30 AM and 10:30 AM, 370 Common cranes split into 7 different
flocks and crossed the Loire river between La Charité-sur-Loire and Marzy.
It is unlikely that these birds halted at the Der Lake, for that would
suppose they had left 2 hours before sunrise; the cranes were more probably
flying directly from Germany, and one can logically deduce that some also
flew over our county at night, between midnight and 7 AM.
An isolated flock was later reported at midday over Tintury, in the Décize axis. The Common crane parade finally started for real at 12:00 over Clamercy, before the first group was reported over the Loire river, over Germigny-sur-Loire at 1:44 PM.
Map displays the places mentioned in this text, as well as the paths the flocks followed at the time of crossing the Loire river..
The main migration axis is only 10 km-wide, from Cours-les-Barres and the Bec d’Allier (confluence of the Allier river and the Loire river) to Marzy. 24,776 Common cranes (about 53% of the total flock) were counted in 103 flocks; two observers, Estelle CHAMPAGNAT (based in Cours-les-Barres) and myself (based in Marzy), numbered 23,188 birds between 1:56 PM and 6:03 PM. We sometimes spoke on the phone so as not to count the same flock twice, knowing that the telescope enabled us both to count the same flocks flying overhead . Generally speaking, we met no difficulty in keeping our flocks separate, as we were both busy counting exclusively what was happening directly overhead. This group of some 25,000 Common cranes was most probably spotted again later above the Tronçais Forest (Allier county), 50 km further south-east (source: Réseau Grues France).
In the afternoon, flocks progressively shifted from north to south, still following the Cours-les-Barres / Marzy corridor as a main axis. Most of the flocks that cut across the Loire river flew over the Val de Loire National Park (between Pouilly-sur-Loire and La Charité-sur-Loire) between 2 PM and 4:30 PM; some data concerning that area were compared some time later in the afternoon, which probably makes the assessment less accurate. In Nevers, the flocks flew over between 2 PM and 6 PM, peaking between 3:30 PM and 4 PM. The birds only started flying over Imphy and Decize, further south, at 4:30 PM finishing at 6:30 PM. The flock was probably counted less accurately over that less inhabited area, with the exception of the Aubigny site (Allier county), where Jean-Christophe SAUTOUR accurately numbered 6,170 Common cranes in 46 flocks.
The north-west of the county was hardly visited, except for one flock in the morning, which was unusual. The part of the corridor used on October 17th was no wider than 70 km, and included two-thirds of the Nièvre county, undoubtedly because of the north-northeast wind blowing that day
The chronology of the flights (see graph) shows, between 3:15 PM and 5:15 PM, a peak of 31,816 cranes –67% of the total flock! On five separate occasions, more than 3,500 cranes every15 minutes were spotted migrating over the Loire. The last flocks were reported before 7 PM, that is to say 40 minutes before night fell completely; one may therefore think that the passage was in fact over by 7 PM.
Wintering sites were visited during this migratory wave.
The Nièvre site was a stopover at 7 PM for a few hundred cranes who
continued their flight during the night after a brief halt. In fact, no
crane was to be seen there the next morning, and the true wintering cranes
did not arrive until November 10th. Similar behaviour was observed at
the Cher site, though it had been visited without interruption since
mid-October. The total number doubled during the passage, before dropping to
its initial level ten days later.
|As an element of comparison, there were as many cranes in
the Nièvre county that day as during the whole autumn of '98 or '01! Cranes
keep breaking records in our county: the October 15th 2003 (24,600 cranes)
and November 5th 2004 (45,000 cranes) records have been shattered. Mass
“flying days” are a fairly recent phenomenon, partly as a result of the
increase in the west-European population. A change in crane behaviour also
seems to be happening: mid-October flights used to take place at night,
whereas now a vast majority of cranes halt in north-eastern France (mainly
at Der Lake), for just a few hours or half a day.
That is what happened on November 17th: At least 60,000 cranes had transited through only one "site" in Hesse (Germany) the two previous days , halting at the Der during the night. They must have left the Champagne-Ardenne county en masse between 11 am and 1 PM. This is confirmed by the thousands of cranes observed while migrating south-west near the Forêt d'Orient Lakes (Aube county) later in the morning. Unfortunately, no data can number these movements prior to the Nièvre county.
The mere size of such huge migratory waves may cast doubts on the accuracy of the census. Depending on the observer's experience and the size of the flocks, overestimations as well as underestimations are inevitable. In my own case, when, at 3 PM, I realized I might be overwhelmed by the sheer number of birds in the flock, I systematically photographed the successive waves. Then I made a rough visual estimation of each flock whenever possible, so as to give myself some standard. I now do this every year. We were later able to compare these figures by counting the birds on the 36 photos taken. Other observers have already used this method. Generally speaking, flocks are very accurately estimated when they do not exceed 400 cranes; even for quick, rough estimates, the error rate being less than 10%. When flocks can be numbered crane by crane –which is the case when there are less than 200 birds-, the error rate is very reasonable (less than 3%). On the other hand, with flocks over 500 cranes, my error was a 20-odd% underestimation, which happened three times – for respectively 792, 890 and 990 cranes. Of course, the problem is aggravated when cranes circle in the sky
That is when photography turned out to be a very useful ally: it helped to count an unbelievable flock of 2,255 Common cranes over Marzy on 9th November 2004, at 2:17 PM, creating an eerie atmosphere overhead for 3 minutes. Naturally, other observers must have made similar -but opposite- mistakes, massively overestimating flocks. However, distributing a sample of 183 accurate data (34 804 cranes) into number groups (graph 2) has shown that 72.6% of the flocks were inferior to 200 individuals, and 40% were composed of less than 100 birds. The distribution into number groups from the Clamecy sample (67 data, 10 265 cranes) is almost identical. Only a limited number of flocks (11) are concerned by major errors, which could be corrected on three occasions thanks to photos. Logically, the error margin of the 47,000 count is estimated to be less than 10 %, hence the margin given in the figures .
|Still, most of the Nièvre observers -including myself-, have a natural tendency to underestimate rather than overestimate huge flocks, and it is conceivable that there were over 50,000 cranes on October 17th. Besides, the Clamecy data (10,265 as counted by Lucien ANGERAND) or those from the Châteauneuf-Val-de-Bargis centre (12,000 cranes, as estimated by Romuald HESLOT), though very useful to the understanding of migratory paths, were not taken into account in the final number results as they were too far away from the Loire river. In fact, the first 200-crane flock (12 AM, Clamecy), though it was not included in the count, was apparently not seen again later, for that would suppose the remaining 60 km to the Loire river (near Germigny) to have been flown at an average speed of 34 km/h, which seems rather slow considering the weather. Besides, one can wonder about the axis that was really followed by the 12,000 Châteauneuf-Val-de-Bargis cranes: they are likely to have flown over Cours-les-Barres, considering the way flocks shifted south (the flock sizes tally), but it cannot totally be excluded that all or some of them did fly over a virtually observer-free zone (between La Charité-sur-Loire and Germigny-sur-Loire). The 47,000 Common crane estimate is therefore unlikely to be overestimated.|
The positioning of Nièvre observers over the Common cranes' main migrating
corridor enables us to detect a large part of the species' post nuptial
migration, especially thanks to a privileged position with regard to Der
Lake, and to a well-structured organization, within the "Réseau Grues
The Spring migration will be quite different, for prenuptial migration has clearly shifted towards the counties west and north of our county, outside our surveillance. It will probably take us almost another year to see anything like this again all the more so as spring flights are largely nocturnal except for flocks coming from Aquitaine.
Many thanks to
|all the observers who transmitted their data to the "Réseau
Grues France" network
ANGERAND Julien, ANGLARET Bernard, BACOT Hugues, BARBIN Marie-Hélène, BARGE Christophe, BARGE Véronique, BAYLE Arnaud, BOLNOT Yves, BOUDEAU Evelyne, CHAMPAGNAT Estelle, CHEVAU Gérard, COMTE Christophe, CONTANT Pascal, COQUERY Stéphane, COQUILLAT Jean, DEVIENNE Gilles, ETTORI Gérard, FRANCK Danièle, GACON Sylvie, GENOUX Liliane, GIRANDE Jean-Claude, GUEGAN Brigitte, GUENY Michel, GUISCHER Valérie, HERENT Jacqueline, HESLOT Romuald, IMBOURG Maïrik, JOST Jean-Paul, JULLIARD Christian, KOCKELKOREN Annie, LACROIX Marc, LALEURE Jean-Claude, LALEURE Nicolas, LEBRETON Stéphane, LECLAND Marcel, MERLE Emmanuelle, PY Jean, PY Nicole, ROUGIER Michel, ROY Jean-Michel, RYF Irene, SABLAYROLLES Jean-Noël, SAUTOUR Jean-Christophe, SIGNORET Lucienne, TAIEB Franck, VIDEUX Jean-Claude, VIEUX Christian,
to Ray LANGFORD for its contribution on this translation in english.
and to Francis DESJARDINS and Emmanuel LE ROY (LPO Champagne-Ardenne)
Sébastien Merle (translated
into English by Cyril Merle)