October 17th 2005: Common Cranes (Grus Grus) exceptional migrating day in the Nièvre county (France)

Summary

Introduction

 

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.


   
Nièvre observers –who have been taking part in the Common Crane project ("Réseau Grues France") for nearly ten years- are therefore situated on a kind of motorway toll, covering a 130 km-wide corridor, and are able to quantify the birds' migrations, particularly when the birds are going away from the Der Lake –on the border of the Marne and Haute-Marne counties

It usually takes Common Cranes 4 to 5 hours to fly from the Der Lake to the Loire River, and observers are regularly informed of flights to come via the "Réseau Grues France" network coordinated by Emmanuel Le Roy (LPO Champagne-Ardenne). Flocks flying directly from Germany generally fly over the Nièvre at night, mostly during the second half of the night, and cannot be counted, even when they detected by observers.

 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.
 

Site

Number minimum

Number maximum 

data minimum

data maximum

Hours

Cosne-sur-Loire

80

80

1

1

15h30

from Pouilly to La-Marche

4000

6100

15

16

10h puis 14h-18h

Germigny / Cours-les-Barres

12149

12149

53

69

13h50-18h

from Fourchambault to Marzy

12627

12627

56

66

09h30-10h30 et 14h-17h30

Nevers

3610

5515

11

23

14h20-18h50

Livry

20

20

1

2

12h puis 18h

axis Imphy / St-Parize

4261

5291

10

16

12h puis 16h-18h15

from Sougy-sur-Loire to Decize

527

600

8

8

16h25-17h15

axis Druy-Parigny / Aubigny

6170

6170

39

39

16h00-18h30

Corbigny

150

150

1

1

17h30

Morvan-Tintury

740

1640

5

6

12h then 14h-16h

Total

44314

50322

199

245

 

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.
  

 

Discussion

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 .


Size of groups in the Nièvre county - October 17th 2005

 
  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.  

Conclusion

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 France" network.

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)
SOBA NATURE NIEVRE
18 place de l’église
58180 MARZY
FRANCE
soba_naturenievre@yahoo.fr

Ref/
Merle S., 2005. 17 octobre 2005 : une journée exceptionnelle de migration des Grues cendrées (grus grus) dans la Nièvre. Nature Nièvre N°13 : 39-44

 

Summary

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