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.

   

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

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.

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 17^th 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 10^th. 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

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

Conclusion

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.

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