Migration et autres types de déplacements de la Mésange noire Parus Ater en transit au col de la Golèze-(2e partie)
Authors: Scherrer Bruno 
Published: November 2022

This study of the activity and migratory behaviour of the Coal tit (Parus ater) is based on observations made at the Golèze pass.

1) The migratory movement of a high percentage of Coal tits starts between 07.00 and 09.00 hours (maxima 08.00 hours). No migratory movement was recorded between 12.00 and 15.00 hours.

2) Birds which have rested several days move earlier in the morning, but «tired » birds which will remain in the area for a number of days are the first to cease movements during the day.

3) The daily movement is variable in duration but averages 4 to 5 hours.

4) Coal tits do not continue their migration every day. Certain days are devoted to rest. The average pause is 3.5 days, but this becomes shorter near the end of the season.

5) Birds feed particularly during the afternoon and evening, but they may also feed during migratory movements.

6) The migratory speed varies according to habitat type. In forests, the speed of movement is relatively slow (5-10 km/hour), but in open areas speed is increased and reaches 20-30 km/hour. From consideration of band recoveries, an average daily movement of 25 km, with maximum of 85 km/day, was estimated. The average daily movement increases at the end of the season.

7) Coal tits are very sensitive to meteorological conditions. High numbers cross the pass in clear weather, but movement is almost completely disrupted during rain or fog. Two meteorological factors, cloudiness and wind, appear to have a direct influence on the passage of Coal tits : clear sky and a small valley breeze are propitious for movement. Temperature, pressure (absolute values) and the passage of a cold front do not have any direct influence. Increasing and decreasing pressure probably act through their effect on cloudiness and wind. Low humidity which would appear favourable for the passage of Coal tits, in fact augments the valley breeze which drives the birds to the top of the pass.

8) Moulting sequences were determined using the english «moult cards » methods.

9) Visual estimation of adiposity proved acceptable. Visual estimations correlated well (r = 0.86) with the percentages of lipids obtained by extraction.

10) Some juveniles finish their moulting at the time when other individuals start to moult.

11) Moulting Coal tits have little or no fat reserve. Fattening starts at the end of the moult. No relation was recorded between the fat reserves of juveniles and the date of capture or the type of movement being undertaken.

12) The timing and duration of the moulting period do not vary according to the type of movement.

13) In 1969, an invasion year, the rate and extent at which fat was stored was greater than in 1968, a year of partial migration.

14) The average fat level in 1969 comprised 8.6 % of the total fresh weight. According to rough estimates, the flight capacity reached 25 to 140 Km/day. A more probable estimate would be 70 to 80 Km/per day.

15) Migration of birds with good fat reserves starts an hour earlier and finishes an hour later than for other individuals.

16) Lean birds rest longer than the «fat» ones. Fat levels increase with the duration of the rest period.

17) The highest migration speeds were registered for individuals with good fat reserves.

To conclude :

a) All intermediates exist between erratism, partial migration and invasion.

b) The different responses observed during these three types of movement relates closely to the balance between population level and food resources.

c) From the evolutionary viewpoint, this scale of responses appears well adapted to ensuring a maximum survival of the species.

d) Coal tits do not have a great facility for migration ; fat reserves are low, the sense of orientation is poorly developed. Journeys of 1500 kilometers are undertaken in a way to minimise risk. Daily flight is short, major obstacles are avoided, the birds remain in favourable habitats, they migrate during fine weather and feed during migration.