Greylag goose (Anser anser)
Earlier migration of greylag geese
Autumn migration for greylag geese peaks in Southern Norway these days, and many greylags are seen crossing the Lista peninsula. We present an analysis of the migration counts from Lista Bird Observatory from 1990 until 2012 showing that the peak of migration has moved forward in time compared to the 90s. While autumn migration seems to be strongly influenced by the start of the hunting season, spring migration could be influenced by the earlier arrival of spring. The shift in spring migration corresponds to about one week, and for autumn migration the shift is approximately two weeks.
Fig.1. Peak migration date for greylag geese from 1990 to 2012
Peak migration date is defined as the date at which half of all individuals during one season has passed. Spring and autumn season is defined as the two periods 15/3-10/6 and 15/7-15/11. The vertical lines indicate when 10% (lower limit) and 90% (upper limit) of all individuals have passed.
Fig. 2. Peak migration date, averaged over tree-year periods.
Peak migration date is here plotted for every third year, where each data point corresponds to the average over three years. Error bars correspond to the standard deviation in the mean.
By M. Wold
It is hard not to notice them, either you hear them first and then see them crossing above your head, or you see them flying in formation in the distance with a steady course toward South-East. Some land to rest in the fields, and others on the lakes or the ocean for a break overnight.
Greylag goose breeds along the Norwegian coast, all the way up to Finnmark. When autumn comes and the young birds are on their wings, they start migrating toward winter quarters in Holland and Spain. The greylag goose is a bird that migrates in family groups, the young ones together with the adults. In the early morning hours at Lista Lighthouse, it is not unusual to see large flocks of up to several hundred individuals passing by. Some come flying along the coast before adjusting the course when passing the tip of the peninsula, others come along the hills, fly over Borhaug village, before setting the course straight toward Denmark.
Greylag goose migrate earlier now then in the 90s.The migration counts at Lista Bird Observatory show that the timing of migration has shifted. The peak migration occurs earlier in both spring and autumn, compared to what it did in the 90s. Figure 1 shows this; the upper part displays the date for peak migration in spring from 1990 until 2012, and the lower part shows peak migration date in the autumn over the same time period.
Graph with higher resolution (2MB)
Spring migrationDaily counts at Lista going back to 1990 show that the average date for peak migration occurred around April 12th (standard deviation of peak migration distribution 1990-2000 is 7.2 days). For the following decade, the date for peak migration has moved forward with approximately eight days to April 4th (standard deviation 2001-2012: 3.4 days). The effect is weak, but significant. A correlation test (Spearman’s rho) also shows that there is a shift toward earlier spring migration, the p-value is 0.00016, i.e. the probability that there is not a shift in peak migration date is 0.0016%.
One possible explanation for the shift is that spring arrives earlier. Greylag geese are short distance migrants, timing their migration to local conditions. When the weather becomes milder and they sense spring arriving, they know it is time to set course to the breeding sites. Birds arriving early in Norway will occupy the best breeding sites, hence it pays off to arrive early.
Whether the shift in spring migration can be explained by an earlier arrival of spring cannot be determined based on these data. One also assumes that a large fraction of geese migrating across Lista breed in the neighbouring counties Rogaland and Hordaland. This far south, the breeding season starts earlier than further North. In addition, the greylag population has increased in later years. If the increase in the southernmost populations is larger than further north, this might also create a shift in peak migration toward earlier dates.
Autumn migrationAutumn migration also occurs earlier. The average peak migration date during the first half of the 90s is late in August, around August 29th (standard deviation 5.5 days), while the average date from 1997 to 2012 has moved to August 14th (standard deviation 4.6 days). Hence the timing of autumn migration has been shifted with approximately two weeks. The shift in the peak autumn migration is most likely caused by the onset of the hunting season for greylags. Up until 1996 the regular hunting season started on August 21st, whereas from 1997 and onwards the starting date was set to August 10th.
The change in starting date for the hunting season 1996/1997 is reflected in the migration counts. After 1996 there is only one year (2001) where the peak migration date appears after August 19th, all other dates appear earlier than this. In the lower part of figure 1, the start of hunting season on August 21st and 10th in their corresponding years is marked with a dashed, red line. The bird observatory’s database on migration counts hence shows that a consequence of the earlier hunting season during later years has resulted in earlier autumn migration of greylag geese.
Furthermore, it also appears that a change from 2001 to 2002 might be present in the data set, as the average peak migration date also here goes through an abrupt change. Autumn 2002 was the first year where an extraordinary period of hunting was allowed, starting up to 10 days before the regular hunting period, and only allowed on farmland. (In 2007 this changed to up to 15 days before regular start of hunting period). If the data set is divided into three equal parts, and the average peak migration date calculated for every three-year period, it becomes evident that a significant change in migration date happened around 2001-2002. The average for 1999-2001 is August 18th, and for 2002-2004 it is August 12th, a change of approximately six days. In the accompanying table, we have listed average peak migration date per three-year period, and the standard deviation in the mean date for each period (gives how well the mean is determined).
Figure 2 higher resolution (1.3 MB)
ConclusionWe conclude that spring migration of greylag geese has shifted to earlier in spring with about one week during the last decade, while autumn migration during the same period has shifted with almost twice as much. A possible explanation for the earlier spring migration is that spring arrives earlier now than in the 90s, whereas the shift in autumn migration can be readily explained by the onset of hunting season.
|Year||Mean migration||Std dev||Mean migration||Std dev|
| ||date spring||mean (days)||date autumn||mean (days)|
|1990-1992 ||15. apr||0.9||29. aug||4.7|
|1993-1995 ||15. apr||4.3||30. aug||2.6|
|1996-1998||13. apr||4.3||21. aug||3.1|
|1999-2001 ||3. apr||1.9||18. aug||3.5|
|2002-2004 ||4. apr||1.5||12. aug||0.7|
|2005-2007||5. apr||3.8||13. aug||2.1|
|2008-2010 ||2. apr||0,9||9. aug||2.6|
|2011-2012 ||4. apr||1.0||13. aug||2.5|