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dimanche 1 décembre 2019
avinews
Breeding success of Bewick’s Swans in 2019: Please take part in the European census on December 14th/15th

As in previous years, a Europe-wide survey of the proportion of young birds in the Bewick's Swan will take place on the weekend of 14-15 December. In recent years we have been able to take a large sample thanks to the well-established network of observers. We hope that this will be possible again this year: Due to the mild weather so far, many Bewick's Swans are again staying with us (see map).

In the future, the recording will always take place on the date of the mid-month count in December. Our colleagues at the European level have thus responded to our request for a bundling of the count dates, which we are very pleased about.

In December 2018, the nationwide survey of the proportion of young birds in the Bewick's Swans resulted in a proportion of 10.6%. After the very low proportion of young birds in the previous year (4.2 %), an average breeding success was again achieved in recent years. At population level, the proportion of young birds was 8.7% in a sample of around 9,000 birds.

As in previous years, the proportion of young birds in Germany in 2018 (10.6 %) was higher than the total proportion of Bewick's Swans wintering in northwestern Europe (8.7 %). This is remarkable because Germany currently forms the eastern border of the core wintering area. Here it is on average colder than in the Netherlands or England. One would expect that adult birds with juveniles would lead them further west into the milder wintering areas. However, there were remarkable differences within Germany in December 2018 that support this assumption: While in Schleswig-Holstein 8.6 % young birds were identified (n = 1,178), in Lower Saxony 11.8 % (n = 1,930).

The number of young birds could be determined for 104 families. Most successful adult birds had one or two young birds "in tow". In contrast to the previous year, families with four juveniles could also be observed on the counting weekend. The average number of juveniles per family was 1.8 (2016: 2.2 juv./fam.).

Many thanks to everyone being involved!

Beneath the percentage of young birds, we would also very welcome taking notes on the type of wintering habitat. Something which many of you are used to do anyway already. If there are motivated birders among you seeking for an interesting challenge, recording the numbers of family groups would be also of great value. For information on how to go about when entering this kind of data please see below.

Background: At this time of the year, assessing the percentage of young birds in wintering flocks of Bewick’s swans is a very good way of getting information on the species’ breeding success. The share of this year’s birds in wintering flocks allows conclusions on the conditions in the species’ arctic breeding grounds during the past summer. It therefore functions as an indicator helping to explain recently observed population changes. The Bewick’s Swan is one of the very few goose and swan species with decreasing population numbers. Among others, low breeding success since the 1990s seems to be one of the contributing factors. Yet, what exactly triggers the parents’ struggle to rear their young is not clear.

Many thanks for your support!

Axel Degen (national coordinator), Wim Tijsen (int. coordinator)
 

Recording age/sex, percentage of young and foraging habitat type:

  • Please enter information on age and sex in the respective data entry boxes and NOT in the entry field for general comments. If you do so, it is very difficult for us to use the information for our analyses!
  • Please always state information on all sexes and ages observed. For example, in a flock of 28 Bewick’s swans you have aged 23 adults and 5 young. If you only submit the 5 young birds, it might mean that all the other birds were adults but it could also mean that there were 5 young birds among 28 birds in total. Concluding all other birds were adults would be wrong. Ambiguous entries as such therefore have to be excluded from the analyses.
    One reason for only submitting the number of young birds or the number of only one of the sexes might be that there is only one row available for entering age and sex data. For entering the other sex or more age groups please click on “add age/sex details for more birds”. Alternatively you’ll find a data entry field beneath in which you can fill in your information easily and conveniently. By clicking in the box a window with all abbreviations and information on how to enter the data opens automatically.
  • Please avoid using the term “immature” when a more exact age can be stated. For birds fledged in the past breeding season please always use the term “1st year” up until December the 31st and after January the 1st please always use “2nd year”. This is essential when aging Mute swans as even birds in their 2nd year can often be distinguished from adults. For differentiating between birds in their 2nd or 3rd year and other adult birds please do so by submitting your entries via extra categories.
  • Please be aware: Small and big bird flocks have different age proportions. Small groups or families, where individuals can be aged often by one glimpse, tend to have a higher proportion of young birds than bigger bird assemblages. Although it is time consuming, please try to age bigger flocks whenever you can take your time to do so. This information is very important for not overestimating the overall percentage of young birds.
  • Please also submit age information details when the flock only consists of adult or only of young birds! We sometimes got the impression that mainly age details of flocks only consisting of mixed age groups are submitted.
  • Recording family group sizes in geese or swan species is also very important. If you would like to submit this kind of information, please do so in the general comments field, e.g. by entering: “families: 3x1, 4x2, 1x5 Juv.”

Wintering or foraging habitat type

  • When recording birds using farmland habitat for roosting or foraging (esp. in wintering geese, swans, cranes and waders) taking notes on the habitat type in which the birds have been seen is important additional information. If you are able to give respective information, please do so via the “(resting) Habitat” data entry box in the “Additional information” section. By using the general “Comments” field for this kind of information it is difficult for us to use it for further analysis. In geese or swan species, the proportions of young birds differ between different kinds of habitats. For being able to account for this, information on wintering or foraging habitat type is very valuable and important.
  • Whenever possible, please also give information in the “Specifications (e.g. behavior)” entry field.  This allows distinguishing clearly between roosting birds and migrating birds / birds flying over. When recording migrating birds, the exact time of the observation is very important to put through. For stating the observation time, just simply add it where you have entered the date separated by a single blank character, e.g. “23.11.2016 09:00”.
  • Differentiating correctly between „foraging“ and „resting“ behaviour is not always possible. If in doubt, please enter “foraging” even if a part of the flock seems to be roosting. Submitting two different data sets for a single wintering flock of birds whose individuals only differ in the way they behave at the time of observation is not useful. Same is true for parts of a single roosting flock which stretches over more than one habitat type. If you have a flock using two different habitat types, please only submit the one which is used by the majority of the flock. However, spatially separated flocks in bigger landscapes should always be recorded with accurate location and separated from each other.

 

The results of the breeding succes of Bewick´s Swans in 2018 can be downloaded here as a PDF file.

Foto: Jörn Clausen, 8.11.2015, Oppendorfer Fledder

posté par Nikolas Prior
 
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page :
 
 
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