Breeding success

breeding_sucess_bewicksswansField-based age assessments were initiated in the 1940s, initially for European White-fronted Geese. This was extended to most other populations between the 1960s and 1980s and currently the two migratory swan species and eleven goose populations native to the UK are all assessed.

The aims of the surveys are to determine the annual reproductive success of each goose or swan population and are mostly carried out at wintering and autumn stop-over sites.

Age assessments of geese and swans comprise two measures of annual reproductive success (or productivity): the proportion of young (first-winter) birds in non-breeding flocks and the average brood size.

Due to differences in plumage characteristics, swans and geese in their first winter are usually easily separated from adult birds, at least for part of the non-breeding season. In many swans and geese, young birds remain in family units with their parents throughout their first winter, and thus it is also possible to measure the number of young produced by successful breeding pairs. Counters, therefore, record the number of young birds present in a flock and also identify family groups making a note of the number of young within each family.

The timing of migration and post-juvenile moult differs between species, meaning that the time period in which data can be collected also varies between species (see below). Plumage differences also vary between species and are more subtle for some species than others, meaning that the level of experience required by the observer is greater.

Age assessments are carried out annually and results from these surveys can be found on the species account pages.

Survey periods for age assessments

A count form and guidance notes are available to downloaded from our GSMP network page.

Getting involved

The skill level required to conduct age assessments varies considerably with the species concerned and most age assessments are conducted by a small network of experienced observers. An observer must be confident in identifying juvenile (first-winter) geese or swans of the particular species and have some experience of counting birds. The survey requires at least one count per month during the priority period (see above).

If you would like to participate in age assessments, please contact WWT’s GSMP Team.