Greater Sand Plover

The statistics presented for this species includes a map and and overview of where most birds have thus far been caught. In addition and only if sufficient data is available to do so, it also includes an age overview and an overview of the juvenile percentages

age overview

The following statistics are being presented:

recaptured: is the total number of individuals for which recaptures exist. The data presented are the last known recaptures of banded individuals (all intermediate recaptures have been filtered out).

Things to consider when viewing these data:

  1. we only conservatively estimate the age of the birds, where 0 means that the bird was recaptured as 0 years old. So, the bird was both banded and recaptured within its first year of life. Similar to how you age people; mind you that these birds were never seen again, since we only used the last known recapture of each individual.

  2. we do not really measure maximum age (only the age at which a bird was recaptured for the last time).

  3. since recently marked birds are also included that likely haven’t seen the end of their life yet, younger birds are overrepresented.

  4. assuming high site-fidelity of birds, the method will see younger birds overrepresented if incidental catching sites are included; only consistent regular catching gives unbiased results.

Juvenile percentages

Juvenile percentages can provide good insights in annual reproductive success and variations therein over the years. The juvenile percentages are calculated using data from all cannon-net catches (since other techniques may be skewed towards juvenile birds), with the exception of winter catches in migratory waders (which typically contain juvenile birds only). What we consider “winter” and “summer” catches may vary from species to species. The dates used take account of when it is judged that all adults and all juvenile birds have arrived in the non-breeding areas which we sample and also the date up to which we believe no adult birds have yet departed for their breeding grounds. These dates are mostly the dates which have been used in recent years for those species which are annually monitored and reported on in Arctic Birds, Stilt and the VWSG Bulletin. For other species then the recommended dates have been derived from our knowledge of the timing of movements of adults/juveniles into and out of the non-breeding flocks.

These dates may differ for some species from those used historically for preparing juvenile data. You may well find therefore that in these species the figures you will derive are not exactly the same as the figures published previously. An example to illustrate this relates to Turnstone and Sanderling. Initially cut-off dates of March 1 were used. Our more detailed knowledge of departure dates for adults on northward migration has enabled us to extend the sampling period by having termination dates in early April for these two species, giving a whole month of extra data collection opportunities. For Greater Sand Plover we only consider catches for these juvenile percentages between 15-Oct and 15-Mar

Seasonal averages (i.e. years starting 1 August) are presented for selected species (i.e. species where enough representative data is available) with 95% binomial confidence intervals. For species where we have >1000 birds caught, we also conduct a logistic regression to study trends in juvenile percentages over time.

Finally, a 10 year right-aligned (i.e. the average of the preceding 10 years is depicted) of (unweighed) juvenile percentages is also presented as a purple line.