Larus cachinnans in the Netherlands

(last update: December 24, 2021)

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Nests at De Kreupel 2021
nest r-02P4 20
nest w-201U 21
nest g-XLHD 21
nest r-KA 21
nest r-KB 21
nest r-KC 21
nest r-KD 21
nest r-KE 21
nest r-KF 21
nest r-KH 21
nest r-KJ 21
nest r-KK 21
nest A01 21
nest C03 21
nest C04 21
nest C06 21
nest D01 21
nest D03 21
nest D05 21
nest D06 21
nest D07 21
nest D08 21
nest E01 21
nest H01 21
nest L01 21
nest L02 21
nest L05 21
nest L06 21
nest L07 21
nest M01 21
nest M04 21
nest M06 21
nest Q01 21

Lelystad:
nest y-PNED
nest y-PESV
nest y-PLDP
nest y-PHSV
nest y-P:A02
nest y-P:U60
nest zy-MT
nest r-FB
nest r-FC
nest 001
nest 003
nest 006
nest 009
nest 011
nest 012
nest 013
nest 014
nest 015
nest 016
nest 016B
nest 017
nest 018
nest 019
nest 021
nest 025
nest 027
nest 028
nest 029
nest 031
nest 032
nest 033
nest 034
nest 035
nest 036
nest 037
nest 038
nest 039
nest 040
nest 041
nest 042
nest 044
nest 045
nest 046
nest 047
nest 048
nest 053
nest 055
nest 056
nest 058
nest 060
nest 062
nest 063
nest 065
nest 066
nest 067
nest 072
pair A
pair B

Cachinnans breeding in the Netherlands - scoring breeding bird phenotypes

PHENOTYPIC CHARACTERISTICS OF BREEDING CASPIAN GULL AT LELYSTAD 2020 & 2021

Every Lelystad visit in 2020 and 2021, we photographed breeding birds, both partners at the nest, both standing and flying as much as possible.
From the images we scored all birds to determine taxon / species, using traits mentioned in Gibbins, Neubauer & Small 2011.
For 2020, the complete set of images was scored by Albert de Jong and Mars Muusse seperately, then put together and checked for differences in 'total score'.
The total scores resulted in 'conclusion' on species ID, and fortunately, conclusions were very close for either Albert or Mars.
For 2021, the complete set of images was scored by Mars, and also included below.

Chris Gibbins, Grzegorz Neubauer & Brian Small - British Birds 2011

The individual trait scores sum to a 'total score'. Classification is:
- score 4 to 12 = Caspian Gull
- score 9 to 20 = Hybrid
- score 12 to 20 = Herring Gull

species composition Lelystad of selected nests (table above)

• This is NOT the species composition for the whole colony, this is the compostion ONLY for selected nests
• As described earlier: nests were selected for:
  - one/both of the birds guarding at lookout post seemed to be cachinnans
  - a nests was built / eggs were present very early in the breeding season indicating cachinnans
  - shell pattern of eggs & egg dimensions indicated cachinnans
  - or (very often) a combination of these 3 points
• For 2020, we estimate 44 Caspian Gulls or Caspian Gull hybrids in colony Lelystad
• For 2020, 85% (44 out of 50 scored birds in the sample) are classified Caspian Gull or Caspian Gull hybrid
• For 2021, we estimate 62 Caspian Gulls or Caspian Gull hybrids in colony Lelystad
• For 2021, 80% (62 out of 74 scored birds in the sample) are classified Caspian Gull or Caspian Gull hybrid

pair composition Lelystad of selected nests (table above)

• This is NOT the pair composition for the whole colony, this is the compostion ONLY for selected nests
• Nest selection criteria for this sample: see above
• For 2020, we estimate at least 22, but up to to 28 nests involving Caspian Gull or Caspian Gull hybrid in colony Lelystad
• For 2021, we estimate at least 30, but up to to 42 nests involving Caspian Gull or Caspian Gull hybrid in colony Lelystad

general notes & remarks

• The ideal situation for us as researchers: guarding breeder + partner present, easy to photograph together, having a full clutch early in the season, with egg dimensions and patterns indicating Caspian Gull.
  Easy peasy. Sometimes that happens, e.g. nest y-PA02, nest 29, or y-PHSV.
• Fortunately, we have nests with both partners present "all the time" (seemingly). But in some nests not, and despite many visits we didn't manage to document a partner sufficiently.
  You find this situation in the table under "Breeding bird not scored (no pictures)". How is that possible?
  Examples are nest 17, with a female Herring Gull present all the time, but a partner never present (suspected Caspian Gull).
  Or nest 55 with a perfect female Caspian Gull, but the partner was never seen, litterally. This nest failed eventually, so maybe there just "was no partner" for her?
• Sometimes, a pair guarded a location "notoriously often". Why didn't they start to breed? Are these prospectors? Floaters maybe?
  In 2020 we had such a couple, both perfect Caspian Gull, called PAIR B. Litterally "always on their lookout spot".
  Also in 2020, a Yellow-legged Gull bonded with Herring Gull g-97, called PAIR A. Always present, but not building a nest (as far as we know).
  In 2021, again a mixed pair, now Yellow-legged Gull with Caspian Gull, always present at dam 2. And it took ages to start nest 41, which almost immediately failed.
  Also worth mentioning is nest 60 - 62 - 63, very close together, with several Herring Gull nests within 3 steps distance, and a nice Blackberry complex for hiding the chicks, to picture the situation.
  Nests this close together proved difficult to monitor, therefore uncertainties in the table with scores, below. Best option seems to trap breeding adults at the nest in 2022, and ring these birds.
• Some nests we just had to let go. For example, a bird deep down in a Blackberry complex (nest 16B). Or close to Spoonbills (nest 40).
  Illustrative is nest 53, of which we have no certainty about the parents. But we managed to ring chicks, which were reported back (see link).
  They nest at T-split dam 1, a 'challenging' situation: loads of Lesser Black-backed Gull nest here, deep down between blocks, invisible from the canoe, and close to a dense Spoonbill section.

SCORING TABLES

RAW DATA 2020

RAW DATA 2021 (scoring Mars)