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 - species composition

ARE THOSE CASPIAN GULLS REALLY CASPIAN GULLS?

As mentioned in the section international trend, Caspian Gull, in their westwards expansion, are known for mixed pairing in areas where they meet Herring Gull, sometimes Yellow-legged Gull, and even Lesser Black-backed Gull. Offspring of such mixed couples may look similar to one of the parents, may show a combination of the parental features, or may in fact resemble a third species. And even more, those hybrid birds will eventually start breeding when adult, and may raise backcrosses, so-called 'F2 hybrids'. What to call birds in such a mixed population?

When ringing chicks in mixed colonies in Poland, it is impossible to ID each and every individual. Therefore, the Polish ringing scheme introduced a new category "Larus cachinnans kolonia", for ringed chicks from colonies with Caspian Gull dominance, and those chicks which proved to have at least one Caspian Gull parent. In analogy, "Larus argentatus kolonia" is used for ringed chicks from colonies with Herring Gull dominance, or when its known at least one parent is Herring Gull.
For other age class (non-chick ringed birds) a Polish life history may read "Herring Gull group Larus argentatus sensu lato" or "Larus
argentatus/cachinnans".
The situation is very similar in adjacent mixed colonies in east Germany. Whatever the category, ID is not immediately clear, and photographic evidence (e.g. from wintering grounds) is necessary to make any firm statements about an individual bird.

We have to accept we are dealing with birds from a contact zone. And with time, hybrids will raise F2, F3, etc backcrosses which eventually are impossible to tell from 'pure' birds, unless we know a birds complete pedigree.

In the Netherlands we call these head-scratching birds 'river' gulls, referring to the situation along the main inland river systems where four gull species meet and breed. Jeroen Nagtegaal, ringer in southern Netherlands, has collected data on such birds, see HERE. As the project continues, he documented all weird kind of combinations, e.g.
- 2021 season: F1 hybrid Yellow-legged x Herring Gull male paired with Lesser Black-backed Gull female.
- 2019 season: Caspian Gull paired with Lesser Black-backed Gull, chicks ringed, but never reported yet.
- see thumbnails righthand side, for more breeding hybrid birds trapped at the nest and ringed with yellow/blue combination tibia rings by Jeroen.

SPECIES CHARACTERISTICS

Lesser Black-backed Gull (Larus fuscus graellsii aka "Dutch intergrades")

• slate-grey upperparts, intense yellow legs, clear iris in most birds, large red gonydeal spot,
• elegant species, with long wings at rest, minority (males) is more bulky,
• most numerous breeder at both De Kreupel and Lelysyad.

Yellow-legged Gull (Larus michahellis)

• pale slate grey uppeprarts with blue cast, yellow legs,
• angular head shape, larger and bulkier than Lesser Black-backed Gull,
• incidental breeder in IJsselmeer district, in mixed pairs.

Herring Gull (Larus argentatus argenteus)

• pale silvery-grey on upperparts, commonly reduced black on P5, short pale tongue on P10 which commonly reach the edge gradually,
• yellow-orange orbital ring, stout fleshy pink legs,
• compact and short-winged, with angular head and fierce look by staring yellow eye,
• together with Lesser Black-backed Gull, the common breeding species in the Netherlands for decades.

Caspian Gull (Larus cachinnans)

• silvery-grey upperparts, orange-red orbital ring, often dark iris, long pale tongue on P10 and full band on P5
• large but still elegant, 'pencil-shaped' bill, long exposed tibia,
• new-comer, but numbers increase rapidly, and likely soon more numerous than Herring Gull.

"River" Gull - hybrids / Gibbins BB

• offspring of mixed breeders like Herring Gull x Caspian Gull, or Herring Gull x Yellow-legged Gull. "River" Gull named after offspring from nests at inland locations along the Dutch river-systems, where the above mentioned combinations have bred at various occasions.

POPULATION SIZE AT DE KREUPEL

POPULATION SIZE AT LELYSTAD

From the kayak, at approx 40 meters distance, we observed guarding birds on the breakwaters. We then landed, searched for the eggs and placed code flags on a bamboo stick near the nest. We monitored 41 coded nests in 2020. Several nests appeared to belong to either Herring Gull or Lesser Black-backed Gull, and were subsequently skipped from the research.
A total of 32 nests had at least one parent Caspian Gull.
We also marked nest codes using paint spray, on blocks that were easily visible from the water.

For most nests, the substantial distance to neighbouring nests made countings easy. However, for some location we found clusters of nests, and especially in these clusters we may have missed breeding pairs.
In 2020, we did not estimate breeding numbers from the canoe, for neither Herring, nor Lesser Black-backed Gull at Lelystad.
However, the breeding gulls at Lelystad colony are counted annually from the airplane.

WHICH NESTS AT DE KREUPEL & LELYSTAD ARE CASPIAN GULL?

How to be sure a nest at De Kreupel or Lelystad belongs to Caspian Gull? We use the following steps.

• guarding birds at potential nest location
  Obviously, Caspian Gull is a relatively early breeding species, and many birds arrive in March, when observing adults at nest locations is not hampered by tall green vegetation. Most guarding birds choose an obvious lookout post, visible from a distance, and attract attention by frequently alarming in a characteristic way and with diagnostic sound, ounce known.

• egg laying date
  Apart from the phenotypics of guarding adults, another strong indication for Caspian Gull presence are the eggs. Egg laying date is obviously early for bulk of the breeding Caspians. We have collected data for egg laying date at colony Lelystad in season 2020 and season 2021. Result will be in the section BREEDING BIOLOGY.
  Results for Lelystad 2020:
  - average egg laying date for 21 selected Caspian nests is April 7th.
  - average hatching date for 21 selected Caspian nests is May 9th.
  

• egg dimensions & pattern
  Shell pattern on eggs is different (more contrasting than Herring) and egg dimensions are different (larger eggs than Herring).
  Nests of Lesser Black-backed Gulls contain much smaller and browner eggs, and egg laying date is weeks later for that species.

• examining photographs
  Many birds, alarming overhead, were photographed and details on head and wingtip were used to classify birds using the Chris Gibbins criteria (British Birds). Therefore, first estimations for the population size are assumed to be pretty accurate.

Ad point 4: in 2020 we collected data for all nests that were classified 'potential Caspian Gull nest'. We photographed the birds as much as possible, and examined each individual. The full table is HERE. Results for 2020 are summarised below.

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.

Yellow-legged Gull michahellis J.N adult, May 21 2013, Wijk bij Duurstede, the Netherlands. Picture: PieterGeert Gelderblom. Breeding in a mixed pair at an inland colony.