Background & Summary

Lake Chany is located in the Baraba forest-steppe of the West Siberian Plain, Russia, between the Ob and Irtish Rivers (54°30′–55°09′ N, 76°48′–78°12′ E). The Lake Chany basin is among the largest in Russia, and the lake itself has the greatest water surface in West Siberia1. The waters, coastal zones, and islands of Lake Chany are favorable for breeding, molting, and feeding of more than 280 species of birds. In spring and autumn, numerous migratory waterbirds (over 300 000), such as swans, geese, ducks, coots, waders, gulls, terns, cranes, stop at the lake during migration1,2. Some of the species are rare and endangered and are listed in the Red Data Book of Russia and the Red List of the International Union for Conservation of Nature and Natural Resources (IUCN)3. Lake Chany is protected by the Ramsar Convention on Wetlands of International Importance, indicating that the lake is an important site for migratory birds, including L. ichthyaetus. Currently, most of the great black-headed gull colonies are in a vulnerable state; therefore, this species is recorded in the Red Data Books of the Russian Federation and Kazakhstan4,5. Therefore, it is important to observe the dynamics of their reproduction. Inter-annual changes in the onset of egg-laying and morphological parameters of eggs were pronounced during the years of our research and were probably determined by the course of spring and feeding conditions6,7. Egg size may be affected by food availability8,9. Some studies suggested a relationship between egg size and morphological traits, survival, and growth rate of chicks hatching from them [reviewed in10,11].

Here, we provide eight years of raw field data (1993, 1994, 1996–1998, 2001–2003) from our egg and chick morphological study on great black-headed gull Larus ichthyaetus breeding on islands of Lake Chany, Russia12.

Methods

We surveyed three islands of Lake Chany: Uzkoredkii (54° 58′ 15′′ N, 77°27′04′′ E), Reden’kii (54° 56′ 05′′ N, 77° 22′ 27′′ 52 E), Korablik (54° 59′ 31′′ N, 77° 40′ 38′′ E). The studied intertidal habitats are rarely reached by humans.

Gull nests were counted in colonies by regular surveys over eight years (1993, 1994, 1996–1998, 2001–2003) on the islands of Lake Chany. Colonies were visited daily or sometimes every other day. To minimize the disturbance caused by the investigation, the time spent working, within view of the gulls was restricted to a maximum of forty minutes per study plots. We noted nest content at every visit for the presence of eggs or chicks. In total, there were 1 164 nests under observation. Nests contained 1 (n = 140), 2 (n = 518), 3 (n = 504) or 4 (n = 2) eggs. Modal clutch size of the great black-headed gull is two or three eggs, varying seasonally. The length and width of the eggs were measured using Vernier calipers (division accuracy 0,1 mm) and numbered with a waterproof marker. Egg volumes were estimated using Hoyt’s equation: Volume = 0.51 * Length * Width * Width/100013. We determined the volume of 2117 great black-headed gull eggs.

As the laying of eggs has already started by the first visit to the colony, the date of the beginning of egg laying was calculated by subtracting the average length of the incubation period of great black-headed gulls (27 days) from the hatching date of first chick in the nest (n = 559 nests). If the hatching date was not known, the clutch initiation date was determined by subtracting the number of days of incubation from the date that the nest was first discovered (n = 469 nests). The stage of incubation was estimated from the change in position of an incubated egg placed in water14,15. The technique’s accuracy varied throughout incubation and mean prediction error fall between 0–4 days. On average, egg flotation estimated an embryo’s developmental age to within 1.9 ± 1.6 days (mean ± 1 SD)16. Only 47 nests were found during egg laying. Great black-headed gulls usually laid eggs at intervals of two days. Incubation started as soon as the first egg was laid, so eggs hatched asynchronously, one or two days apart.

Whenever possible, we determined the within-clutch laying sequence of eggs (1st, 2nd, 3rd, and 4th). A complete laying sequence was established by observation in 47 cases. In about 48% of clutches the position in laying sequence was established on the basis of the sequence of hatching. In other cases, if we could distinguish within-clutch distinct flotation levels of eggs, we numbered eggs according to the stage of incubation. Sometimes this technique for distinguishing egg laying order were used in other seabirds17,18.

We recorded the pipping date (i.e. appearance of star-like bursts) and the actual hatching date of the individual eggs. Wet chicks were registered as hatchlings of that day; dry chicks were registered as 1 day old. Chicks older than two days left the nest and moved to a location nearby. Newly hatched gull chicks were captured by hand at nests, ringed, and measured. We determined wing, tarsus, and head length using a ruler with zero-stop and vernier calipers and body weight measured using Pesola spring balances for 747 chicks of great black-headed gulls, and 457 of them hatched from eggs that were measured.

Data Records

The all data were deposited in Zenodo https://doi.org/10.5281/zenodo.517555112. The data name, unit and method for the data were described in Table 1.

Table 1 Data name, unit and method for the data.
Full size table

Technical Validation

The long-term monitoring program for was the egg and chick size in the nests of Larus ichthyaetus designed by A.K.Y. A.K.Y. identified the nest and measured the values which we provided in the database. Then A.K.Y. recorded all data in an Excel file. deceased before preparing the data base and this paper. The other authors carefully prepared the data sheet with double checking by the authors.