We monitored breeding colonies of Caspian terns on the islands of Lake Chany: Perekopnyi (54°47′40′′N, 77°31′15′′E) in 1985; Redkii (54°55′49′′N, 77°21′20′′E) in 1989 and 1993; Uzkoredkii (54°58′15′′N, 77°27′04′′E) in 1992, 1993, 1994, 1995, and 1997; Cheremukhov (54°56′26′′N, 77°21′01′′E) in 1999 and 2000; and Kolpachek (55°01′23′′N, 77°28′03′′E) in 2011 (Table 1; Figure 1). We observed the colonies repeatedly in the breeding season from May to June. At every visit, we examined the nest contents for the presence of eggs or chicks. We observed a total of 2,128 nests with one (n = 319), two (n = 1,388), or 3 three (n = 421) eggs. Egg length and width were measured using a Vernier caliper (division accuracy = 0.1 mm), and the eggs were numbered using a waterproof marker. Egg volume was estimated using Hoyt’s equation: volume = 0.51 × Egg Length × Egg width × Egg width / 1000 (Hoyt, 1979). We determined the volume of 4,137 Caspian tern eggs. As egg laying had 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 Caspian terns (26 days) from the hatching date of the first chick in the nest. If the hatching date was unknown, the clutch initiation date was determined by subtracting the number of incubation days from the date when the nest was first observed. The incubation stage was estimated from the change in position of an incubated egg placed in the water (Westerskov, 1950; van Paassen et al., 1984). The accuracy of this method varied throughout the incubation period, and the mean prediction error was between 0 and 4 days. On average, embryonic age estimated using egg flotation was within 1.9 ± 1.6 (SD) days (Ackerman & Eagles-Smith, 2010). Eggs in the nest hatched asynchronously within 2–4 days. Whenever possible, we determined the within-clutch laying sequence of eggs (1st, 2nd, and 3rd). Position in the laying sequence was established on the basis of hatching sequence in approximately 27% and 30% of the two- and three-egg clutches, respectively. In other cases, if we could distinguish the flotation levels of eggs, we numbered the eggs according to the stage of incubation (in approximately 33% and 36% of the two- and three-egg clutches, respectively). This method has been commonly applied in other seabirds to determine the egg-laying order (Nisbet, 1975; Gochfeld, 1977). Furthermore, we recorded the pipping date (i.e., appearance of star-like bursts) and the actual hatching date of individual eggs. Wet chicks were considered hatchlings of that day, and dry chicks were registered as a day old. Chicks older than two to three days left the nest and moved to locations nearby. Newly hatched chicks were hand-captured at nests, ringed, and measured. Head, bill, and tarsus length was determined using a Vernier caliper, and body weight was measured using a Pesola spring balance (On ~60 and 60~100-g measurement, the divisions were 0.5 and 1.0 respectively) in 570 chicks; of the 570, 486 (85%) chicks had hatched from the measured eggs.