Basing on the long-term observations carried out over the period 1989-2015 during excursions, from hide and by the method of direction finding (Markin, 1978) in three study areas of the Left-bank Ukraine it was analyzed the use of 779 se-ries of unisonal duets performed by the Common Crane (Grus grus grus).
1. Intensity of duets during the breeding season. The maximal intensity is recorded in the third ten-day period of March, whereas in April and May it correspondingly reduced 4.8 and 15.6 times that is also proved by increasing intervals between the duets from March to May.
The minimal intensity of duets in May is probably connected with the hatching of chicks that sharply changes the character of physical activity and interrelations between partners in breeding pairs. The quiet feeding in nesting sites and incubation of eggs (with rare territorial conflicts of adult birds busy with themselves and their clutch) is replaced by permanent movements through foraging territory with chicks, their feeding, heating and protection, building for them «children» nests, heating nests and roosting nests.
Due to the restricted observation period in June no duets were registered; in July and August they were heard approximately 2 times more often than in May, and in September their frequency was similar to that in May.
Basing on chronometry of the crane behaviour measured from hide, the maximum intensity of duets in breeding pairs took place during the 23d (21‑25.04) and the 25th (1-5.05.) five-day periods, about a week before the median hatching time (11.05; on the base of 228 from 120 nests, in 1989‑1998). At this, in April the number of unisonal duets was 1.4 times higher than in May.
Distribution of duets of the breeding pairs over the period of egg incubation has shown that their intensity increased from the second to the third ten-day periods of incubation, reaching maximum by the end of incubation (73.1% of duets). Probably, that was evidence of increasing synchronization of physiological status of the incubating partners.
Frequency of unisonal duets was undoubtedly determined not only by the physiological status during the breeding period, but also by environmental parameters, for example, by different start of the daylight hours. A more early start of the first unisonal duets was revealed from the last ten-decade period of March to April (sunrise shifted 1 hour earlier) and May (sunrise shifted again 41 minutes earlier).
2. Distribution of duets by timing. Analysis of distribution of 212 duets of breeding pairs (chronometry from hide) has shown that in April during the first 4 hours after sunrise (4-8 a.m.) pairs issued 61.1 % of all the daytime duets, and in May – only 48.8 %. During next 3 hours (8-11 a.m.) in April they issued another 29.3 % duets, in May – 18.6 %. Thus, only after 11 a.m. the duet activity sharply decreased, comprising until darkness in April 9.6 %, and in May –32.6 %. Thus, the duets in May were distributed by the daytime hours more evenly than in April.
«Direction finding» of duets and other calls in the last ten-day period of March has shown that the highest number of duets was recorded during the first hours after sunrise (81.6 %), and after that noticeably declined. The comparison of the relative number of the unisonal duets and other calls shows that in the third decade of March the first compiled 19.5 % of the total repertoire of the calls of this period.
3. Connection of duets with the rotation of incubating partners. During the incubation period pairs did almost 3/4 replacements (72.7 %) without duets, and only about quarter (27.3 %) was accompanied by them. A half of duets (53.7 %) took place during rotation, and slightly less number – in other situations, for example, during territorial conflicts in nesting sites (46.3 %).
4. Inter-pair variability of the duet activity. According to observations from hide, five pairs did not issue any duet during 11 days, while other five pairs issued 54 duets for 15 days, and the average for all 10 pairs constituted 2.08 duets per day. The extreme options of duet intensity in different pairs were as follows: one issued 26 duets for 5 days (48.1 % of all registered from hides in 10 breeding pairs), and the other pair for the same time issued no duets.
5. Participation of partners in duets of breeding pairs. During 26-day observations from hide, in 13.0 % cases the duet was only issued by one of the partners! Though, it is doubtless, that the meaning of the duet is to support contacts between partners that they exactly did in 87.0 % of other cases.
The most frequent option of the duet performance (66.7 %) is when birds from one pair stand and cry 3 - 30 m apart from each other, often one of them being in the nest. Much rarer (13.0 %) were the cases when during these calls one bird was sitting in the nest and other bird was standing 3 - 6 m away (4 times a male was sitting, 3 times – female).
Single performance of its own part of the duet in all cases was recorded only for males. The most frequent option of such a «semiduet» (5 cases): an incubating male left the nest and went aside at the distance of 3–15 m from the nest and issued its own part of duet, a female at the same time was presented within their feeding area but we did not know if the partners saw each other or not.
6. On the duration of duets and time of observations. On average, the duet lasted about 70 sec and occupied a very insignificant part of time budget. Judging from the period of complete 25-day observations from hide (23994 min), the breeding pairs spent 0.26 % of time (70 sec х 54 = 3780 sec, or 63 min) for duets, and together with the nearest neighbours – 1.03% (70 sec х 212 = 14840 sec, or 247 min. 20 sec) of the observation time.