(135) OBSERVATIONS IN A ROOKERY DURING THE INCUBATION PERIOD C. M. OGILVIE.

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1 (135) OBSERVATIONS IN A ROOKERY DURING THE INCUBATION PERIOD BY C. M. OGILVIE. METHOD OF OBSERVATION. FOR the purpose of the observations here described a clear day was chosen and a date when incubation was the main activity in the rookery March 24th, A colony of eight nests was observed continuously from a.m. (G.M.T.) to 6.45 p.m. (dusk) from a house window near the level of the nests. For purposes of reference the nests were numbered from left to right (the eighth nest was situated well to the rear of the others). The position of each member of the rookery was recorded at five-minute intervals: either "sitting," "standing by the nest" or "away." Apart from these three positions, every activity noted within the colony was recorded. OBSERVATIONS. The Nests. In six nests, incubation was proceeding (1, 3, 4, 5, 6 and 8); the seventh nest was under construction (2) and an eighth unoccupied (7). PLAN OF THE GROUP OF EIGHT NESTS. Frequency of Visit. The visits of the cocks with food and the builder (No. 2) with material were variable in frequency throughout the day. Prolonged sojourns by the nest or absences from the rookery alternated with periods of foraging and of frequent visits. For example, the building bird brought material to the nest 11 times in the half-hour period of p.m. (G.M.T.). In the subsequent hour, seven visits only were paid. The longest period of absence recorded was for No. 3 cock, who remained away from the nest from 2.35 until 3.10 p.m. (G.M.T.). Several cocks remained inactive by the nest for periods up to minutes.

2 136 BRITISH BIRDS. [VOL. XL. The following figures, derived from my five-minute "roll-calls" of the whole colony, indicate the attendance rate of the cocks (beside the nest), the hens (actually sitting), and of the building bird : Cocks present 198 times (out of a possible 324). Hens present 276 times (out of a possible 324). Builder present 28 times (out of a possible 42). The number of visits per hour of each cock bird and of the builder is indicated in the figures tabled below. TIME NO (Builder) p.m Not noted P-m The Ritual of Feeding. In most instances, the ceremony conformed to a typical pattern : (a) The hen heralds the approach of her mate with loud calls and shifts about the nest in obvious excitement. (b) The cock alights beside the hen and feeds her upon the nest; she accepts the food with her feathers quivering, the loud crowing now curiously muffled. On two occasions, the cock landed several yards from the nest; after prolonged and fruitless protest, the hen left her eggs and flew up to solicit the food (No. 8). Although I have often witnessed "soliciting," this was the only instance of a hen leaving the nest to receive food on this particular day presumably a vital time in the incubation period. (c) The cock side-steps away from the nest and is often followed by the hen. A curious performance would often ensue : the beak is drawn to and fro across a branch as a knife is sharpened on a stone. The purpose of this procedure is not clear ; although both birds would do it, there does not appear to be any mutual significance in the act. Yeates refers to the performance after feeding as "beak-wiping" ; although it frequently occurs after feeding, I have seen it carried out by cock birds at other times.* (d) In most instances, the cock leaves after a few minutes and the hen, if she does not accompany him, immediately returns to the nest. Both birds may remain beside the nest or fly around together, but the female returns to the eggs within a few minutes. Courtship and coition may occur in this postprandial period, and on several occasions the cock attended to the nest (?twig adjusting). No. 5 cock often hopped on to the unoccupied nest(7) and fed on something taken from among the twigs ; his mate stood nearby, but made no attempt to join in the repast. On no occasion during the day did I witness any effort by the female to feed herself; this may of course, have occurred during the few brief absences of the hen birds with their mates. Incubation. Throughout the eight hours of observation, none of the six cocks made any effort to incubate the eggs; at all events, Bill-wiping occurs as an accompaniment of sexual excitement in many species of birds. EDS.

3 VOL. XL] OBSERVATIONS IN A ROOKERY. 137 I witnessed no change-over take place. On several occasions, however, the cock probed about in the nest, presumably adjusting twigs or turning eggs. Incubation was neglected only during the period after the cock's visit with food and, on one occasion, during a "general alarm." The hen birds were not seen to leave the nest at any other time. Excepting four occasions during the day, no nest was left uncovered for more than five consecutive minutes; the four instances were: (a) All nests deserted for about ten minutes during a general alarm (human disturbance). (5) Nest 5 neglected for 20 minutes (both birds beside the nest). (c) Nest 8 neglected for 15 minutes (both birds beside the nest). (d) Nest 1 neglected for 20 minutes (both birds away). As recorded above, the hen bird was seen actually sitting on the nest 276 times in 324 five-minute observations. Mating. Courtship display and pursuit occurred on several occasions when the female left the nest after feeding. The performance was essentially brief and hurried, however, and actual coition ensued on only five occasions during the day. Four times the act was accomplished on the nest (pairs 1, 1, 6 and 8) and in the fifth instance coition was attempted beside the nest (pair 5). With one possible exception, coition between mated pairs did not provoke attacks by other members of the colony. The case about which I am doubtful occurred as the birds returned to the rookery after a "general alarm"; No. 1 pair appeared to be mobbed whilst mating, but I am not sure that the male partner was in fact the lawful mate. Intrusions and other disturbances. I recorded over 20 disturbances in the normal routine of the colony during the day. My observations support the view of Yeates that "fights" within the rookery during this period are often the outcome of promiscuous sexual attacks. I witnessed these battles on 13 occasions and I have no doubt that the initial assault in each instance was sexual in nature; the following sequence was noted in every case: {a) A cock bird flies down upon a sitting hen and at once attempts coition. (b) The hen usually resists the assault with vigour; she would often fly from the nest and defend herself in the air.* *I have long felt somewhat sceptical of the. view that mobbings of Rooks by other individuals during coition or attempted coition are mainly confined to promiscuous or "stolen" matings, but the evidence that this is so is difficult to resist. The idea that such behaviour is somehow resented as "antisocial" can hardly be entertained seriously ; it seems likely that Mr. Ogilvie's observations provide the clue and that birds other than the rightful mate are stimulated to interfere by the resistant behaviour of the female. It is no doubt the instinct of such strongly colonial birds as Rooks as Lorenz (Proc. Eighth Internat. Ornithol. Congress, Oxford, 1934, pp } showed is the case with the Jackdaw to react in defence of fellow -members of the colony which are in danger or difficulties, and if the female's behaviour is consistently resistant to the intruder, this would no doubt suffice to evoke the action described from neighbouring birds. B.W.T. J

4 138 BRITISH BIRDS. [VOL. XL. (c) Other members of the rookery intervene; in the first instance the nearest cock or the legitimate mate, but ultimately several may join in the affray. The "mediators" are not always impartial, for on one occasion No. 4 cock himself assaulted the hen whom he had just delivered. The sitter on No. 8 nest (rather remote from the main colony) was the only hen not attacked in this manner during the day. An unmated bird was often present in the colony (recorded in about half of my five-minute "roll-calls") and was responsible for a number of these assaults. He was driven away from No. 4 hen by the returning cock; undaunted, he immediately attacked No. 6 hen and was again repelled by No. 4 cock. Later in the day this same bird made several assaults on No. 1 hen, each being frustrated by the lawful mate. Aggressor. No. 6 cock,, 1? 1» 4 Unmated rook 3t >> (3 attacks) Marauder (non-member of colony) Unidentified Victim No. 5 hen, ,. 1..,. 1 «,,. 3,. 6 2 (builder) Mediators. No. 2 (builder) 6 cock Two rooks No. 4 cock 4 1 Two rooks Many rooks Intrusions other than promiscuous sexual attacks also evinced an aggressive response from the male birds : (a) The unmated Rook visited the colony frequently and always perched a few feet below Nest 3. He was repeatedly driven away by No. 3 cock and once they fell to the ground in fierce combat. (b) No. 1 cock, returning from a foraging sally, tried to settle beside No. 6 nest, which was unattended. He was immediately driven off by No. 4 cock and returned forthwith to his own nest. (c) Strangers to the colony were dealt with on three occasions by the local cocks (1, 6 and 5). Each time the intruders, although never obviously aggressive, were closely pursued into the surrounding trees. Building. One nest (No. 2) was under construction throughout the day. The frequency with which material was brought to the nest has been noted. Twigs were taken sometimes from the tree itself but more often from a nearby copse. It is certain that most if not all the work was carried out by the same bird, which rarely ventured far in its search for material. Two birds were present beside the nest on one occasion only, just 30

5 VOL. XL.] OBSERVATIONS IN A ROOKERY. 139 minutes before sunset; the builder, after waiting by the nest for about 20 minutes, was joined by a second Rook carrying a twig. The latter remained by the nest for a few moments and then departed. An assault by an unidentified Rook upon the building bird was undoubtedly of a sexual nature; this suggests that the builder was a hen bird. SUMMARY AND CONCLUSIONS. A rookery of eight nests (one unoccupied) was closely studied throughout a day in the incubation period. The hen birds applied themselves almost exclusively to incubation; excursions were rare and very brief. Feeding, and coition when this occurred, took place upon the nest. The cock bird was responsible for feeding the hen, a duty performed with characteristic ceremony at irregular intervals. Various intrusions within the colony and invasions from without were combated by the male birds. Coition was more often promiscuous than between mated pairs and took the form of an assault resisted by the hen and countered by other cocks. This phenomenon may result from the disparity of duties during this period: the intense demands of incubation suppressing the mating instinct of the hen, whilst idleness enhances this desire in the male bird. An unmated cock, with instincts neither blunted by duty nor requited in wedlock, was thus the main offender. The building of one nest was observed; the male was not seen to participate directly in this work.