British Birds VOLUME 74 NUMBER 4 APRIL I 98 I

Rob Berry and Colin J. Bibby

he Nightjar Caprimulgus europaeus is one of those birds whose decrease in T numbers and contraction of range in Britain and Ireland has been going on for a long while, perhaps 100 years but certainly 50 (Parslow 1973, Stafford 1962). The factors responsible are unknown, but habitat loss and climatic change are the two most often suggested. Neither seems all that satisfactory. Afforestation, mainly of moorland, has benefited the Hen Harrier Circus cyaneus and the Short-eared Owl Asioflammeus, but not the Nightjar, in spite of the frequency of its occurrence in young forestry plantations (Sharrock 1976). The decline is sometimes bracketed with those of the Red-backed Shrike Lanius collurio (Ash 1970) and Wryneck Jynx torquilla (Monk 1963) as possible candidates for the effects of climatic change. All these species were, however, declining in the period of climatic

[Brit. Birds 74: 161-169, April 1981] 161 162 A breeding study of Nightjars amelioration in the early decades of this century. In Sweden, Stolt (1972) considered road deaths and pesticides to be possible factors in the decline there. If this bird is declining for climatic reasons, we might expect to find that its breeding productivity is markedly influenced by variations of weather. The present study examined breeding chronology and success, and growth rates of chicks to see if such effects were evident. The ecology of the Nightjar has been remarkably little studied, the most detailed work being that of Lack (1929, 1930, 1932), so there is little background against which to assess the present findings. A previous paper from this study (Berry 1979) described Nightjar habitats. Methods The study took place at Minsmere, Suffolk, the site being described previously by Berry (1979). During 1976-79, two to seven territories were investigated, with up to 30 visits each per year. Care was taken to record first and last sighting dates of occupants and nesting details. In 1978 and 1979, chicks were uniquely marked, and weighed with a spring balance at the same time each evening. Pairs, once included, were followed consist­ ently throughout the season. Maximum and minimum air temperatures were read daily from a screened thermometer on site. Further information on first-egg dates was extracted from the Nest Record Cards of the British Trust for Ornithology. These findings are used briefly for comparative purposes, but not presented in full as they will be the subject of a future paper (D. E. Glue in prep.). Time of arrival and delay before breeding For 12 pairs, the time of arrival of both adults and the date of laying of first eggs was known (table 1). These data included five cases of the same territories being occupied in successive years. There was suggestive evidence that territories were occupied in a similar order each year. A was the earliest and N the latest, but the correlations were not significant, perhaps because of the small sample size. Sparser data from the previous two years (1976-77) supported but could not confirm this suggestion. Table 1. Arrival and first-egg dates of Nightjars Caprimulgus europaeus at Minsmere, Suffolk, in 1978 and 1979

Site Male Female First egg laid

A 10 May 11 May 23 May B 20 May 1 June 3 June C 23 May 26 May 10 July D 1 June 14 June 20 June M 19 May 31 May 3 June N 24 May 5 June 12 June O 31 May 14 June 22 June A 15 May 29 May 1 June B 22 May 3 June 8 June C 17 May 22 May 26 May M 23 May 5 June 9 June N 27 May 16 June 23 June 'A'breeUing study o]~Nightjars 163 Males arrived on their breeding areas between 10th May and 1st June. They were, on occasions, recorded earlier, but always in places subse­ quently abandoned. Females arrived one to 20 days later, between 11th May and 16th June, the mean interval between arrival dates of the two sexes being 10.9 days. First eggs were laid two to eight (once 12) days after the female's arrival or nine to 22 (once 27) days after the male's. Not surprisingly, first-egg dates were correlated with arrival dates of males (r9 = 0.950) or females (r9 = 0.963). The delay from arrival of the female to the laying of the first egg was inversely correlated with mean minimum (fig. 1, r9= —0.719)—but not maximum—temperature. Inter­ vals from the female's arrival to first laying were often so short that the pre-laying accumulation of reserves by the female must have started before her arrival on site, but this too may have been temperature-related.

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6 8 10 12 °C Fig. I. Interval between arrival of female Nightjars Caprimulgus europaeus on breeding territories at Minsmere, Suffolk, in 1976-79, and laying of first egg, plotted against mean minimum temperature at time. Fitted line: y = 16.4 — 1.05x, r, = —0.719. Chronology of breeding The timings of 20 breeding attempts are set out in fig. 2. First eggs of first clutches were laid between 17th May and 28th June (mean 8th June ± 12.2 days, n= 19). In a further, exceptional case, first laying was recorded on 10th July, even though the female had been present since 26th May (table 1); it is, however, just possible that an earlier attempt had failed without being discovered. Even without this record, the spread of commencement of breeding was considerable. Eggs (always two) were laid at 24-hour intervals in the three cases where this was fully recorded. Incubation started immediately, which is probably invariably true for an open-ground nesting species whose egg is not cryptic. There was thus an age difference between the young, which hatched after 17-21 days of incubation (mean 19). The young could fly by 16-18 days, but were probably dependent on the parents until about 30 days. 164 A breeding study of Nightjars

~r~ T~ MAY JUN JUL AUG Fig. 2. Breeding schedules of pairs of Nightjars Caprimulgus europaeus at Minsmere, Suffolk, in 1976-79, whose territories observed throughout season. Each line represents pair identified by letter for site and last digit of year. Solid lines = successful nests; dashed lines = failed nests In only four cases were pairs found to have reared two broods. The intervals between attempts (first egg to first egg) were 34, 37, 38 and 39 days, so the males had been left single handed to rear the first-brood young from 14-19 days old to independence. The four pairs which attempted a second brood had started their first clutches on 23rd May, 26th May, 1st June and 3rd June. The latest observed nests—both of which reared young—started on 11th July. This suggests that, with a 37-day repeat interval, the latest starting date to permit the rearing of two broods would be 4th June, which agrees well with the above observations. The Nest Record Cards do, however, include details of clutches started as late as the

95. Nightjar Caprimulgus europaeus, Surrey, August 1974 (Michael It'. Richards) A breeding study of Nightjars 165 end of July, so it might be possible for a pair to rear two broods after a late-June start. Of the 19 Minsmere pairs, ten laid their first eggs too late to allow time for a second brood. Four of the earlier starters attempted a second brood after a first success, and one had a second attempt after losing its first-brood young. Four pairs, however, with successful first broods starting on 17th May, 25th May, 31st May and 2nd June, did not attempt a second, so late starting was not the only factor to inhibit double broodedness. A further three pairs which lost their eggs on 29th June and 6th July (twice) did not make a further attempt, though earlier failures on 23rd June and 28th June were followed by another clutch. Nesting success and breeding productivity Young were fledged from 16 of the 28 nests found, eight of the failures being of eggs and four of young. One area (sites M, N and O in fig. 2) suffered nine failures and no successes, while the other group of adjoining territories did significantly better (16 successes and three failures). The causes of the failures were unknown, though adders Viperus hems were notably abundant—-and often close to nests—in the unsuccessful area, which had rather few clearings in the heather Calluna vulgaris either for nesting Nightjars or for basking adders. At five nests, only a single young was reared, although starting with two eggs. In three cases this was because only one egg hatched, and in two cases one of the chicks died when half grown. The mean brood size of successful nests was therefore 1.69, and of all nests 0.96, with an average of 1.35 young reared per pair per season. Growth rates of chicks The growth pattern of chicks is shown in fig. 3. Growth was virtually linear

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 days Fig. 3. Weights of nestling Nightjars Caprimulgus europmus at Minsmere, Suffolk, in 1976-79, plotted against age. Fitted line for ages 1 to 10: y = 2.0 + 4.48d 166 A breeding study of Nightjars at 4.48g/day for the first ten days and thereafter weights stabilised at 50-60g. Individual growth rates were calculated for 16 young (from nine nests) up to ten days of age in 1978 and 1979. There was a significant positive correlation between growth rates of young and hatching date (fig. 4, r14 = 0.519): young hatched later in the season had higher growth rates. This is not surprising, since numbers of moths, and probably other night flying insects such as craneflies and some beetles eaten by the Nightjar (Gollinge 1924-27), increase in abundance throughout the summer. A significant alternative correlation was found between growth rates and mean minimum temperature during the first ten days of the chicks' lives (r14 = 0.553), but not mean maximum temperature. This might partly have reflected a suppression of insect activity on cold nights, but, since minimum temperature and date were (not surprisingly) highly correlated (r = 0.866), it is not possible to sort out what may have been a complicated interaction between date and temperature as a predic­ tor of food availability and hence growth rates of young Nightjars. Suffice it to say that growth rates of young Nightjars were rather variable, that conditions improved in the warmer nights of later summer and that the growth rates could be so poor as to result in the death of one of the chicks, as was witnessed in two nests.

10 20 30 10 20 30 Jun Jul Fig. 4. Growth rates of nestling Nightjars Caprimulgus europaeus aged 0-10 days plotted against hatching date. Each point represents single individual, pairs on same date being siblings. Individual with lowest growth rate died in nest at 13 days old. Fitted line: y = 4.1 + 0.13x, where x is the date in June, r,4 = 0.519 Discussion The most striking feature of this study was that such a small proportion of pairs (four out of 20) were double brooded. This was not because of poor nesting success, but because most pairs started too late to allow time for rearing a second brood. Lack (1930) described how female Nightjars abandoned care of the first young at about 13 days old to start a second clutch. The timing was such that achievement of independence of the first A breeding study oj Nightjars 167 brood coincided with hatching of the second so that the male could switch from feeding one to feeding the other. Lack found, as in this study, that second clutches were not laid later than the early days of July, but he also found that double broodedness was normal rather than exceptional. There seem to be three features of difference between these two studies separated by some 50 years in time but only 50 miles (80km) in space. First, although Lack did not give first-egg dates, they must have been earlier than in this study to allow pairs to attempt a second clutch not later than early July. Secondly, in this study, young were left in the care of the male not at 13 days but at 14-19 days, by which time they were near flying. Thirdly, four early-starting pairs did not attempt a second clutch at all, but, instead, both adults continued to care for the first-brood young. Why were Minsmere Nightjars breeding so late and why were young of the first brood tended for so long by both adults? Much of the reason for late breeding was late arrival of both sexes, but especially of the females, which was, on average, 11 days after their mates. Since eggs were laid so soon after the arrival of the females, it seems unlikely that their detection on the breeding territories coincided with their arrival from sub-Saharan winter quarters. It would be surprising if they could accumulate reserves to recover from migration, lay two moderate-sized eggs and prepare for incubating for all buthalfanhouraday (Lack 1932) in a period of two to eight days. Thus, there seems to be some question as to where females were and what they were doing in this period preceding egg laying. Perhaps, at this time of year, when nights may be cold and flying insects scarce, there are some specially favourable feeding places associated with particular habitat features. There was certain inconclusive evidence in this study that some territories might have been better suited than others for early breeding. This could be akin to another heathland bird, the

96. Nightjar Caprimulgus europaeus at nest, Czechoslovakia, June 1977 (Lubomir Hlasek) 168 A breeding study of Nightjars Dartf'ord Warbler Sylvia undata, in which some territories were, for vegeta- tional reasons, better suited than others for early feeding and egg laying and thus double broodedness (Bibby 1979). It is also possible, as suggested by fig. 1, that late starting may be associated with delayed springs, which seem to have become so characteristic a feature of recent years. Though it was not possible to separate the effects of advancing season from rising night temperatures, the increasing growth rates of young with season suggested that feeding the young might have been quite difficult for some Nightjars early in the season (June). This was confirmed by the deaths of two young, one of which reached 14 days of age but grew very slowly (fig. 2). If this was the case, it is perhaps not surprising that both adults had to continue to rear their young, or alternatively that it might have been difficult for the females to acquire reserves for a second breeding attempt before early July. It is not known to what extent the weather might be an important factor. Early breeding and double broodedness were, however, not evident in the hot summer of 1976 when this study was starting, albeit on a small scale. On the other hand, abundance of potential prey such as moths might have been depressed in recent years by long-term climatic effects, though the evidence is rather thin (Heath 1973).

97 & 98. Nightjars Caprimulgiti tumpatus: left, singing from tree top, Suffolk, July 1960 {John Mark/mm); below, at nest with young, Czechoslovakia, June 1977 (Lubnnur Hlasek) A breeding study of Nightjars 169 In conclusion, this small study has pointed to some possible factors which might be giving rise to lowered breeding success and hence declining numbers of Nightjars in Britain. The census about to take place (Brit. Birds 73: 195; 74: 189; BTO News 108: 1-2) is long overdue and so too is a detailed ecological study to investigate the interaction of weather and habitat on food abundance and breeding biology of the Nightjar.

Summary A study which started in 1976 and intensified in 1978 and 1979 is reported. Male Nightjars Caprimulgus europaeus arrived in the second halfof May and females on average 11 days later. The delay from the female's arrival to the laying of the first egg was inversely correlated with mean minimum—but not maximum—temperature at the time. In contrast with a study conducted 50 years previously, second broods were reared in only 25% of territories; these were achieved by the earlier starters. Pairs reared 1.35 young to fledging per year. Later- hatched young had higher growth rates, though this effect might have been attributed to the comparably strong correlation between growth rate and minimum temperature. It is sug­ gested in discussion that the study population was less productive than that observed 50 years ago, largely due to the paucity of second broods. The influence of night temperature, presumably via food supply, on the timing of laying and the growth rate of chicks suggested that the Nightjar could be susceptible to climatic deterioration. A detailed ecological study with emphasis on the influence of habitat and weather on food supply and breeding success seems overdue.

References Asu, J.S.I 970. Observations on a decreasing population of Red-backed Shrikes. Brit. Birds 03: 185-205, 225-239. BERRY, R. 1979. Nightjar habitats and breeding in East Anglia. Brit. Birds 72: 207-21!!. BIBBY, (:. j. 1979. Breeding biology of the Dartlbrd Warbler Sylvia undata in England. Ms 121: 41-52. ' CIOLUNGE, W. E. 1921-1927. The Food of Some Britisk Wild Birds. York. HEATH, J. 1973. A century of change in the Eepidoptera. In HAWKSWORTH, D. L. (ed.l. The Changing Flora and Fauna of Britain, pp. 275-292. London. LACK, D. L. 1929. Some diurnal observations on the Nightjar. London Nat. (1929): 47-55. 1930. Double-brooding of the Nightjar. Brit. Birds 23: 242-244. 1932. Some breeding-habits of the European Nightjar. Ibis 74: 206-284, MONK. J. V. 1963. The past and present status ol the Wrvneck in the British Isles. Bird Study 10: 112-J32. PARSLOW.J. L. !•'. 1973. Breeding Birds of Britain and Ireland. Berkhamsted. SHARROCK.J. T. R. 1970. The Atlas of Breeding Birds in Britain and Ireland. Berkhamsted. STAFFORD,]. 1902. Nightjar Enquiry. 1957-58. Bird Study 9: 104-115. STOI.T, B.-O. 1972. Ormnattskarrans Caprimulgus europaeus lorekomst i Sverige 1970. Var Fagelv. 31: 111-116.

Rob Berry and Colin j. Bibby, RSPB, The Lodge, Sandy, Bedfordshire SG192DL