Loose Nesting Aggregation in the Red-Necked Nightjar Caprimulgus Ruficollis

Ardeola 48(1), 2001, 11-18

LOOSE NESTING AGGREGATION IN THE RED-NECKED NIGHTJAR CAPRIMULGUS RUFICOLLIS

Juan ARAGONÉS, Pilar RECUERDA & Luis ARIAS DE REYNA*

SUMMARY.—Loose nesting aggregation in the Red-necked Nightjar, Caprimulgus ruficollis. The existence of loose nest aggregations in the Red-necked Nightjar Caprimulgus ruficollis is scarcely documented and very little is known about it. Here, we study whether the existence of loose nest aggregations in this species co- rresponds to a colonial behaviour or it is just related to habitat availability. We carried out a study in two different sites of Córdoba (Southern Spain) with apparently different patterns of nest dispersion. At each site we recorded: distance between neighbouring nests, breeding delay in relation to the earliest nest, defensive response, tolerance to conspecifics and breeding success. Our results show that nests seem to be aggregated in one site and isolated in the other. Compared to isolated nests, aggregated nests show a smaller breeding delay, a stronger communal defence response, a higher tolerance of intrusions and a breeding success similar to isolated nests. We failed to find benefits/costs typical of coloniality, indicating that nest aggregations are a response to habitat features. Key words: Caprimulgiformes, Caprimulgus ruficollis, coloniality, nest aggregation, Red-necked Nightjar.

RESUMEN.—Agregación dispersa de nidos en el Chotacabras Cuellirrojo, Caprimulgus ruficollis. Se co- noce muy poco sobre las agregaciones reproductoras en el Chotacabras Cuellirrojo Caprimulgus ruficollis. En este artículo estudiamos si la existencia de agregaciones reproductoras en esta especie está relacionada con un comportamiento colonial o si es una derivación de la mejor explotación de los recursos del hábitat. Estudia- mos dos zonas con patrones de dispersión de nidos aparentemente diferentes y en cada nido registramos la distancia entre nidos vecinos, retraso reproductor en relación al nido más temprano, respuesta comunal defensiva, tolerancia y éxito reproductor. Nuestros resultados muestran que los nidos parecen estar agregados en una de las zonas estudiadas y que, en comparación con la otra zona, tienen una gran sincronía reproductiva, respuesta defensiva intensa, gran tolerancia hacia parejas vecinas y un éxito reproductor similar al de los nidos aislados. Nuestros resultados no apoyan que esta agregación de nidos se deba a un contexto colonial ya que no hemos encontrado los beneficios/costos típicos de la colonialidad y pensamos que la agregación de nidos se debe posiblemente a diferencias en disponibilidad de hábitat para nidificar. Palabras clave: agregación de nidos, caprimulgiformes, Caprimulgus ruficollis, Chotacabras Cuellirro- jo, colonialidad.

INTRODUCTION bers of the group (Benkman, 1988; Eckman & Hake, 1988; Bekoff & Scott, 1989), a higher Colonies of birds are assemblages of largely probability of ectoparasite transmission (Hoo- unrelated individuals that generally exhibit no gland, 1979; Wittenberger & Hunt, 1985; helping or cooperative breeding among adults Brown, 1987; Møller, 1987), parental invest- (Brown & Brown, 1996). The nests of a co- ment mistakenly directed to non-offspring (Hoo- lony can be very conspicuous in a restricted gland & Sherman, 1976; Marzluff & Balda, area (Møller, 1987), although these nests can be 1992), infanticide and intraspecific brood para- fairly separated in some cases (see Alvarez & sitism (Hoogland & Sherman, 1976) and increa- Arias de Reyna, 1974; Birkhead, 1991; Marz- sed conspicuousness to predators (Treisman, luff & Balda, 1992). To live close to other 1975; Marzluff & Balda, 1992). conspecifics can have disadvantages like increa- However, the benefits of nesting in aggrega- sed competition for different resources (Sas- tions can be due to increases in the vigilance vary, 1993), agonistic interactions among mem- rate of the group and a decrease of the indivi-

* Departamento de Biología Animal (Etología). Universidad de Córdoba. Campus de Rabanales. Ctra. Madrid-Cádiz, km 396. E-14071 Córdoba, España. E-mail: [email protected] 12 ARAGONÉS, J., RECUERDA, P. & ARIAS DE REYNA, L. dual vigilance rate, which allows a better use of loose nesting aggregations in this species is time (Hoogland & Sherman, 1976; Caraco, scarcely documented and there is just one refe- 1979; Caraco et al., 1980; Elgar & Catterall, rence about this matter (see Cramp, 1985) re- 1981; Lima & Dill, 1990), decrease of the pro- porting loose nesting aggregations of 4 to 12 bability of being preyed upon due to a dilution pairs in Tunisia with a mean distance between effect, either by an increase in group size (Ha- nests of 2-15 m. However, there is no published milton, 1971), by breeding synchronisation study on this subject so the causes and implica- (Hoogland & Sherman, 1976) which produce a tions derived from the aggregation of their saturation of predators’ capabilities (Pulliam & nests are unknown. Loose nesting aggregations Caraco, 1984) or by their collective defence in the Red-necked Nightjar could be explained (Hoogland & Sherman, 1976; Andersson & by a colonial breeding behaviour or could just Wicklund, 1978; Wicklund & Andersson, result from differences in the suitability of dif- 1980), and increased probability of food loca- ferent habitats for breeding. tion whenever it is patchily distributed (Krebs In this paper we describe behavioural adjust- et al., 1972; Brown, 1987). Other important ments to dense breeding (tolerance and collec- benefits are increases in probabilities of parasi- tive defence) and reproductive success in two tizing alien parental care and aggregation in a different habitats with different nest dispersion limited nesting habitat (see a complete review patterns. We also evaluate if the aggregation of cost and benefits of coloniality in Brown & pattern found can be considered as strict colo- Brown, 1996). niality. In the Red-necked Nightjar it is possible Colonial breeding is a phenotypically plastic to find both isolated or aggregated breeding trait in many species. In certain populations and this could be explained by the existence of most birds breed colonially, while in others adaptations that allow the nesting aggregations only part of the population is colonial. Moreo- to be possible. One of this adaptations is a high ver, individuals in a population may be colonial degree of tolerance towards other pairs bree- in a season and breed solitarily in the next sea- ding nearby. A higher degree of nesting aggre- son. The size and structure of colonies is also gation, and the tolerance towards other pairs, very variable ranging from just a few indivi- may involve advantages in terms of a more in- duals to several hundreds of them, concentrated tense, collective defence against nest predation. on a small area or dispersed over a large zone If nesting aggregation could be explained by (see Brown & Brown, 1996). tolerance and collective defence, we expect to Coloniality in Caprimulgiformes is very un- find for aggregated nests compared to isolated common and there is just one true colonial spe- nests: 1) a greater number of birds approaching cies, the Oilbird Steatornis caripensis (see the playback song of an unknown male, 2) a Snow, 1961; 1962), but other authors also re- greater number of birds defending against the port the existence of reproductive groups in approximation of the observer, 3) a higher bree- both American and Euro-Asiatic species (the ding success, 4) a greater breeding synchroni- Sand-coloured Nighthawk Chordeiles rupes- sation, and 5) a lower number of agonistic in- tris, Groom, 1992; Sick, 1993; the Lesser teractions. Nighthawk Ch. acutipennis, Stiles & Skutch, 1989; the Antillean Nighthawk Ch. gundlachii, Cleere, 1998; 1999; the Little Nightjar Capri- MATERIAL AND METHODS mulgus parvulus, Hilty & Brown, 1986; the Pauraque Nyctidromus albicollis, Aragonés, During the 1995 breeding season (from the 1997a). In the case of the Sand-coloured Night- end of May to the end of August), we conduc- hawk, these reproductive associations can be ted a study on the reproductive biology of the either monospecific or multispecific (Groom, Red-necked Nightjar in Córdoba (Southern 1992). Spain), in two different but nearby sites (at the Studies on the Red-necked Nightjar Capri- nearest points they are only separated by 750 mulgus ruficollis are scarce and very little is m). We are going to refer to these as A and B, known about its breeding habits (Cuadrado & respectively. Site A is characterised by a high Domínguez, 1996; Aragonés, 1997b; Arago- density of trees (Quercus ilex spp. ballota) and nés et al., 1998; 1999; 2000). The existence of bushes (mainly Q. coccifera, Pistacia lentis- LOOSE NESTING AGGREGATION IN THE RED-NECKED NIGHTJAR CAPRIMULGUS RUFICOLLIS 13 cus, Myrtus communis, Chamaerops humilis Tolerance: Response to the song and Crataegus monogyna; see Table 1), whe- of an unknown male reas Site B has low density of trees (Q. ilex spp. ballota) and a domain of crops (sunflo- In order to measure tolerance towards other wer and wheat; see Table 1). However, they pairs we played a tape recording of a singing conform to different stages of succession due to male (maximum time: five minutes) at a dis- human management. Both sites have been ex- tance of 20 m from each nest (n = 14) between haustively monitored during five years, and the 20:30 and 21:30 when chicks were one to four Red-necked Nightjar population is well known. days old. The latency time was registered in We intensively searched both sites and loca- seconds from the moment when the emission ted all breeding pairs by monitoring the sin- started to the moment when an individual ap- ging activity of males, as well as by diurnal proached the tape recorder closer than 5 m. Du- and nocturnal observations of individuals. In ring the next two minutes, we registered the Site A we observed a density of breeding pairs number of birds approaching as well as the that, apparently, was higher than in Site B and number of birds showing agonistic interactions the densities observed in previous years. There among them. As control, we repeated the expe- were apparent differences in nest density bet- riment in the same conditions, but using recor- ween areas, which allowed us to carry out a dings of the Common Nightjar C. europeus factorial analysis in order to look for a possible (n = 13) and the Stone Curlews Burhinus oe- classification of nests in two types: «aggrega- dicnemus (n = 10). All replications were cated» or «isolated». At each nest we registered rried out using a single recording (Roché, the distance to the nearest neighbour nest, the 1966) for each one of the three species, which breeding success (proportion of chicks that re- corresponded to the song of birds unknown to ach the fledgling stage relative to the number of the study population. eggs laid) and the breeding delay in days with respect to the earliest laying date at each site. Data were not normal (tested with the Sha- Defence: Response towards a potential piro-Wilk test for normality), so Kruskal-Wallis threat to the brood tests were used to compare each variable with respect to if nests were aggregated or isolated. When the chicks were five to six days old, We log-transformed data in order to perform we registered the total number of adult passing the factorial analyses (extraction method: prin- flights over an observer approaching each nest cipal components; rotation: normalized vari- at sunset. We recorded the number of passing max). All statistics were two-tailed and the re- flights by the individual attending the nest and sults were considered significant at P < 0.05. also by other individuals that appeared in the

TABLE 1

Description of the two studied sites. [Descripción de las zonas estudiadas.]

Site A Site B [Zona A] [Zona B]

Area [Superficie] (ha) 11 43 Nests [Nidos] n 86 Pair density (pairs/ha) [Densidad de parejas (parejas/ha)] 0,73 0.14 Tree density (trees/ha) [Densidad arbórea (árboles/ha)] 20.27 9.93 Tree cover [Cobertura arbórea] 5% 2.5% Bush cover [Cobertura de matorral] >50% <1% Grass cover [Cobertura herbácea] 90% <5% Crops [Cultivos] <2% 95% Fallow [Barbecho] 10% <5% 14 ARAGONÉS, J., RECUERDA, P. & ARIAS DE REYNA, L. territory. As a control, we registered the num- lues for two variables («number of birds de- ber of passing flights over the observer when fending with passing flights» and «number of he approached a randomly chosen spot in the birds approaching»), and low values for the o- territory. ther variables («distance to the nearest nest» Adult members of the population were not and «breeding delay»). The other nests have banded; therefore, it was not possible to recog- low values for «number of birds defending with nise them individually. To minimize the possi- passing flights» and «number of birds approa- bility of error we used the following criteria ching» and high values of «distance to the nea- on each trial: 1) in relation to approaching rest nest» and «breeding delay» (Fig. 1). «La- birds, we considered the maximum number of tency time» and «breeding success» do not individuals detected at the same time, and 2) in influence nest discrimination. Factorial analysis relation to the number of passing flights, we lumped one nest from Site A with those in Site considered the sum of all passing flights ca- B due to their similar values. Hereafter, from rried out by all the approaching birds. In both the factorial analysis results, we are going to experiments, and to avoid pseudoreplication, talk about nests as «aggregated» (all of the only one trial was carried out at each nest. nests in Site A, n = 7) and «isolated» (located in both Site B, n = 6 and Site A, n = 1). The aggregated nests show shorter distances RESULTS to neighbouring nests than isolated nests (H = 9.86, df = 1, n = 14, P < 0.001; see Table 2). Nest classification Breeding delay was lower in aggregated nests (H = 9.90, df = 1, n = 14, P < 0.001; see Ta- The results of the factorial analysis (Factor 1: ble 2), all nests hatching within five days (0.7 explained variance = 56.75 %, eigenvalue = days as the mean interval between nests in this 3.40; Factor 2: explained variance = 22.58 %, category), as compared to isolated nests, with eigenvalue = 1.35; see Table 2) show that all all nests hatching within 24 days (a 3.42 days but one nest located in Site A have high va- interval between nests in this category).

TABLE 2

Summary of the variables considered and factor score coefficients of the factorial analysis. We never observed agonistic interactions so this variable has not been included here. *** P < 0.001,** P < 0.01, NS = not significant, n = 14 for all comparisons. [Resumen de las variables consideradas y relación de las variables con cada eje. Nunca observamos inte- racciones agresivas por lo que esta variable no se ha incluido aquí. *** P < 0.001, ** P < 0.01, NS = no sig- nificativo, n=14 para todas las comparaciones.]

Aggregated nests Isolated nests. Mean (SD) Mean (SD) Factor 1 Factor 1 [Nidos agrupados. [Nidos aislados. P [Factor 1] [Factor 2] Media (SD)] Media (SD)]

Distance to nearest nest 59.28 (18.53) 235.14 (56.63) *** –0.27 0.05 [Distancia al nido más cercano] Breeding delay 1.57 (1.29) 20.42 (8.63) *** –0.25 –0.05 [Retraso reproductor] Breeding success 0.71 (0.45) 0.64 (0.44) NS –0.03 –0.56 [Éxito reproductor] Latency time 5.71 (2.37) 4.57 (2.19) NS –0.08 0.62 [Tiempo de latencia] No. of birds approaching 2.85 (0.83) 1.14 (0.34) ** 0.31 –0.12 [No de aves que se acercan] No. of birds defending with passing flights 6.71 (2.25) 2.57 (1.17) ** 0.28 0.03 [No de aves que defienden con pasadas] LOOSE NESTING AGGREGATION IN THE RED-NECKED NIGHTJAR CAPRIMULGUS RUFICOLLIS 15

FIG. 1.—Factorial analysis results with normalized varimax rotation (open circles = nest at Site A; solid squa- res = nest at Site B; black arrows = factor 1 loading > 0.8; white arrows = factor 1 loading < 0.2; LT = latency time; BS = breeding success; DNN = distance to the nearest nest; BD = breeding delay; NBA = number of birds approaching; NBD = number of birds defending with passing flights). [Resultados del análisis factorial con rotación varimax normalizada (círculos vacíos = nidos de la Zona A; círculos rellenos = nidos de la Zona B; flechas negras = carga del factor 1> 0,8; flechas blancas = carga del factor 1< 0,2; LT = tiempo de latencia; BS = éxito reproductor; DNN = distancia al nido más cercano; BD = retraso reproductor; NBA = número de aves que se aproximan; NBD = número de aves que defienden con vuelos de pasada).]

Clutch size of aggregated nests (n = 7) was Tolerance: Response to the song of an 1.85 compared to 1.43 for isolated nests (n = 7), unknown male but differences were not significant (H = 2.60, df = 1, n = 14, P = 0.106). There were no signi- We obtained significant differences (H = ficant differences in clutch size (H = 0.39, df = 9.63, df = 1, n = 14, P < 0.01) in the number of 1, n = 14, P = 0.843) between successful (1.80, birds approaching the tape recorder, as a higher n = 10) and unsuccessful nests (1.50, n = 4). We number of birds came closer to it when pla- did not find significant differences in breeding cing it near an aggregated nest than when pla- success (H = 0.14, df = 1, n = 14, P = 0.705) cing it near an isolated nest (Table 2). There between aggregated and isolated nests (Ta- were no differences in «latency time» (H = ble 2). Only nine out of 12 eggs from aggrega- 0.82, df = 1, n = 14, P = 0.364) between both ted nests (75%) produced fledglings, whereas types of nests (Table 2). No response was ob- seven (64 %) produced fledglings successfully tained when the recordings of Common Night- from 11 eggs in isolated nests. Aggregated nests jar (n = 13) and Stone Curlew (n = 10) were produced 1.43 fledglings per clutch (SD = 0.97) played. However, all trials with Red-necked and isolated nests produced 0.86 fledglings per Nightjar recordings (n = 14) obtained a respon- clutch (SD = 0.69) but differences were not sig- se (H = 26.0, df = 2, n = 37, P < 0.001). nificant (H = 1.85, df = 1, P = 0.173). 16 ARAGONÉS, J., RECUERDA, P. & ARIAS DE REYNA, L.

Defence: Response towards a potential mon Nightjar together at a nest site (Schlegel, threat to the brood 1969) or at roost sites (Lack, 1932; Schlegel, 1969) showing no intolerance. In three cases There were differences in the defensive res- we have observed family parties of several ponse to the approach of an observer to the birds (n = 7, n = 7, n = 6) in late August. Fa- nest, with a higher number of passing flights mily parties of six individuals could be explai- when approaching aggregated nests (H = 8.39, ned by double brooding (i.e. two parents and df = 1, n = 14, P < 0.01, Table 2). On the other four young), but observations consisting of se- hand, if defence intensity was related to a colo- ven individuals can be hardly explained by nial response for aggregated nests, we would double brooding considering that nightjars lay expect a higher defence intensity than that ex- only one or two eggs per clutch. In at least pected from density alone. If we extrapolate three pairs of the Common Nightjar, a second from the observed defence intensity on Site B, male helping to feed the young was recorded we would expect for aggregated nests a defence (Schlegel, 1969), which even suggests the pos- intensity of 13.4 birds defending with passing sible existence of helpers. flights, but we observed only 6.4, differences being not significant (χ2= 3.34; P = 0.076). The response to an approaching observer (number Defence of birds approaching) was significantly different when approaching a nest compared to a The speed of reaction towards the tape recor- spot (n = 16) chosen randomly (H = 20.42; der shows no differences between both kinds n = 30; df = 1; P < 0.001). of nests. The similarity in latency times can be caused by the fact that the tape recorder was located at a constant distance from each nest and DISCUSSION that the registered time is probably what it takes for one of the owners of the territory to come We have found that seven pairs located in near, which would be a response of the «isolated Site A are tolerant to the approach of neigh- kind». The difference between both kinds of bours, show a high defence intensity, have nests lies in the fact that, in aggregated ones, short distances to the nearest nest and are other individuals besides the owner also appro- highly synchronised compared with the other ach the nest. The fact that not a single agonistic pairs, which are less tolerant, have a lower in- encounter was detected can be due to the exis- tensity of nest defence, show a greater distance tence of a great tolerance or mutual recognition to the nearest nest and are not so synchronised among adults when in aggregated nests. In iso- in breeding dates. lated nests, the number of Red-necked Nightjars that approach the tape recorder (Table 2) suggests that they are paired birds, which would Tolerance rule out the appearance of aggression among them when the stimulus was played. Tolerance seems to be high between pairs in We cannot tell whether a Red-necked Night- aggregated nests and this tolerance to neigh- jar that responds to tape recordings, or reacts to bours has also been noted in the Common the observer approaching a neighbouring nest Nigthjar. In this species, males can be found or territory, is defending the neighbouring nest resting outside the territory and sometimes even (communal response), or only its own nest be- closer to foreign nests than to their own cause the stimulus is near enough to be consi- (Cramp, 1985; Cleere, 1998). In case of danger, dered as threatening. However, the point re- the presence of neighbour males can be tolera- mains that a higher defence response may ted without apparent aggression, as they appro- increase breeding success in aggregated nests. ach the alarm calls of the territory owner (Sch- legel, 1969). We observed three adults (male, female and an individual of unknown sex) to- Nesting aggregation gether in a diurnal resting place at Site A just four days prior to laying. These data agree with The nest aggregations reported here can observations of two or three males of the Com- hardly be explained as strict coloniality because LOOSE NESTING AGGREGATION IN THE RED-NECKED NIGHTJAR CAPRIMULGUS RUFICOLLIS 17 of the absence of the typical benefits/costs as- by Cuadrado & Domínguez has vineyards and sociated to colonial breeding. Group size in the orchards with some sparse trees. This differen- Red-necked Nightjars seems not to be related to ce between habitats could explain the different predation risk due to: 1) distances between nests patterns of aggregation, as happens with the are large enough in this case, 2) they show very Common Nightjar (Schlegel, 1969). cryptic plumage and 3) have unconspicuous It seems that the nesting aggregation repor- nests. Therefore, an increase in group size is ted here is more related to habitat than to colo- not necessarily related to an increase in group niality. However, whatever the reason for the conspicuousness to predators. Breeding syn- nest aggregation, there is a higher hatching chronisation and defence intensity seem to synchronisation among aggregated nests, and represent small benefits due to the fact that bre- the overlap of the defence areas around the eding success is not increased, but the small nests, or the closeness between nests, together sample size may have influenced this result. We with the absence of agonistic interactions bet- did not find evidence of decreased individual ween neighbouring pairs, facilitates a more in- vigilance rate, parental care mistakenly addres- tense defensive response. The loose nesting ag- sed to alien chicks, infanticide or brood parasi- gregations during reproduction documented in tism among other benefits/costs associated to other species (Alvarez & Arias de Reyna, 1974; coloniality. Increased competition for food, ma- Møller, 1987; Birkhead, 1991; Sasvari, 1993; tes or nest sites could be the only cost derived Brown & Brown, 1996) allow a balance bet- from nest aggregation but we do not have evi- ween the two options of breeding isolated or dence to support or discard these possibilities. aggregated, and provide a range between both All nests having characteristics of a loose extremes. To choose one or another option may nesting aggregation are found in Site A. The depend on different factors like parental and greater structural complexity of this site (Ta- territory quality, habitat availability, exploita- ble 1) could favour the existence of a higher tion of temporary resources, etc. availability of both insects and nesting places, which would mean a higher density of bree- AKNOWLEDGEMENTS.—Many friends and collea- ding pairs. Since the vegetation structure is ho- gues helped with the fieldwork, contributing with both practical ideas and suggestions. We want to mogeneous at this site, we expected to find a thank E. López, M. C. Casaut, J. Marín, M. A. Nú- higher number of breeding pairs, or a more ho- ñez, M. Morales, F. and M. Marín, and J.I. Redondo mogeneous nest distribution covering the who- who offered valuable comments and field support le area, but all the nests were located in about during the realization of this study. M. Cuadrado half of the area. (E.B. de Doñana-CSIC) and F. Domínguez (Grupo In the Cuadrado & Domínguez area in Los Ornitológico del Sur) kindly allowed us to use their Palacios (Sevilla province, Spain) (mean dis- unpublished data on nest dispersion at their study tance between nests = 96.80 m, SD = 58.31, n = area in Los Palacios (Sevilla). We also thank J. Mo- 45; Cuadrado & Domínguez, unpubl.), there reno, J. L. Tella and M. Díaz for the improvements made on early versions of the manuscript. were no significant differences among the four years studied (H = 0.72, df = 3, n = 45, P = 0.869). Mean distance between nests at Los BIBLIOGRAPHY Palacios is shorter than for our isolated nests (H ÁLVAREZ, F. & ARIAS DE REYNA, L. 1974. Repro- = 2.63, df = 1, n = 52, P = 0.653 ), but higher ducción de la Urraca (Pica pica) en Doñana. Do- than for our aggregated nests (H = 13.61, df = ñana, Acta Vertebrata, 1: 77-95. 1, n = 52, P < 0.01). Nest densities at Los Pala- ANDERSSON, M. & WICKLUND, C. G. 1978. 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