Should I brood or should I hunt: a female barn owl s dilemma

1011 Should I brood or should I hunt: a female barn owl s dilemma Joël M. Durant, Jean-Paul Gendner, and Yves Handrich Abstract: While brooding, many female raptors rely exclusively on food provisioning from males. Thus, they may forego hunting until young are about half grown before exiting the nest to undertake a first foraging trip. To investigate the mechanisms that trigger this first foraging exit, we analysed nest food provisioning, female body mass change, and nestling and female food requirements in regard to exit date in five pairs of barn owls, Tyto alba (Scopoli, 1769), nesting in eastern France. Adult mass and behaviour were monitored using an automated weighing system and a video camera. Our results indicate that the first foraging exit of the female occurs about 15 days after the hatching of the first egg. This reinitiation of foraging occurs at about the same time that male food provisioning no longer matches nestling food requirements about 17 days after the hatching of the first egg. Thus the timing of the female s first hunting trip may be primarily adjusted to a discrepancy between brood food requirements and available food supply. Additionally, we found that females started to lose mass, on average, 6 days before their first hunting trip through a reduction of food intake, and we discuss the potential mechanisms and implications. Résumé : Au cours de l incubation, les femelles de rapaces dépendent quasi-exclusivement de l apport alimentaire du mâle et ne sortent chasser pour la première fois que lorsque les jeunes ont terminé environ la moitié de leur croissance au nid. Afin d isoler les mécanismes qui conduisent la femelle à sortir chasser pour la première fois, nous avons analysé les apports de proies au nid, les changements de masse corporelle de la femelle, ainsi que les besoins alimentaires de la nichée (avec ou sans ceux de la femelle) en relation avec la date de sortie. La masse et le comportement de reproduction de cinq couples de chouettes effraies, Tyto alba (Scopoli, 1769), ont été suivis dans l est de la France grâce à l utilisation d un système de pesée automatisée couplé à une acquisition vidéo. Au cours de notre étude, la sortie de la femelle a eu lieu en moyenne 15 jours après l éclosion du premier œuf de la couvée au moment où les besoins de la nichée dépassent l apport en proies au nid par le mâle, soit environ 17 jours après la première éclosion. Nous montrons que la première sortie de chasse de la femelle pour ses poussins a lieu principalement en réponse à une insuffisance des apports en proies par le mâle par rapport aux besoins de la nichée. Nous avons aussi montré que les femelles commencent à perdre de la masse du fait d une réduction de leur prise alimentaire en moyenne 6 jours avant leur première chasse et nous discutons les mécanismes et implications de ce changement de masse. Durant et al. 1016 Introduction Life-history theory assumes trade-offs between investment in current reproduction, survival, and future reproduction (Williams 1966; Stearns 1992). The quality of care given by parents to the nestlings is thus one parameter that optimizes the parental fitness. However, an offspring s interest is not necessarily the same as its parents (the parent offspring conflict hypothesis ; Trivers 1974; Stearns 1992) or its siblings (Mock and Parker 1997; Roulin 2001). Indeed, the willingness of the parents to invest in current reproduction depends on the expected global benefits of this investment for lifetime reproduction (Stearns 1992). Because parents Received 3 December 2003. Accepted 8 June 2004. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 21 August 2004. J.M. Durant, 1,2 J.-P. Gendner, and Y. Handrich. Centre d Écologie et Physiologie Énergétiques, Centre National de la Recherche Scientifique, 23 rue Becquerel, F-67087 Strasbourg CEDEX 02, France. 1 Corresponding author: (e-mail: joel.durant@bio.uio.no). 2 Present address: Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, PO Box 1050, Blindern, 0316 Oslo, Norway. should balance their investment in current offspring against expected future reproduction (Williams 1966), parental investment may be in conflict with offspring interest. The barn owl, Tyto alba (Scopoli, 1769), is a monogamous species that commonly lays clutches of 4 7 eggs depending on food availability (up to 18 eggs; Taylor 1994). Eggs hatch asynchronously at an average interval of 2.3 days (Durant and Handrich 1998) because the female starts incubation after laying the first egg of the clutch. Consequently, and because of a fast growth rate (Durant and Handrich 1998), barn owl nestlings exhibit a large size hierarchy (Taylor 1994; Durant 2002). During reproduction, female owls are usually heavier than males (Mikkola 1983). This reverse sexual dimorphism is common in raptors (Korpimäki 1986; Marti 1990) and is associated with a marked division of parental duties (Bunn et al. 1982; Shawyer 1994; Taylor 1994). This marked division is evident in barn owls, where the female stays in the nest without foraging during incubation and early rearing. It is only several days after hatching that she starts to undertake short hunting trips and leaves the nestlings alone for increasingly longer time periods (Newton 1979; Taylor 1994; Durant 2002). Because of the hatching asynchrony, nestlings are left unattended at different stages of development when the female first starts to hunt (Durant 2002). Before 15 20 days of age, barn owl nestlings cannot Can. J. Zool. 82: 1011 1016 (2004) doi: 10.1139/Z04-078

1012 Can. J. Zool. Vol. 82, 2004 maintain their own body temperature (Taylor 1994; Durant and Handrich 1998). As a consequence, the female cannot leave the nest before this date without adverse consequences for the youngest nestlings. This is especially the case for the younger and less developed nestlings that are confronted by both increased thermoregulatory costs and food intake constraints in comparison to their older siblings (Durant 2002). The timing of the female s first foraging exit after brooding is therefore of crucial importance because it represents compromise between the nest food supply, the female s own food requirements, and the care of the youngest chicks. The aim of this study was to determine the factors underlying the timing of a female barn owl s first hunting trip. We hypothesized that the female exit is triggered by discrepancies between brood food requirements and food provisioned by the male. Specifically, we predicted that the first exit would correspond to the moment when the food supplied by the male no longer matched brood requirements. We report here the changes in the daily food quantity brought to the nest by both the male and the female for five pairs of freeranging barn owls during reproduction. In addition, we report the frequency of round-trips to the nest and adult body mass changes. Fig. 1. Illustration of weighing and video systems. The access corridor was devised to leave the nest cavity in darkness and to take several seconds to be crossed by the birds (3.44 ± 0.09 s (±SE), n = 7319). The passage of a bird and its direction were indicated by infrared photocells installed in the corridor near the entrance of the nest box and near the exit. Each time a bird (with or without prey) walked on the balance, a mass was recorded, with a frequency of one measurement every 0.03 s (on average, 115 times during a passage). Cells and weighing data were associated and sent to a remote computer, its clock recording the time and duration of each passage. The computer controlled and synchronized the video recorder. Video recording speed was set 80 times slower than real time and was accelerated to normal speed for 2 min at each exit or entrance of a bird. Materials and methods The study was conducted in Alsace, eastern France (48 20 N, 7 45 E) on five pairs of barn owls. We defined an owl-day as the period between 1600 and 1559 the following day to put the foraging events occurring before and after midnight within the same day. Hereafter, day refers to an owl-day. Days were numbered from the hatching of the first egg to synchronize the broods. Results encompass data from egg laying to 40 days after the hatching of the first egg. After this date, the nestlings moved frequently to the nest exit where they received their meal, preventing accurate monitoring. Monitoring design Data on mass and behaviour were obtained using nest boxes equipped with an electronic balance and an infrared video camera (Durant 2000, 2002). Figure 1 is a diagram of the nest box device and shows the relative locations of the weighing device, photocells, video camera, video recorder, and computer. An average adult body mass for each passage over the weighing platform (Fig. 1) was calculated using custom-made software (J. Lage, Jensen Software Systems, Laboe, Germany). The mass of prey brought by males was obtained by calculating the difference between mass at entrance with prey and mass at exit without prey (average SE of ±0.9 g). To follow individual changes in body mass over time, we used the minimum body mass measured each day, on the assumption that it represented mass with an empty gastrointestinal tract. We observed in-nest activity (laying, hatching, prey deliveries) during reproduction using a video time-lapse recorder controlled by the computer (Fig. 1). Sex identification was made during copulation events, at which time we determined visual criteria used later to recognize single individuals. Nest monitoring began, at the latest, 27 days before the laying of the first egg (range 27 39 days) and finished 58 ± 8 days (range 43 95 days) after hatching of the first egg. However, only the periods of incubation (from laying until first hatching) and rearing (from hatching to fledging) are presented and analysed in this study. Nest food requirements The nest food requirement is the sum of both the female s and her nestlings requirements. The food requirement was

Durant et al. 1013 estimated for each brood (estimated brood requirement, EBR) from food intake of captive barn owl nestlings reared under ideal conditions, i.e., thermoneutrality, ad libitum food, and growth at its characteristic rate (Durant and Handrich 1998). EBR is the sum of the food intake of the individual nestlings composing the brood and takes into account the differences in age among nestlings due to hatching asynchrony, and the occurrence of brood reduction. Female food requirement (g day 1 ) was estimated from metabolized energy (ME) (for Strigiformes ME = 8.630 body mass 0.578 ; Wijnandts 1984) transformed into prey mass required per day, assuming a caloric value of prey of 7.7 kj g 1 and an energy assimilation efficiency in barn owls of 72.3% (Durant et al. 2000). Fig. 2. Nest food supply and female body mass change. Data are averages for five barn owl (Tyto alba) pairs, the days being numbered from the date of the first hatching to synchronize the years. Data for body mass change of captive females are presented in the inset of panel c (Durant 2000). The asterisks indicate the locations of the average inflection points of the female body mass curves (Table 1). Statistical analysis Day-to-day changes in mass and behaviour of females and males were estimated by linear mixed effects models fitted to the daily data, with pair and date entered as random variables, using the function LME in the statistical package S- Plus (Venables and Ripley 2002). Means (±SE) are presented and results were considered significant at P < 0.05. Dates of events were compared using Friedman s or Wilcoxon s tests. Results Daily nest food supply The number and mass of prey brought to nest by males and females were determined for each day (Fig. 2a). Food load, which always consisted of only one prey item, was of the same mass throughout our study. The average mass of food loads brought by males and by females was 22.8 ± 0.46 g and 22.9 ± 0.72 g, respectively (Table 1). During the period of incubation, food supply brought by the male did not change significantly in number (3.0 ± 0.1 prey day 1 ) or in mass (53.7 ± 3.7 g day 1, Table 1). However, after hatching, the male daily food supply significantly increased to a maximum of 11.0 ± 2.6 prey items, equivalent to 256.8 ± 22.5 g day 1 (Table 1). The daily number and mass of prey brought to nest by the female after her first exit to hunt steadily increased to 11.3 ± 2.2 prey items and 251.9 ± 59.5 g, respectively (Table 1). Round-trip frequency During the incubation period, the frequency of nest entry and exit (round-trip frequency) was constant for both females and males (2.5 ± 0.3 and 7.7 ± 0.9 round-trips day 1, respectively; Table 1). After hatching, the round-trip frequency significantly increased for both females and males (Table 1). In males, the ratio of visits with a prey to the total number of visits per day (ranging from 0 to 1) changed over the course of the reproductive cycle (Fig. 2b, Table 1). This ratio was constant during incubation (0.50 ± 0.02, t [140] = 0.652, P = 0.52), whereas it increased to an average of 0.82 ± 0.02 (t [144] = 15.414, P < 0.001) from hatching to the female s first exit to hunt (+15 days, Table 2). Body mass change (Fig. 2c) Among females, change in body mass was highly consistent. Inflection points in the body mass curves were localized using change of sign of the rate of body mass change (dm/dt), and analyses were conducted for periods separated by these points (Table 2). During the few days following egg laying, female body mass decreased by 56.8 ± 7.2 g (Table 1) but then remained constant (387.2 ± 7.2 g, Table 1) during the following 22 days, when females incubated and began to take care of hatched chicks. Body mass then sharply decreased again by 87.9 ± 6.1 g (22.7% ± 1.4% of initial body mass) over a 20-day period (Table 1). Thereafter, female body mass remained constant (299.3 ± 5.6 g, Table 1). Body mass of males exhibited a small decrease of only 0.05% during the first 30 days following hatching (Table 1) and then remained more or less constant (average male body mass, 289.7 ± 1.8 g).

1014 Can. J. Zool. Vol. 82, 2004 Table 1. Results of fitting linear mixed effects models to the daily behavioural and body mass data for pairs of European barn owls (Tyto alba) during incubation and rearing. Variable df t df P Females Food load, mass (g) 1,1008 0.974 0.331 Nest food supply, mass (g) 1,197 5.421 <0.001 Nest food supply, no. of prey 1,197 8.174 <0.001 Round-trip frequency during incubation (trips day 1 ) 1,139 0.575 0.566 Round-trip frequency during rearing (trips day 1 ) 1,198 3.046 0.003 No. of visits with prey / total no. of round-trips 1,198 4.947 <0.001 Body mass before 1st inflection point (g) 1,38 2.607 0.013 Body mass between 1st and 2nd inflection points (g) 1,32 0.134 0.894 Body mass between 2nd and 3rd inflection points (g) 1,95 8.264 <0.001 Body mass after 3rd inflection point (g) 1,95 1.459 0.148 Males Food load, mass (g) 1,2034 1.832 0.067 Nest food supply, mass (g) 1,264 7.163 <0.001 Nest food supply, no. of prey 1,297 4.213 <0.001 Round-trip frequency during incubation (trips day 1 ) 1,140 1.717 0.088 Round-trip frequency during rearing (trips day 1 ) 1,151 2.593 0.01 No. of visits with prey / total no. of round-trips 1,297 4.465 <0.001 Body mass during incubation (g) 1,153 0.685 0.495 Body mass during first 30 days of rearing (g) 1,142 6.036 <0.001 Body mass at end of rearing (g) 1,44 2.446 0.019 Note: The inflection points in the females body mass (BM) curves and other variables used in the models are found in Table 2. Table 2. Timing of events during incubation and rearing (days before (negative values) or after (positive values) hatch of the first egg). Brood Brood size End of laying* Start of IMC, End of IMC, Female s 1st exit EBR = male food supply Nest = male food supply End of BM loss, A 5 25 16 6 15 20 20 28 B 6 20 12 7 14 16 16 28 C 6 19 14 13 16 17 15 27 D 6 20 16 9 14 18 18 32 E 4 24 8 8 17 16 8 29 Mean 5.4±0.4 22±1a 13±1b 9±1c 15±1d 17±1d 15±2d 29±1e Note: Mean values followed by different letters are significantly different (Friedman s test, χ 2 = 28.3261, df = 6, P < 0.001; Wilcoxon s signed-ranks test, P > 0.05). *Date when the last egg of the brood was laid. IMC, incubation mass constancy (period when the female body mass is constant). Note that the End of IMC was not significantly different from the hatching date of the last egg (Wilcoxon s signed-ranks test, Z = 0.41, P = 0.682). The inflection points of the body mass curves were localized using the change of sign of the rate of body mass change, dm/dt. Date after which the estimated brood requirement (EBR) exceeded the male daily provisioning. EBR is an average based on observed data (number of chicks, age, hatching asynchrony, brood reduction) to which is applied the daily food intake of captive chicks fed ad libitum. Date after which the nest food requirement exceeded the male daily provisioning. Nest food requirement is the sum of the female food requirement and EBR. Date after which the female body mass (BM) remained constant until the end of rearing. Female s first exit to hunt and nest food requirements (Fig. 3) The females first exit to hunt occurred about 15 days after hatching (range of 14 17 days; Table 2). This date was not significantly different from the date when the nest food requirements (15 days after hatching; Wilcoxon s signedranks test, Z = 1.49, P = 0.136) and the EBR (17 days after hatching; Wilcoxon s signed-ranks test, Z = 0.270, P = 0.787) exceeded male food provisioning. Specifically, before the female s first exit, the EBR was less than male food provisioning (t [69] = 2.732, P < 0.01). Thereafter, the EBR was greater (t [75] = 3.945, P < 0.001). Likewise, the total nest food requirement was not significantly different from male food provisioning before the first exit of the female (t [109] = 1.745, P = 0.084) but was greater thereafter (t [121] = 3.006, P > 0.01). Discussion The change in female behaviour from taking care of nestlings in-nest to sharing the foraging effort with males is a crucial step in the rearing of nestlings because it can affect nestling development. We investigated the mechanisms responsible for a female s first exit to hunt after brooding in a

Durant et al. 1015 Fig. 3. Food provisioning and nest requirements in relation to rearing stage. The area in grey is the period before the female s first exit to forage (ca. 15 days after hatching). Each data point is a mean (±SE) over 4 days. The female food intake was evaluated using the observed body mass and corresponding food requirement. Total nest food requirement (black curve) is the sum of the female and brood requirements (estimated brood requirement (EBR), black dashed curve). After the female exit, the nest food requirement is equal to EBR. EBR is an average based on observed data (number of nestlings, age, hatching asynchrony, brood reduction) to which is applied the daily food intake of captive nestlings fed ad libitum. The dotted lines represent the food requirements for broods of 1 6 nestlings. free-ranging raptor, the barn owl, using for the first time simultaneous direct measurements of food provisioning to the nest and changes in body mass of both members of a pair. We found a strong synchrony between the first exit of the female and the moment when the nest food requirements exceeded the male s provisioning of the nest (Fig. 3, Table 2). Thus, we have confirmed by empirical methods the hypothesis that the timing of a female s first exit is adjusted to the discrepancy between brood energy requirement and available food supply (Whittingham and Robertson 1993; Taylor 1994). In support of our results, one female barn owl was observed to leave her nest to hunt for the first time only after the fledging of her single nestling: all food requirements of both mother and nestling were provided by the male (J.M. Durant and Y. Handrich, personal observations). Further, in Eurasian kestrels (Falco tinnunculus L., 1758), females decrease their hunting effort and prey delivery rate in response to supplementary feeding (Wiehn and Korpimäki 1997). In raptors, it seems that the female hunting effort is commonly adjusted to that of the male. In barn owls, the inability or unwillingness of males to supply enough food for their brood may (i) increase the aggressive behaviour of nestlings (Roulin 2001) and (ii) make it impossible for brooding females to obtain their daily energy requirements (Fig. 3). Both cases may induce the female to begin hunting (Durant 2002). During incubation, the male provides food for himself and his mate. After hatching and before the female reinitiates foraging activities, the male has to provide for the growing nestlings through increased foraging efforts (Fig. 3). In the five broods examined here, male food provisioning to the nest reached a maximum while the EBR continued to increase. This observation is consistent with the prediction that in iteropareous species such as the barn owl, parental effort should be circumscribed to a fixed level that is independent of the offspring s needs (Ricklefs 1992; Sæther et al. 1993; Wiehn and Korpimäki 1997) so that adult survival and future reproduction are not jeopardized (Goodman 1974; Charlesworth 1980). Indeed, male body mass was more or less constant over the period examined. The level of male investment may be partly determined by food availability, which in turn is determined by territory quality and variations in weather (Hakkarainen et al. 1997). During rearing, males provided up to 11 prey per night a level of food provisioning that did not include their own food requirements (ca. 2 prey; Durant 2002). The time available to hunt for such a large quantity of prey may be a constraint for the male. In support of this hypothesis, we found that the ratio of visits with prey to the total number of nest visits reached a maximum over the same period (Fig. 2). Is the change in body mass observed in females an important parameter for explaining the timing of their first hunting trip? Our results show that females started to lose mass, on average, 6 days before their first exit (Table 2). This body mass loss, beginning at the hatching of the last egg (Table 2), corresponds to ca. 4.7 ± 0.4 g day 1 (body mass loss during a prolonged fast is 6.6 g day 1 at 5 C (Handrich et al. 1993; Thouzeau et al. 1999)). In captivity, incubating females showed a similar decrease in body mass even when fed ad libitum (Fig. 2c). These results suggest that body mass loss at the end of incubation is a consequence of a spontaneous reduction in food intake and is not due to a reduction of food availability. Thus, this reduction in body mass may be a means to reduce wing loading and optimize foraging after exit (Sanz and Moreno 1995). This hypothesis is supported by the finding that female barn owls caught more prey when their body mass reached their initial nonbreeding mass (Taylor 1994; Durant 2000). On the other hand, the increased hunting effort, driven by the increased brood requirements, may affect body mass owing to an increase in energy expenses. Furthermore, we have observed a female barn owl that continued to lose mass well after reaching her prebreeding mass as a consequence of very poor male food provisioning (Durant and Handrich, personal observation). This suggests that female food intake is at least partly dependent on brood energy requirements and food availability (provided either by both parents or by the male alone). In fact, 10 days after eggs hatch, the combined requirements of the brood and of a female that would maintain her body mass would became greater than the food provided by the male (Fig. 3). It is at this point that females start to decrease food intake and hence lose mass (Table 2). In addition to a reduction in wing loading, the reduction of food intake by the female could add a level of flexibility to the timing of her first exit. By reducing her consumption, the female postpones the moment when food provisioning by the male becomes insufficient to satisfy brood requirements (ca. 4 days; Fig. 3) and, hence, the timing of her first exit to hunt. Indeed, the quantity of food consumed daily by the female corresponds to the consumption of the three

1016 Can. J. Zool. Vol. 82, 2004 youngest nestlings of a brood of five of up to 17 days of age. However, a decrease in female food intake is not always sufficient for chick survival. Brood reduction occurred some days after the female s first exit to hunt in two of the nests examined. In conclusion, the timing of the female s first hunting trip (9 15 days after eggs hatch) appears to be of utmost importance for the energy balance of the brood. This event may be crucial to successful fledging because energy supply at this time is bottlenecked and could initiate a growth delay leading to brood reduction when the female leaves the nest (Durant and Handrich 1998). Indeed, during years of low food availability, increased begging behaviour of nestlings would lead the female to begin hunting earlier. However, in large broods, older siblings give the youngest a certain amount of care, e.g., by feeding younger siblings (Epple 1979; Marti 1989) or transferring heat (Durant 2002), therefore minimizing deleterious effects of an early female exit. Past research indicates that the timing of the hunting exit creates a different thermal and food environment for the youngest nestlings compared with the environment that older siblings had at the same age (Durant 2002). Here, we show that the exit may be triggered by the discrepancy between increasing brood requirements and the supply of food brought to the nest by the male, which in turn may depend on environmental conditions. Acknowledgments This study was supported financially by the Ministère de l Environnement, Service de la Recherche des Etudes et du Traitement de l Information sur l Environnement, France. The experiments were done in compliance with current French laws and after program acceptance by French authorities (authorization of the Ministère de l Agriculture et de la Pêche, No. 04196). We thank C. Plumeré for her help in developing the automatic nest and M. and B. Bertrand for the maintenance of the system. We are grateful to J. Lage of Jensen Software Systems (Lammertzweg 19, D-24235 Laboe, Germany, JLage.JSS@t-online.de) for his computer expertise and D.Ø. Hjermann for his help with the statistical analysis. We also thank C.-A. Bost, N. Pettorelli, I. Taylor, C. Marti, and an anonymous reviewer for their useful comments and M. Landys for editing help. References Bunn, D.S., Warburton, A.B., and Wilson, R.D.S. 1982. The barn owl. Poyser, Carlton, United Kingdom. Charlesworth, B. 1980. Evolution in age-structured populations. Cambridge University Press, Cambridge, United Kingdom. Durant, J. 2000. Energétique de la reproduction chez la Chouette effraie (Tyto alba). 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