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Provisioning behavior of male and female eastern screech-owls during the post-brooding period.

INTRODUCTION

Although owls exhibit biparental care, the relative contributions of males and females during different life cycle stages, and particularly the breeding season, are poorly understood. Sunde et al. (2003) found smaller male Tawny Owls (Strix aluco) provisioned nestlings more often than females during the post-brooding period and had larger home ranges than females but did not deliver larger prey. Eldegard and Sonerud (2010, 2012) reported no differences in size of prey fed to offspring by male and female Tengmalm's Owls (Aegolius funereus) during the post-brooding period, although smaller males provisioned offspring more, particularly when food was abundant. Davis and Cannings (2008) found, during the breeding season, pellets of smaller, male Western Screech-Owls (Megascops kennecotti) contained more small prey items (i.e., fish and insects) and those of larger females had larger prey (i.e., small mammals). In contrast Abbruzzese and Ritchison (1997) found no difference in hunting behavior or type of prey hunted by male and female Eastern Screech-Owls during the nonbreeding season.

Given the substantial fitness benefits associated with successfully provisioning young during the post-brooding period, selection pressure for optimizing this behavior is expected to be significant (Ydenberg, 1994). Understanding relative contributions of males and females during this critical period when both parents feed nestlings could contribute important information to our understanding of the feeding ecology and life history of owls and, in particular, to the long-standing debate (Mueller, 1986; Kruger, 2005) about the evolution of reversed sexual dimorphism (RSD; i.e., females larger than males). Two of the prevailing hypotheses for the evolution of RSD are the Ecological Hypothesis that suggests intersexual differences in size evolved to enhance niche partitioning by males and females and reduce competition for food (Reynolds, 1972; Mueller, 1986) and the Role Differentiation Hypothesis that suggests larger females are better able to protect and incubate eggs, whereas smaller males forage more efficiently (von Schantz and Nilsson, 1981). Support for the Ecological Hypothesis would include evidence that males and females deliver prey of different types or sizes and, according to the Role Differentiation Hypothesis, males would be expected to make more visits to the nest or deliver larger prey than females. A third related hypothesis focuses on the benefits of a faster ingestion rate of smaller prey during the breeding season ("Ingestion Rate Hypothesis"; Slagsvold and Sonerud, 2007). According to this hypothesis, males benefit by being small enough to capture agile prey that require less handling time during the breeding season but large enough to capture larger food items (i.e., mammals and birds) outside of the breeding season. In addition utilizing smaller prey allows young to feed on food items unassisted at an earlier age and relieves the female from duties associated with dismembering prey, therefore allowing her to participate in prey capture. According to this hypothesis, both sexes would be expected to capture smaller prey during the post-brooding period and intersexual differences in provisioning behavior would be minimal.

Eastern Screech-Owls (Megascops asio) are slightly size dimorphic and found throughout Eastern North America in tree-dominated landscapes (Gehlbach, 1995). On average females are 17% greater in mass than males (mean adult male body mass = 166 g; Gehlbach, 1995) and larger than males in length (means = 21.5 cm vs. 20.7, respectively) and wing chord (means = 15.2 vs. 15.8 cm); but, within populations, measurements of males and females overlap (Gehlbach, 1995). During the breeding period owls feed on a diverse array of vertebrate and invertebrate species with rodents and small birds contributing the greatest biomass (Gehlbach, 1995). Male Eastern Screech-Owls typically feed their mates during incubation and both mates and young early in the nestling period (Gehlbach, 1995). Beginning about 2 wk post-hatching, when young are able to thermoregulate, and continuing until fledging, males and females share provisioning duties (Gehlbach, 1994, 1995). The objective of our study was to assess possible intersexual differences in provisioning behavior (visit rate and prey size) between male and female Eastern Screech-Owls during the 2 wk post-brooding period.

METHODS

We studied the provisioning behavior of 17 different pairs of Eastern Screech-Owls (hereafter Screech-Owls) over a 3 y period (1992-1994) at the Miller Welch-Central Kentucky Wildlife Management Area located 17 km southeast of Richmond, Kentucky (37.6345[degrees]N, -84.2007[degrees]W). Nest boxes used by Screech-Owls were checked weekly from late March through May to monitor the status of eggs and young.

When nestlings were 12-14 d old, we transferred them and adult females to specially constructed boxes placed in either the same tree or an adjacent tree. As part of the transfer process, adult females and nestlings were banded with a USGS numbered aluminum band and adult females were marked with a dark spot on their foreheads (using a black Sharpie[R] marker) to permit identification of males and females during subsequent video-recording sessions. Transferring owls prior to this age could have caused females to stop brooding young and, potentially, influence survival of the young because they are unable to thermoregulate until about 12 d old (Gehlbach, 1994). Owls were transferred to boxes that had two chambers separated by a plywood partition that could be removed during observation sessions. Owls were placed in a nest chamber (20 x 31 x 41 cm) that had a battery-powered light attached to the ceiling for use when taping, and behind the nest chamber was a larger chamber (20 x 31 x 61 cm) where a camcorder was placed. No nests were abandoned following the transfer of females and nestlings to the new nest boxes.

We videotaped nests (n = 17; 4 in 1992, 4 in 1993, and 9 in 1994) for a total of 424 h, or an average of 24.9 h per nest. We recorded each nest nightly for 2-4 h starting at dusk, except during inclement weather {e.g., heavy rain). When taping, camcorders were placed in the chamber behind the nest chamber and were focused on the box entrance. Taping at each box continued until young fledged, approximately 2 wk after nest-transfer.

Tapes were subsequently reviewed and, for each visit by an adult, we noted sex (only females were marked with Sharpie[R] marker; see above), type of prey (identified to lowest taxonomic category possible), and the size of prey delivered. Prey size was categorized relative to the length of an owl's bill (small = <2x bill length, medium = 2-3x bill length, and large = >3x bill length).

Mean visitation rates were calculated (feeding trips per hour) for all individuals. Because multiple observations were made at each nest, we used repeated measures ANOVA (SAS Institute, 2002-2004) to examine the possible effects of sex on visitation rate and mean size of prey delivered. Values are presented as means [+ or -] SE (standard error).

RESULTS

Screech-Owls delivered 1260 prey items to young during our study. We identified 16 different types of prey, with insects and crayfish the most common prey (Table 1). The most common insect prey was beetles and beetle larvae (Coleoptera) and Orthopterans (i.e., katydids and crickets). Overall, Screech-Owls made 5.2 [+ or -] 0.3 visits per hour (n = 140 observation periods). Mean brood size was 3.4 nestlings, with two nests having two nestlings, seven with three, seven with four, and one with five. The mean provisioning rates of females (6.2 [+ or -] 0.4 visits per hour) and males (4.3 [+ or -] 0.3 visits per hour) did not differ ([F.sub.1,16 = 0.3, P = 0.09). In addition brood size had no effect on provisioning rates ([F.sub.3,18] = 0.2, P = 0.88), and the interaction between sex (visit rates) and brood size was not significant (P = 0.42). Mean size of prey delivered was 1.4 [+ or -] 0.03. We found no significant difference between male and female Screech-Owls in the mean size of prey delivered ([F.sub.1,15] = 0.1, P = 0.80). In addition brood size had no effect on the mean size of prey delivered by adults ([F.sub.3,16] = 0.1, P = 0.97), and the interaction between sex and brood size was not significant (P = 0.51).

DISCUSSION

We found no difference in the size of prey delivered to nestlings by male and female Screech-Owls. Screech-Owls are opportunistic feeders, and although the increased availability of smaller prey {i.e., invertebrates) during the post-brooding period (Ritchison and Cavanagh, 1992) may have masked potential differences in size of prey delivered between sexes, it seems more likely that males and females capture the same types and sizes of prey. These findings are consistent with provisioning data reported for Tawny Owls during the post-brooding period (Sunde et al, 2003) and suggest male and female Screech-Owls do not exhibit food niche partitioning during the post-brooding period, which has been proposed as one possible explanation for RSD in raptors (i.e., Ecological Hypotheses; Reynolds, 1972; Kruger, 2005).

We also found no difference in the provisioning rates of male and female Screech-Owls during the 2 wk post-brooding period. Therefore, our results do not support the Role Differentiation Hypothesis for RSD development and suggest, at least during the last 2 wk of the nestling period, female Screech-Owls provision nestlings at rates similar to that of males. One possible caveat is that we were unable to account for the possibility smaller males, being more energetically efficient flyers, foraged at greater distances from nests, whereas larger females foraged closer to nest sites. Consistent with this potential caveat, Buhay and Ritchison (2002) reported male Screech-Owls may hunt in different areas of their ranges during different breeding periods, and Sunde et al. (2003) reported male Tawny Owls had larger home ranges than females during the post-fledging period. It is unlikely differences in male home range size or observed provisioning behavior were explained by males seeking additional mates because there is little evidence of extra-pair paternity in Screech-Owls (Gehlbach, 1995; Lawless et al., 1997). It should also be noted that prey abundance was not directly measured in our study, and although we have no reason to suspect our study period was anomalous, prey delivery rates of females, in particular, can decrease in years of high food abundance (Zarybnicka, 2009; Eldegard and Sonerud, 2012).

The most common prey items adults delivered to young were insects (Table 1). In 65% of visits, insects were delivered to young. In contrast insects only make up 33.3% of the diet of adult Screech-Owls during the breeding season (Ritchison and Cavanagh, 1992). During the breeding season, adults appear to feed primarily on larger prey items (i.e., birds and small mammals; Ritchison and Cavanagh, 1992) but provision young with small prey (i.e., invertebrates). Given this result, we find it likely food deliveries we observed in our videos were made from the actual parent that captured the food and not transferred from male to female outside the nest box as has been reported in some raptors that feed larger prey to their young (Sonerud et al., 2013; Sonerud et al., 2014b). We found no evidence of Screech-Owls feeding on Chiropterans (Janos and Root, 2013).

Our results indicate smaller male Screech-Owls did not provision nestlings at higher rates than females during the post-brooding period. Such results are inconsistent with predictions made by the Role Differentiation and Ecological Hypotheses for the evolution of RSD, although it seems likely intersexual differences in provisioning behavior would be less pronounced in slightly size-dimorphic owls (Olsen and Olsen, 1987; Kruger, 2005). Our findings are most consistent with the Rate Ingestion Hypothesis for RSD development which predicts both sexes would take smaller prey and provision at a similar rate during the post-brooding period to increase ingestion rate and decrease handling time (Slagsvold and Sonerud, 2007; Sonerud et al., 2013). This hypothesis also predicts that difference in sex roles should increase with prey size. According to this line of reasoning, selection pressure for RSD may be minimal during the post-brooding period for a raptor such as the Eastern Screech-Owl that tends to feed smaller prey to its young and exhibits minimal intersexual differences in provisioning behavior. Similar trends have recently been reported in an interspecific comparison of nine raptor species (Sonerud et al., 2014a) and in an intraspecific analysis of the Eurasian Kestrel (Falco tinnunculus; Sonerud et al., 2014b). Taken together our results improve our understanding of the feeding ecology of the Eastern Screech-Owl and contribute to an understanding of factors that govern natural and sexual selection in slightly dimorphic owl species.

Acknowledgments.--We thank S. Carr for assistance with the field work, the EKU Research Committee for financial support, and Malone University for providing a Summer Scholars Grant to complete this manuscript.

LITERATURE CITED

Abbruzzese, C. M. and G. Ritchison. 1997. The hunting behavior of Eastern Screech-Owls. Gen. Tech. Rep. NC-190, U.S.D.A. Forest Service, North Central Research Station.

Buhay, J. and G. Ritchison. 2002. Hunting behavior of and space use by Eastern Screech-Owls during the breeding season. J. Rapt. Res., 36:194-199.

Davis, H. and R. J. Cannings. 2008. Diet of Western Screech-Owls in the interior of British Columbia. British Columbia Birds, 18:19-22.

Eldegard, K. and G. A. Sonerud. 2010. Experimental increase in food supply influences the outcome of within-family conflicts in Tengmalm's owl. Behav. Ecol. Sodobiol, 64:815-826.

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Gehlbach, F. R. 1994. The Eastern Screech-Owl: life history, ecology, and behavior in suburbia and the countryside. Texas A&M University Press, College Station, TX U.S.A.

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von Schantz, T. and I. N. Nilsson. 1981. The reversed size dimorphism in birds of prey: a new hypothesis. Oikos, 36:129-132.

Slagsvold, T., and G. A. Sonerud. 2007. Prey size and ingestion rate in raptors: importance for sex roles and reversed sexual size dimorphism. J. Avian Biol., 38:650-661.

Sonerud, G. A., R. Steen, L. M. Low, L. T. Roed, K. Skar, V. Selas, and T. Slagsvold. 2013. Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors. Oecologia, 172:93-107.

--,--, V. Selas, O. M. Aanonsen, G. H. Aasen, K L. Fagerland, A. Fossa, L. Kristiansen, L. M. Low, M. E. Ronninc, S. K. Skouen, E. Asakskogen, H. M. Johansen, J. T. Johnsen, L. I. Karlsen, G. C. Nyhus, L. T. Roed, K. Skar, B. A. Sveen, R. Tveiten, and T. Slagsvold. 2014a. Evolution of parental roles in provisioning birds: diet determines role asymmetry in raptors. Behav. Ecol, 25:762-772.

--, --, L. M. Low, L. T. Roed, K. Skar, V. Selas, and T. Slagsvold. 2014b. Evolution of parental roles in raptors: prey type determines role asymmetry in the Eurasian kestrel. Anim. Behav., 96:31-38.

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SUBMITTED: 8 JULY 2016

ACCEPTED: 21 AUGUST 2016

JASON R. COURTER (1)

Department of Natural Science and Mathematics, Malone University, Canton, Ohio 44709

GARY RITCHISON

Department of Biological Sciences, Eastern Kentucky University, Richmond, 40475

AND

W. RUSSELL MCCLAIN

Department of Biology and Environmental Science, Davis & Elkins College, Elkins, West Virginia 26241

(1) Corresponding author: e-mail: jcourter@malone.edu
TABLE 1.--Total number of prey delivered by male and female
Eastern Screech-Owls identified to the lowest taxonomic
category possible and arranged in order of abundance

                   Prey                        N

Insects
  Coleoptera (N = 166 larval individuals)    490
  Orthoptera                                 220
  Lepidoptera                                 64
  Odonata                                     10
  Cicadidae                                    3
  Tettigoniidae                                1
  Unidentified insects                        44
Crayfish
  Family Astacidae                            81
Amphibians
  Family Ranidae                              23
Leeches
  Subclass Hirudinea                          20
Mammals
  Peromyscus spp. or Microtus spp.            15
  Family Sciuridae                             1
Fishes
  Class Osteichthyes                          12
Birds
  Unidentified passerine                       1
Earthworms
  Class Oligochaeta                            1
Unknown                                      294
Total                                        1280
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Author:Courter, Jason R.; Ritchison, Gary; McClain, W. Russell
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2017
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