Evidence of nest material kleptoparasitism in Worm‐eating Warblers (Helmitheros vermivorum) in east‐central Arkansas, USA

Abstract Nest material kleptoparasitism likely evolved in birds to reduce the cost of searching for and collecting material themselves. Although nest material kleptoparasitism has been reported commonly in colonially nesting species, reports for solitary breeding species are infrequent, especially for neotropical migratory species. Here, we report potential and actual nest material kleptoparasitism in the Worm‐eating Warbler (Helmitheros vermivorum). We deployed video camera systems at passerine nests (n = 81) in east‐central Arkansas during summers 2011–2012. In one video, we observed a Worm‐eating Warbler stealing nesting material from a Hooded Warbler (Setophaga citrina) nest. One day later, we later observed a Worm‐eating Warbler landing within 0.5 m of the same warbler nest when the female was incubating, which possibly deterred a second theft of nesting material. In a third video recording, we observed another Worm‐eating Warbler landing within 1 m of an Indigo Bunting (Passerina cyanea) nest. As far as we could determine, neither of these latter two nest visits resulted in nest material kleptoparasitism. Potential benefits of nest material kleptoparasitism include reduced competition for limited nest materials, easy access to suitable material, reduced travel distance, and reduction of nest predation risk; however, costs include risk of attack by host or introducing parasites to one's nest. Importantly, this behavior could ultimately affect the behavioral and life history evolution of a species. We suggest further work should be conducted to determine the prevalence of nest material kleptoparasitism in Worm‐eating Warblers and other solitary breeding passerines, including efforts to quantify the benefits and costs of this behavior.


| INTRODUC TI ON
Avian behavior affects individual fitness, reproductive success, and ultimately shapes the evolved life history of a species. For example, Lack (1968) suggested avian offspring exhibit developmental adaptations that are driven by strong selective pressures.
These pressures include depredation risk and food availability (Sibly et al. 2012). These adaptations theoretically increase fitness and reproductive success and enable individuals to cope with various ecological challenges during the breeding season. One challenging process that likely increases the exposure and risk to nest depredation, is time-consuming, and energetically costly for adults is nest construction (Mainwaring & Hartley, 2013 and references therein). During this period, birds must collect suitable nest material, which may be arduous to gather either because material is located far from the individual's nest or pair's territory, rare (Slager et al. 2012), or perhaps needed in large quantities that are difficult to collect (Jones et al. 2007).
Importantly, a trade-off exists between the amount of time a bird invests in nest-building and later reproductive stages (i.e., incubation and brood rearing). For example, Moreno et al. (2010) demonstrated females that collected less nest material spent more time incubating eggs and feeding nestlings, resulting in improved nestling growth. Further, research has demonstrated nest construction may play a role in sexual selection and in resulting reproductive success (Mainwaring & Hartley, 2013 and references therein). Moreover, Jones et al. (2007) emphasized kleptoparasitism of nest materials may reduce the number of trips to a nest during construction, which could reveal the location of the nest to a brood parasite or predator. Thus, birds may avoid the cost of searching for dispersed and hard-to-find material if they can gather relatively large quantities of suitable material from a nest of another nearby individual (either intra-or interspecific); albeit the thief must locate a suitable nest first. This nest material kleptoparasitism ultimately may influence the evolution of a species' life history and our understanding of a species' reproductive investment (Mainwaring & Hartley, 2013 and references therein).
Nest material kleptoparasitism is frequently reported in colonialnesting birds that may engage in aggressive competition for nest materials (e.g., Moreno et al. 1995;Siegfried, 1971;Slager et al. 2012 and references therein). However, nest material kleptoparasitism in solitary breeding species (i.e., noncolonial-nesting birds) is infrequently reported in the literature (but see Fulton, 2006;Jones et al. 2007;Lindell, 1996;Slager et al. 2012)-especially for neotropical migratory species. Because of the high avian species richness in the neotropics, Slager et al. (2012) found it surprising that this behavior has not been reported more frequently in this region. This lack of reporting both in the neotropics and with breeding migratory species in North America either may be due to available sampling methodology or it may be true biological phenomena, that is, occurs infrequently (Slager et al. 2012). Specifically, most previous reports of nest material kleptoparasitism (e.g., Jones et al. 2007) are based on opportunistic anecdotal observations at nests during the sensitive nesting period. In such cases, both the avian nest occupants and potential nest material kleptoparasites often will be wary of the presence of a human observer and will modify or interrupt natural behavior. Therefore, instances of stealing nest material may be severely under observed and under reported. Thus, nest material kleptoparasitism incidentally has been reported only for a few neotropical migrants breeding in the USA (e.g., Jones et al. 2007), and we found no records on nest material kleptoparasitism for the Worm-eating Warbler (Helmitheros vermivorum).
This monotypic, rather secretive species is a neotropical migrant that breeds in deciduous and mixed forests of eastern USA and winters in Central America (Vitz et al. 2020). Worm-eating Warblers construct their nests on the forest floor; females use skeletonized leaves and moss (Polytrichum spp.) stems and may line their nests with white-tailed deer (Odocoileus virginianus) hair, pine needles, fine grass, horsehair, or maple (Acer spp.) seed stems (Vitz et al. 2020 and references therein). Previous observations reported a Worm-eating Warbler stole prey from another passerine (Kellner & Cooper, 1998); however, stealing nesting material has not been reported. To our knowledge, we present the first reported (and recorded) incidence of nest material kleptoparasitism by Worm-eating Warblers.

| RE SULTS
We observed three occasions where Worm-eating Warblers vis- female Indigo Bunting returned to feed her nestling 1 min after the warbler left (i.e., 10:37:46); however, no buntings were seen on video during the duration of the warbler's visit.

| D ISCUSS I ON
The evolution of nest material kleptoparasitism may have several adaptive advantages including reduced time and effort in searching for and gathering material for the kleptoparasite (Jones et al. 2007), easy access to suitable material, reduced travel distance, and reduced competition for limited supply of materials (Slager et al. 2012). Thus, by stealing nest material individuals may spend more time defending nests, laying eggs (Jones et al. 2007), constructing nests, caring for young (Mainwaring & Hartley, 2013 and references therein), and foraging. Moreover, individuals may reduce depredation risk by not descending to the ground to collect material from the forest strata or by spending less time in less familiar territory (Jones et al. 2007;Slager et al. 2012). Additionally, depredation and brood parasitism risk may be alleviated by individuals taking shorter flights to pirate material or making fewer visits to collect material (Jones et al. 2007;Ley et al. 1997).
There are risks, however, to stealing material; hosts may defend their potentially valuable nest material and attack kleptoparasites, causing injury or perhaps death, or nest parasites may be transmitted to the kleptoparasite's nest (Jones et al. 2007;Ley et al. 1997).
Yet, in other studies and likely in ours, kleptoparasites returned to the same nests to steal material on multiple occasions, suggesting the benefits of kleptoparasitism, at least in some circumstances, outweigh the costs (Jones et al. 2007;Slager et al. 2012).  (Jones et al. 2007). All these observed kleptoparasitism incidences were interspecific.
Here we document actual and potential nest material kleptoparasitism in another warbler species-the Worm-eating Warbler.
Notably, this species is relatively uncommon in our study area (USFWS 2020). In fact, in our larger avian breeding ecology study, we found 282 active passerine nests in the forests' lower strata, but not one Worm-eating Warbler nest. Thus, these observations suggest nest material kleptoparasitism may be relatively common in Worm-eating Warblers at least in our study areas.
All three of our actual or potential nest material kleptoparasitism events were interspecific as well. Interestingly, our observations suggest the Worm-eating Warblers were not always successful in stealing nesting material. The incubating Hooded Warbler prevented the Worm-eating Warbler from stealing more material, although it was not for a lack of trying. The short duration of the third incident (i.e., 4 s) could indicate an Indigo Bunting parent may have been present nearby off camera, deterring this Worm-eating Warbler from taking nesting material. The female bunting returned to her nest within 1 min of the warbler departing; therefore, it may have been in the vicinity and startled or chased the warbler off camera. As our video did not record audio, we cannot ascertain if any buntings were chipping (i.e., vocalizing warnings) nearby. Further, the bunting video resolution was limited; thus, we could not determine if the warbler already had nest material in its bill. We also cannot rule out that the warbler landed near this nest for another purpose unrelated to nest material kleptoparasitism. for nest material theft likely are limited. Specifically, Worm-eating Warblers use relatively delicate nesting material such as fine grass, mammalian hair, and moss compared to more coarse nesting material such as heavy grasses and small sticks used by Northern Cardinals.
For these reasons, we predict Worm-eating Warblers would unlikely attempt to parasitize materials from a Northern Cardinal nest.
Notably, our sample size was small and limited to individuals inhabiting two forested regions in east-central Arkansas; therefore, we encourage researchers to examine whether this nest material kleptoparasitism behavior exists in other populations throughout the Wormeating Warbler's range. Further, given nest material kleptoparasitism is infrequently reported in the literature, we encourage researchers to investigate this behavior throughout the noncolonial breeding avian community to determine if it is indeed more widespread than presently recognized. With the advent of remote nest camera technology, as used in our study, we suggest more systematic and accurate assessments can be made on the incidence and costs and benefits of nest material kleptoparasitism. Specifically, to study this phenomenon nest cameras might be installed during the nest-building stage and after nestlings are depredated or fledged. Importantly, quantification of behavior and aspects of natural history are necessary for the scientific community and managers to better understand the selective forces and factors influencing life history traits and population viability. Moreover, natural history studies are needed to increase public awareness, general knowledge, and scientific progress (Xiao et al. 2017 and references therein).

ACK N OWLED G M ENTS
We thank C. Foshee for invaluable field assistance and V. Rolland, reviewers, and editors for their helpful and insightful comments.

CO N FLI C T O F I NTE R E S T
None declared.

AUTH O R CO NTR I B UTI O N S
AW and JB conceived the ideas and designed the methodology, JB secured funding, AW collected and extracted data from video recordings and led the writing of the manuscript. AW and JB contributed critically to drafts and gave final approval for publication.

DATA AVA I L A B I L I T Y S TAT E M E N T
Original data and video pertaining to this research are available on the Dryad Digital Repository: