University of Groningen The functional significance of multiple nest-building in the Australian Reed Warbler Acrocephalus australis Berg, Mathew L.; Beintema, Nienke H.; Welbergen, Justin A.; Komdeur, Jan Published in: Ibis DOI: 10.1111/j.1474-919X.2006.00482.x IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2006 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Berg, M. L., Beintema, N. H., Welbergen, J. A., & Komdeur, J. (2006). The functional significance of multiple nest-building in the Australian Reed Warbler Acrocephalus australis. Ibis, 148(3), 395-404. https://doi.org/10.1111/j.1474-919X.2006.00482.x Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 27-09-2021 Ibis (2006), 148, 395–404 BlackwellThe Publishing Ltd functional significance of multiple nest-building in the Australian Reed Warbler Acrocephalus australis MATHEW L. BERG,1,2* NIENKE H. BEINTEMA,1 JUSTIN A. WELBERGEN2 & JAN KOMDEUR1,2 1Animal Ecology Group, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands 2Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia The vast majority of bird species build a nest in which to breed. Some species build more than one nest, but the function of most multiple nest-building remains unclear. Here we describe the unusual nest-building behaviour of the Australian Reed Warbler Acrocephalus australis, and test experimentally the hypotheses that multiple nest-building is related to individual condition or territory quality, and plays a role in mate assessment. Australian Reed Warblers built two types of nest structures: ‘type I’ nests, which were used for eggs and nest- lings, and ‘type II’ nests, which were structurally distinct from type I nests, did not support eggs, nestlings or adults and were not essential for successful breeding. The number of type II nests built in each territory varied. Type II nests were only built before breeding had commenced in a territory and females were not observed participating in their construction, supporting a role in female mate choice. Birds provided with supplementary food built sig- nificantly more type II nests than control birds. However, supplementary-fed birds did not have greater pairing success, and the addition of further type II nests to territories did not increase the pairing rate or type II nest construction in those territories. There was no rela- tionship between the presence of type II nests and either reproductive success or likelihood of nest predation. We discuss the implications of these results in light of previous suggestions regarding the function of multiple nest-building in birds. Nest-building is an essential activity for successful excess energy (Forbush 1929), (2) to practise nest- reproduction in most birds, and many species expend building (Hunter 1900), (3) to outline territory considerable time and energy in the construction of boundaries (Allen 1923), (4) to provide shelter for one or more elaborate nests for breeding (Verner & adults or fledglings (Verner 1965) and (5) to act as Engelsen 1970, Collias & Collias 1984, Metz 1991). decoys to reduce predation (Shufeldt 1926, Leonard There is evidence that both the selection of a nest-site & Picman 1987). However, the most commonly invoked and the quality of a nest can have important effects hypothesis is that of mate attraction (e.g. Verner & on breeding success (Thompson & Furness 1991, Engelsen 1970, Collias & Collias 1984, Evans & Burn Hoi et al. 1994, 1996, Weidinger 2002). Although 1996, Soler et al. 1998, Brouwer & Komdeur 2004). most species typically build only one nest per breed- If this is the case, individuals might signal their ing attempt, some species simultaneously build more current condition or the quality of their territory (e.g. nests or nest-like structures than are used for breed- food availability, nest-site quality or the absence of ing (e.g. Verner & Engelsen 1970, Garson 1980, nest predators) through constructing multiple nests Savalli 1994, Friedl & Klump 1999). To date, there (e.g. Leonard & Picman 1987, Moreno et al. 1994, has been remarkably little empirical work on this Savalli 1994, Evans & Burn 1996, Kusmierski et al. unusual behaviour, but several hypotheses have been 1997, Friedl & Klump 2000). In this way, even non- proposed to explain the adaptive significance of breeding nests may be considered as (non-bodily) multiple nest-building. These include: (1) to reduce ornaments in the same way as other secondary sexual characters (Andersson 1994). Moreover, when both *Corresponding author (present address): School of Biological sexes participate in nest-building it may be a sexually Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK. selected display that allows each sex simultaneously Email: [email protected] to gain reliable information on the condition or quality © 2006 The Authors Journal compilation © 2006 British Ornithologists’ Union 396 M. L. Berg et al. of the other (Soler et al. 1998), and may function in In this study, we describe the unusual nest-building mutual mate assessment or pair bonding. behaviour of the Australian Reed Warbler, which has The Australian Reed Warbler Acrocephalus austra- not been documented in detail previously. As has lis arrives at breeding sites in southern Australia from commonly been proposed in other species (e.g. Verner September to December. Males arrive before females & Engelsen 1970, Evans & Burn 1996), previous and defend small breeding territories throughout the workers have suggested that multiple nests may be breeding season (Cramp 1992, M. Berg, J. Welbergen built by male Australian Reed Warblers for mate & R. Kats unpubl. data). Pair-bonds typically persist attraction purposes (so-called ‘cock nests’, Courtney- throughout the season and the social breeding Haines 1991). We predicted that if type II nests system is primarily monogamous, although polygyny function in mate assessment, these nests will only be and extra-pair fertilizations can occur (Brown & Brown built during the period of pair formation, whereas if 1986, J. Welbergen & M. Berg unpubl. data). Nests they have a different function or are by-products of are constructed in beds of Common Reed Phragmites type I nest-building attempts they should appear australis, where potential nest-sites are extremely throughout the breeding cycle or during re-nesting abundant. The birds frequently build more nest attempts. We also conducted two experiments designed structures than are actually used for breeding (Courtney- to examine further the functional significance of Haines 1991, this study); however, in contrast to multiple nest-building in this species, with particular similar systems well documented previously, there is attention to a potential role of type II nests in mate a clear and consistent structural difference between and/or territory assessment and pair formation the nests that are used for breeding (‘type I nests’, (‘sexual selection’ hypothesis). First, we conducted a Fig. 1a) and the nests that are not used for breeding supplementary-feeding experiment to examine the (‘type II nests’, Fig. 1b). Type I nests are typically role of territorial food availability on nest-building. If supported by three or more closely placed reed stems the construction of multiple nests is energetically and have a distinct cup shape suitable for containing constrained, the number of type II nests built by an a clutch or brood. Type II nests are much smaller, individual or pair may be a signal of condition or rudimentary structures that are always built on only territory quality (Zahavi 1987). Secondly, we per- two reed stems and which, lacking a cup shape, are formed a nest-addition experiment (Leonard & not suitable for egg-laying, or for the shelter of off- Picman 1987) using artificial type II nests (Fig. 1c). spring or adults (see Verner 1965). Several broods With this experiment we could test directly the are usually attempted in a season, with a new type I relationship between type II nest number or density nest being built for each breeding attempt (Brown & and subsequent nest-building or pairing success, Brown 1986, this study). without the confounding effects of associated Figure 1. Type I nest (a) and type II nest (b) of an Australian Reed Warbler, and artificial type II nest (c) used in the nest-addition experiment. © 2006 The Authors Journal compilation © 2006 British Ornithologists’ Union Multiple nest-building in Australian Reed Warblers 397 variation in nest quality, individual quality or terri- (our pers. obs.). Nine type I nests were filmed in the tory quality. construction stage (3.09 h ± 0.02 sd each), and 18 newly discovered type II nests were filmed (02.33 h ± 0.03 sd each). From these videos we recorded all MATERIALS AND METHODS nest-building activity by both sexes. We also marked some nest material in each of five type II nests with Study area and data collection spray paint or an indelible marker to determine The study was conducted on a colour-banded popu- whether this material was later used for the con- lation of Australian Reed Warblers in 10 ha of Com- struction of other nests.