Brood parasites may use gape size constraints to exploit provisioning rules of smaller hosts: an experimental test of mechanisms of food allocation

Behavioral Ecology doi:10.1093/beheco/arr202 Advance Access publication 20 November 2011 Original Article Brood parasites may use gape size constraints to exploit provisioning rules of smaller hosts: an experimental test of mechanisms of food allocation Karen L. Wiebea and Tore Slagsvoldb Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada and bCenter for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, P.O. Box 1066 Blindern, Oslo NO-0316 , Norway a We investigated whether a mechanism of gape size limitation could increase the competitive ability of a large brood parasite in a brood of smaller host nestlings. The gape size of hatchling birds may limit the size of prey they can swallow and hence parents should bring larger more profitable prey as their nestlings grow. The relatively large gape of a brood parasite in a brood of smaller hosts may 1) increase provisioning rate to the brood, 2) allow the parasite to swallow large prey, or 3) cause parents to start bringing larger prey at an earlier nestling stage. We added 1 large same-aged great tit Parus major nestling to broods of smaller blue tits Cyanistes caeruleus to simulate a naive brood parasite system and filmed them when 2–3 days old. The cue of a single large nestling did not cause parents to increase provisioning rates nor to change the species of prey, but prey items were larger than in control broods. Large prey were ‘‘tested’’ more often than small prey, that is offered and then removed from the gapes of small nestlings, and 17% of the prey that the great tit nestling received had been previously offered to a blue tit. Our results revealed that prey size brought by parents could further increase the competitive ability of brood parasites. Key words: begging, brood parasite, cowbird, prey size, provisioning. [Behav Ecol 23:391–396 (2012)] INTRODUCTION he size of a nestling’s gape imposes a physical limit on the size of prey items it can swallow. For those bird species that deliver indivisible prey items, this constraint could explain a general pattern that parents bring larger items as nestlings grow (Quinn 1990; Slagsvold and Wiebe 2007). This occurs because the greater energy demands made by larger nestlings can be met more efficiently by delivering larger prey, and so parents begin to deliver large items when the nestlings’ gapes are wide enough to swallow them (see review in Slagsvold and Wiebe 2007). However, the clutches of many birds hatch asynchronously that creates a range of ages and sizes in the brood. In this case, parents may face a conflict between providing small prey items for small nestlings and larger more profitable prey for large nestlings. This conflict, the ‘‘feeding constraint’’ or, more precisely, the ‘‘gape size constraint hypothesis’’ can explain the paradox of early nestling mortality because small nestmates are offered large prey but are unable to swallow them (Slagsvold and Wiebe 2007; Wiebe and Slagsvold 2009). Brood parasitism is another situation which may generate a range of nestling sizes in a brood. For example, brownheaded cowbirds Molothrus ater, generalist brood parasites, T Address correspondence to K.L. Wiebe. E-mail: . Received 9 February 2011; revised 2 September 2011; accepted 25 October 2011.
The Author 2011. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: take advantage of a wide variety of hosts, which may be larger or smaller in body size (Lowther 1993). However, most of these hosts are smaller in adult body size than the cowbirds (Dearborn 1998) and cowbird nestlings may have a size advantage when competing for food. Brood parasites have evolved morphological and behavioral traits such as bright gape colors and exaggerated begging (Kilner et al. 1999; Grim 2008) to elicit investment by the foster parents, but in some nonevicting parasite species, it is the relative body size of the parasite nestling versus the host nestlings that often appears to be the strongest determining factor of intrabrood food allocation in mixed broods (e.g., Lichtenstein 2001; Rivers et al. 2010). Not surprisingly, parasite nestlings are stronger competitors for food when they are larger than host nestlings than when they are smaller because parents tend to prefer to feed larger nestlings (Slagsvold 1998). The effect of a nestling’s body size on food allocation has been studied mainly in older broods where nestlings jostle for position, vocalize, and stand and stretch toward the parent to be the first to reach the food (Lichtenstein and Sealy 1998; Budden and Wright 2001; Glassey and Forbes 2003). However, swallowing ability in the first hours and days after hatching when nestlings are quite immobile may determine early growth rates, dominance hierarchies, and ultimately which of the nestlings survive (Slagsvold and Wiebe 2007; Mock et al. 2009), whether in parasitized or natural broods. Indeed, brown-headed cowbird nestlings have relatively larger gapes for their body size at hatching compared with some hosts, for example, the larger host yellow-blackbird Xanthocephalus Behavioral Ecology 392 Table 1 Summary of studies of avian brood parasites that have quantified characteristics of prey items delivered to parasitized broods versus control (host) broods Parasite species Relative size of host SHCO Larger BHCO Similar Rivers et al. (2010) BHCO Martı́n-Gálvez et al. (2005) Hauber and Moskát (2008) Grim and Honza (2001) Grim and Honza (1997) Brooke and Davies (1989) COCU Small, medium, and large Smaller COCU Source Lichtenstein (2001) Glassey and Forbes (2003) Brood sample sizea Nestling age (days) Result of prey testing Gape size reported? Prey type differenceb Prey sizec 9 7–8 Yes — — 22 2–8 No — — 13, 17, 16 3–5 Taken from parasite No loss or gain for parasite — No No — 15 5–10 — No Yes Larger Smaller 12 4–8 — No — Larger COCU Smaller 29 Not given — Yes Yes COCU Smaller 3 Not given — No Yes? Larger then smaller with age Smaller COCU Smaller 23 5–12 — No No — SHCO, shiny cowbird Molothrus bonariensis; BHCO, brown-headed cowbird Molothrus ater; COCU, Common Cuckoo Cuculus canorus. Sample of parasitized nests. Sample of controls (host) nests may have differed slightly. b Whether the parasite received a different diet than host (control) nestlings. c Average size difference of prey provided to a parasite or parasitized nest relative to control (host) nestlings. a xanthocephalus nestlings (Ortega and Cruz 1992), and large gape size of nestlings may increase the success in securing food when competing with siblings (Gil et al. 2008). A large gape may benefit a parasite by several mechanisms. Some have suggested a large and colorful gape is a supernormal stimulus that triggers the host to increase provi (...truncated)