Flock Formation and the Role of Plumage Colouration in Ardeidae
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683 Flock formation and the role of plumage colouration in Ardeidae M. Clay Green and Paul L. Leberg Abstract: It has been hypothesized that white plumage facilitates flock formation in Ardeidae. We conducted four ex- periments using decoys to test factors involved in attracting wading birds to a specific pond. The first three experi- ments tested the effects of plumage colouration, flock size, and species-specific decoys on flock formation. The fourth experiment examined intraspecific differences in flock choice between the two colour morphs of the reddish egret, Egretta rufescens (Gmelin, 1789). Wading birds landed at flocks of decoys more often than single or no decoys (P < 0.001) but exhibited no overall attraction to white plumage (P > 0.05). White-plumaged species were attracted to white decoys (P < 0.001) and dark-plumaged species were attracted to dark decoys (P < 0.001). Snowy egrets (E. thula (Molina, 1782)), great egrets (Ardea alba L., 1758), and little blue herons (E. caerulea (L., 1758)) landed more often at ponds that contained decoys resembling conspecifics. At the intraspecific level, all observed reddish egrets selected flocks with like-plumaged decoys. Our results suggest that plumage colouration is an attractant for species with similar plumage, but white plumage is not an attractant for all wading bird species. White plumage may facilitate flock forma- tion in certain species but does not serve as a universal attractant for wading birds of varying plumage colouration and size. Résumé : On a avancé l’hypothèse qui veut que le plumage blanc des ardéidés facilite la formation des volées. Nous avons fait quatre expériences à l’aide de leurres afin de déterminer les facteurs impliqués dans l’attirance des échassiers pour un étang particulier. Les trois premières expériences vérifient les effets de la coloration du plumage, de la taille de la volée et des leurres spécifiques à l’espèce sur la formation des volées. La quatrième expérience examine les dif- férences intraspécifiques du choix de la volée chez deux morphes de couleurs différentes de l’aigrette roussâtre, Egretta rufescens (Gmelin, 1789). Les échassiers se posent plus fréquemment près de groupes de leurres que près de leurres isolés et que dans les sites sans leurres (P < 0,001); ils n’ont pas, cependant, d’attirance générale pour le plumage blanc (P > 0,05). Les espèces à plumage blanc sont attirées par les leurres blancs (P < 0,001) et celles à plumage foncé par les leurres foncés (P < 0,001). Les aigrettes neigeuses (E. thula (Molina, 1782)), les grandes aigrettes (Ardea alba L., 1758) et les aigrettes bleues (E. caerulea (L., 1758)) se posent plus souvent dans des étangs qui contiennent des leurres qui leur ressemblent. À l’échelle intraspécifique, toutes les aigrettes roussâtres ont choisi des groupes de leurres qui possèdent leur propre couleur de plumage. Nos résultats indiquent que la coloration du plumage attire les espèces de plumage semblable, mais que le plumage blanc n’attire pas toutes les espèces d’échassiers. Le plumage blanc peut faciliter la formation des volées chez certaines espèces, mais il n’attire pas de façon universelle tous les oi- seaux échassiers qui varient en taille et en couleur de plumage. [Traduit par la Rédaction] Green and Leberg 693 Introduction birds is the occurrence of species with all-white plumage in both sexes. The adaptive value of colouration in animals has been de- bated and studied intensively (Huxley 1938; Hamilton White plumage, although not absent in other bird orders, 1973). Cryptic colouration is common in some species of is prevalent in many species of waterbirds (Tickell 2003). In birds, presumably for concealment — either protection from the order Ciconiiformes, numerous groups of closely related predators or crypsis to prey (Gill 1990). Lack of crypsis in species differ considerably in plumage colouration (dark ver- birds is often associated with bright colouration in adults, sus white) (Sibley and Ahlquist 1990; Sheldon et al. 1995). which is used for mate attraction. In these mating systems, Within the family Ardeidae, six species exhibit two distinct one sex is usually brightly coloured while the opposite sex is colour morphs (dichromatism), where an individual is either dull in colour. One exception to plumage colouration in all white or dark-plumaged. Currently accepted phylogenies suggest that white plumage arose independently several times (Sibley and Ahlquist 1990; Sheldon et al. 1995). Nu- Received 21 October 2004. Accepted 28 April 2005. Published on the NRC Research Press Web site at merous hypotheses for the adaptive significance of white http://cjz.nrc.ca on 28 June 2005. plumage have been proposed, including crypsis to prey, thermoregulation, and sociality (Kushlan 1977; Ellis 1980; 1 M.C. Green and P.L. Leberg. Department of Biology, Mock 1980; Caldwell 1986; Tickell 2003). As part of an in- University of Louisiana at Lafayette, Lafayette, LA 70504, vestigation into the ecological significance of white plumage USA. (Green 2003), this study focused on the influence of white 1Corresponding author (e-mail: [email protected]). plumage on sociality in wading birds. Can. J. Zool. 83: 683–693 (2005) doi: 10.1139/Z05-058 © 2005 NRC Canada 684 Can. J. Zool. Vol. 83, 2005 Ciconiiformes often form large single- or mixed-species were drained annually in July and reflooded in October from aggregations for both foraging and nesting. Foraging aggre- the same water source (Bayou Teche). gations of wading birds may form for several reasons, in- Four sets of decoys and one control (no decoys) consti- cluding increased foraging success and reduced energy tuted the five treatments used in this experiment (Fig. 1). expenditure. Increased foraging success, or “positive inter- Our decoy sets were as follows: one great egret (white plum- ference” (see Mock 1980), refers to the situation where an age), one great blue heron (Ardea herodias L., 1758; dark individual’s foraging efficiency is significantly enhanced by plumage), five great egrets, and five great blue herons. All the presence of another forager. Foraging wading birds in decoys were commercially available, moulded plastic flocks may experience reduced energy expenditure owing to “heron” decoys of either great blue heron type or great egret a reduction in stepping rate (Kushlan 1978a) or the break- type (Cabellas Sporting Goods, Inc.). These decoys were down of territoriality (e.g., less energy expended chasing chosen because they were all similar in size, thereby control- conspecifics; Wiggins 1991). The formation of bird aggrega- ling for any “size” choice made by wading birds. The exper- tions may also occur for protection from predators. Flocking iment was conducted during April–May 2001. behaviour may enhance predator detection through shared Each day, treatments were randomly assigned to five adja- vigilance or by a dilution or confusion effect on the predator cent ponds. In the ponds assigned five decoys, the decoys (Hamilton 1971; Pulliam 1973). In Ardeidae, incidents of were placed 1 m apart. The decoys were set up before sun- predation are scant and anecdotal (Monson 1951; Lowe rise on each observation day. The observation period began 1954; Cottrille and Cottrille 1958), making it difficult to test at local sunrise and ended approximately 2 h later (e.g., whether predation is a major impetus for flocking in 0730 US Central Daylight Time (CDT) to 0930 CDT) or long-legged wading birds (Mock 1980; but see Caldwell when regular arrivals of birds from night roosts diminished. 1986). The observer (M.C.G.) was situated on a levee perpendicular Kushlan (1977) suggested that white plumage functions as to the ponds, approximately 50 m from the closest pond. The a signal for the formation of wading bird aggregations. He observer monitored all ponds and recorded the species and observed that significantly more wading birds were attracted time of arrival for each wading bird that landed in the ponds. to white decoys than to dark decoys, and he argued that Percent cloud cover was also recorded, as observations were white plumage might be a way of attracting other birds to grouped into “sunny” (<50% cloud cover) and “cloudy” foraging aggregations. All species attracted to the flocks (>50% cloud cover) days for analysis. Once the first bird were lumped together for the analysis and there was no dis- had landed at one of the ponds, all birds were flushed from cussion of which species (and hence plumage colours) were the area and the ponds were randomly assigned new treat- attracted to the white or dark flocks. Using snowy egret ments; no birds were present at the beginning of the next ob- (Egretta thula (Molina, 1782)), great egret (Ardea alba L., servation. Only the first bird arriving at a pond was used in 1758), and mixed flock (white and dark-plumaged birds) de- the analysis. Species that had fewer than five individuals coys, several studies (Caldwell 1981; Master 1992) have land at our ponds were dropped from our analyses. shown that snowy egrets are the “attractive force” in drawing We treated each landing by an individual bird as an inde- in several species of herons, regardless of plumage colour- pendent observation. The Crawfish Research Center is lo- ation. However, these studies failed to offer flocks of cated less than 16 km from several large rookeries (e.g., dark-plumaged decoys as an alternative choice to incoming Lake Martin, >15 000 breeding pairs) and roost sites, result- wading birds. ing in high densities of wading birds flying over and landing In this study, we examine the possible influences that at the research center (Martin and Lester 1990; M.C. Green, plumage colouration may have on flock formation in personal observation; J. Huner, personal communication). Ciconiiformes. We test the hypothesis that white plumage is This high number of overflying wading birds presumably more attractive than dark plumage to wading birds of both minimizes the chance of the same bird landing repeatedly at plumage colourations.