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Breeding Biology of Red-winged Blackbirds in South Florida
John W. Prather and Alexander Cruz

Southeastern Naturalist, Volume 5, Number 3 (2006): 547–554

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2006 SOUTHEASTERN NATURALIST 5(3):547–554 Breeding Biology of Red-winged Blackbirds in South Florida John W. Prather1, ** and Alexander Cruz2,* Abstract - We studied the breeding biology of two Agelaius phoeniceus (Redwinged Blackbird) populations in south Florida. Red-winged Blackbirds in our study bred at low relative abundance in a wide variety of habitats, were rarely socially polygynous, and were highly aggressive toward intruders. The breeding season extended from March through August and coincided with the rainy season. Nesting periods were similar to those reported for other subspecies of Red-winged Blackbirds, but clutch sizes were small, with 2- and 3-egg clutches predominating. Nesting success varied from 28% on Sanibel Island to 63% on the Florida Keys. Both populations accepted a majority of artificial eggs into their nests, although cowbird parasitism has been rarely reported in south Florida. Introduction Agelaius phoeniceus L. (Red-winged Blackbird; hereafter redwing) is a widespread breeding species in North America, with a continuous range from southern Alaska and Canada throughout the continental United States and Mexico (Beletsky 1996, Yasukawa and Searcy 1995). Disjunct populations also exist on the Bahamas and in Central America as far south as Costa Rica (Dickerson 1974, Jaramillo and Burke 1999, Yasukawa and Searcy 1995). A closely related species, A. assimilus Jaramillo and Burke (Red-shouldered Blackbird), which was formerly considered conspecific with the Red-winged Blackbird, occurs in Cuba (Jaramillo and Burke 1999). Over their range, redwings show wide variation of morphological and plumage characteristics, and at least 26 subspecies have been recognized (Yasukawa and Searcy 1995). Additionally, populations vary in their breeding biology, including degree of polygyny, territoriality, and aggressiveness (Beletsky 1996, Dyer et al. 1977, Orians 1973, Prather et al. 1999, Yasukawa and Searcy 1995). In south Florida, two subspecies are recognized, A. p. mearnsi Howell and van Rossem (Florida Red-winged Blackbird) and A. p. floridanus Maynard (Maynard’s Red-winged Blackbird) (Stevenson and Anderson 1994). These subspecies are of interest for several reasons. First, they represent the southernmost breeding populations in eastern North America. Second, most redwing populations breed primarily in Typha Linnaeus (cattail) marshes and/or wet grasslands, with breeding in woody vegetation being uncommon (Yasukawa and Searcy 1995). South Florida redwings, in contrast, breed extensively in mangroves and other woody vegetation in flooded areas (Prather and Cruz 2002, Stowers et al. 1968). Third, while 1Center for Environmental Science and Education, Northern Arizona University, Flagstaff, AR 86011-5694. 2Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334. *Corresponding author - alexander.cruz@colorado.edu. **Deceased. 548 Southeastern Naturalist Vol. 5, No. 3 redwing populations are generally well studied, especially during the breeding season (Beletsky 1996, Yasukawa and Searcy 1995), most of the available information on the breeding biology of the Florida subspecies is anecdotal (e.g., Curnutt 1989, Harrison 1981). Finally, in contrast to other populations, south Florida redwings have not historically been sympatric with Molothrus Boddaert (cowbirds). Breeding Molothrus ater Boddaert (Brown-headed Cowbird) have been recorded in south Florida only since 1985 (Cruz et al. 1998, 1999; Hoffman and Woolfenden 1986), and Molothrus bonariensis Gmelin (Shiny Cowbird) is also currently colonizing south Florida (Cruz et al. 1998, 1999; Post et. al. 1993). We present here information on the breeding biology of two south Florida subspecies, and compare our data to those available from other subspecies of redwings. Methods Breeding biology From 1990–1993, we studied populations of A. p. floridanus nesting in mangroves on the Florida Keys (especially on Key Largo) and on small mangrove islands in Florida Bay (25o00'N, 80o30'W). In 1994 and 1995, we studied populations of A. p. mearnsi nesting on the J.N. “Ding” Darling National Wildlife Refuge and properties owned by the Sanibel-Captiva Conservation Foundation on Sanibel Island (26o25'N, 82o11'W). Site descriptions are provided in Prather and Cruz (1995, 2002). Our arrival time varied from year-to-year, with our earliest arrival dates in early April of 1993 and 1995. In areas that we visited frequently, we observed and recorded behavior of redwings on and around the nest and located nests and checked their contents every 2–3 days until they were no longer active. We documented social polygyny by observing the movements of individual male redwings in relation to nest sites and recording which male redwings defended each nest. During counts, and other visits to breeding areas, it was relatively easy to identify the number of individual male redwings in each habitat patch visited. We could then observe male redwings, both while visiting nests, and from a distance, to see which nests they defended or visited regularly. In addition, it was possible to identify the area within each habitat patch in which the male redwing responded to intrusion or spent time singing. These areas were considered to lie within the male’s territory. A male was considered socially polygynous if it defended more than one nest and/or more than one nest fell within the territory it typically defended. Typically, due to the low numbers of redwings on these sites, it was easy to find every nest in a habitat patch and identify which male was responsible for each nest. Since only one female was responsible for each nest, we defined harem size for males as being equal to the maximum number of concurrently active nests within their territory. We did not record any males that failed to have an active nest in their territory for the entire breeding season. Once nests were no longer active, we recorded the substrate in which the nest was placed, the height of the nest above the ground or water, and an estimate of the percent vegetation cover over the nest. 2006 J.W. Prather and A. Cruz 549 Territory abundance We undertook point counts to determine the abundance of redwings in various habitats on the Florida Keys and Sanibel Island, using 120-m fixedradius (Hutto et al. 1986) counts of 10-minute duration. Each male redwing seen or heard during the count was considered to have a territory within the count radius. As individual males could have been polygynous, we counted the number of singing males and assumed that each singing male represented one territory. Egg rejection We undertook egg-rejection experiments on 20 redwing nests, 10 on Sanibel Island and 10 in the Florida Keys, using artificial eggs that mimicked cowbird eggs in size, ground color (white), and in having dark spotting. Real Brown-headed Cowbird eggs from a study in Colorado averaged 21.1 x 16.3 mm (Ortega and Cruz 1988), while real Shiny Cowbird eggs from a study in the West Indies averaged 20.7 x 16.5 mm (Cruz and Wiley 1989). Eggs of both species are white to off-white and heavily speckled with brown. The artificial eggs used in our experiment were made of hollow plastic and measured 21.1 x 16.1 mm. Each egg was filled with water to mimic the weight of a real egg and painted with acrylic polymer paint to simulate the color and spotting density of real cowbird eggs (Ortega and Cruz 1988). The eggs differed from real cowbird eggs in texture and reflectance, but were considered suitable for obtaining a general indication of how Florida redwings would respond to brood parasitism. In order to mimic patterns of cowbird parasitism as effectively as possible, we added eggs to nests only during the morning hours and only to nests in the egg-laying stage. Responses were scored as “accepted” if the egg model remained in the nest for 5 days, and “rejected” if the model disappeared from the nest during the 5-day period. All eggs were removed from nests if they were still present after day 5. Statistical analyses We used Wilcoxon two-sample tests with normal approximation to compare nest-site characteristics of redwings on the Florida Keys with those on Sanibel Island. We used chi-squared tests to compare between-population clutch size, fledging success, and egg-rejection rates. We used logistic regression to test for effects of nest height and canopy cover on nesting success at each site. Means and standard deviations are provided for all appropriate data. Statistical procedures were carried out using JMP Statistical Software Ver. 4.04 (SAS Institute 1998, Cary, NC), and significance was set at P ≤ 0.05. Results The nesting season in Florida started as early as April 21 on Sanibel Island and April 29 on the Florida Keys, and extended into July. New nests, many of which were second nesting attempts after an initial success, continued through June 23 on the Florida Keys and July 14 on Sanibel Island. The July 14 record appears to be a very late date, with no other new nesting 550 Southeastern Naturalist Vol. 5, No. 3 attempts recorded after July 1 despite a continuous presence in the field until late July in both 1994 and 1995. On Sanibel Island, our point-count data indicate that redwing relative abundance ranged from 6 to 13 territories/ha in marshes dominated by Spartina alterniflora Loisel (cord grass) and Typha spp. Yasukawa and Searcy (1995) report a mean 6.15 territories/ha (based on mean territory size) in other studies (n = 9) in marshes. Relative abundances were lower in mangroves, however, averaging only 4.9 territories/ha on Sanibel Island, and ranging from 1.5 to 3.0 territories/ha in 7 habitat patches on the Florida Keys. Few Florida redwings appeared to be polygynous. Of 22 territorial males in frequently monitored areas on Sanibel Island, only 7 (31%) were confirmed to be polygynous. On the Florida Keys, we confirmed polygyny in only 3 males out of 20 (15%) in frequently monitored areas. Average harem size was 1.41 on Sanibel Island and 1.20 on the Florida Keys. Eighty-five of the 89 nests on the Florida Keys were placed in mangroves. Rhizophora mangle Linnaeus (red mangrove), Laguncularia racemosa Gaertn (white mangrove), and Avicennia germinans Linnaeus (black mangrove) were all used regularly as nest sites. The remaining four nests were located in Conocarpus erecta Linnaeus (buttonwood) and Coccoloba uvifera Linnaeus (sea grape) trees adjacent to mangroves. On Sanibel Island, 18 nests were found in mangroves, 19 in cattails (Typha spp.), and 16 in shrubs of various species, especially Cephalunthus occidentalis Linnaeus (buttonbush). Mean cover was significantly higher (n = 93, z = -6.96, P < 0.001) at redwing nests in the Florida Keys (50.2 ± 18.1%, range 20–90%), than on Sanibel (21.0 ± 12.5%, range 10–60%). Mean nest height was significantly higher (n = 93, z = -4.93, P < 0.001) on the Florida Keys (1.87 ± 0.56, range 1.01–3.14 m) than on Sanibel Island (1.44 ± 0.45 m, range 0.70–3.23 m). Both of these differences are primarily attributable to the large number of nests in cattails on Sanibel Island. Clutches ranged from 2 to 4 eggs, but 4-egg clutches were found only four times (Table 1). Clutch sizes were similar on the Keys (2.81 ± 0.53, n = 52) and Sanibel Island (2.77 ± 0.48, n = 43), and the relative frequency of clutch sizes did not differ significantly between the two sites ( χ2 = 0.723, v = 2, P = 0.693). At both sites, the incubation period averaged 12 days and the brooding period averaged 10 days (Table 1), and were within a range reported for other Red-winged Blackbird subspecies (Martin 1995, Nero 1984). We found almost no instances of cowbird parasitism (2 records from Sanibel, none from the Keys) of redwing nests in Florida despite the presence of both Brown-headed and Shiny Cowbirds at some of our sites (see also Prather and Cruz 2002). The proportion of successful nests was significantly higher (v = 1, χ2 = 0.013, P = 0.910) on the Florida Keys (33 of 52, 63.5%) than on Sanibel (12 of 43, 27.9%). However, the mean number of fledglings per successful nest was similar at the two sites (Florida Keys: 2.67 ± 0.48, Sanibel Island: 2.50 ± 0.52; Table 1). We found no significant within-site relationships between nest success and nest height (Florida Keys: v = 1, χ2 = 0.384, P = 0.535; Sanibel 2006 J.W. Prather and A. Cruz 551 Island: v = 1, χ2 = 0.013, P = 0.910) or vegetation cover (Florida Keys: v = 1, χ2 = 0.011, P = 0.912; Sanibel Island: v = 1, χ2 = 0.096, P = 0.757). Redwings accepted artificial eggs designed to mimic cowbird eggs in 7 of 10 cases in the Florida Keys and 8 of 10 cases on Sanibel Island. Eggrejection rates did not differ between the two Florida redwing populations (v = 1, χ2 = 0.268, P = 0.605), but our statistical power was too low to make strong conclusions regarding this result. Discussion Redwings have been documented nesting in Florida from mid-March through early August (Stevenson and Anderson 1994), and we found active nests mainly from late April through early July. In our study areas, nesting appeared to coincide with the summer rainy season, which normally begins in May (South Florida Water Management District 1993). This was especially true on Sanibel Island, where redwings began breeding only when areas beneath the cattails and shrubs, in which they normally nest, were flooded. Relative abundance of redwings in our study sites were within a range reported in other studies (Yasukawa and Searcy 1995). However, relative abundance in mangroves were on the low end of this range. In addition, the incidence of social polygyny in Florida redwing populations is low as compared to other studies in which over 50% of territorial male redwings were reported to be socially polygynous (Searcy and Yasukawa 1995, Yasukawa and Searcy 1995). In a review of 26 studies, Beletsky (1996) reports harem sizes ranging from 1.6 to 6.2, well above the numbers in our study. These data suggest that territory quality may have been low in our study sites (Orians 1973, Robertson 1973, Weatherhead and Robertson 1977), especially in mangroves. We note that West Indian Agelaius species that nest heavily in mangroves are socially monogamous (Post 1981, Whittingham et al. 1996). Two- and 3-egg clutches are typical of Agelaius blackbirds breeding in tropical climates. The mean clutch sizes from our study areas are similar to Table 1. Aspects of the breeding biology of Maynard’s and Florida subspecies of Red-winged Blackbirds in south Florida. Variable Florida Keys1 Sanibel Island2 2-egg clutches 13 (25.0%) 11 (25.6%) 3-egg clutches 36 (69.2%) 31 (72.1%) 4-egg clutches 3 (5.8%) 1 (2.3%) Total complete clutches 52 43 Mean ± SD clutch size 2.81 ± 0.53 2.77 ± 0.48 Incubation eriod (days) 12.0 ± 1.3 (n = 8) 12.5 ± 0.9 (n = 9) Nestling period (days) 10.5 ± 1.1 (n = 8) 9.5 ± 1.2 (n = 8) Total nesting period (days) 22.5 ± 2.1 (n = 8) 21.0 ± 1.2 (n = 8) Percent successful nests 63.5 27.9 Mean # fledglings / nest 1.69 ± 1.35 (n = 53) 0.70 ± 1.17 (n = 43) Mean # fledglings / successful nest 2.66 ± 0.48 (n = 33) 2.51 ± 0.52 (n = 12) 1Maynard’s Red-winged Blackbird (Agelaius phoeniceus floridanus). 2Florida Red-winged Blackbird (A. p. mearnsi). 552 Southeastern Naturalist Vol. 5, No. 3 those reported for Red-winged Blackbird populations in Costa Rica (2.75 ± 0.55, n = 144; Orians 1973) and for West-Indian Agelaius species, such as A. xanthomus Sclater (Yellow-shouldered Blackbird), in Puerto Rico (3.03 ± 0.65, n = 30; Post 1981), A. icterocephalus Linnaeus (Yellow-hooded Blackbird) in Trinidad (2.41 ± 0.71, N = 189; Cruz et al. 1990), and A. humeralis Vigors (Tawny-shouldered Blackbird) in Cuba (3.5 ± 0.3, N = 5; Whittingham et. al. 1996). Clutch sizes from our study were generally smaller than those found for temperate subspecies of the redwing (mean of 3.28 for 20 studies), where 3- and 4-egg clutches often predominate (Dyer et al. 1977). Even as far south as Louisiana, clutch size averages 3.82 eggs (Brown and Goertz 1978). This pattern suggests that either the quality of nesting habitats in Florida is poor, or that Florida redwings follow a pattern of reduced clutch sizes observed in some passerines in more tropical climates (Cody 1966). Dendroica discolor paludicola Vieillot (Florida Prairie Warbler) and D. petechia gundlachi Linnaeus (Cuban Yellow Warbler) that breed in southern Florida also have smaller clutch sizes than their northern conspecifics (Prather and Cruz 1995). We cannot exclude either hypothesis, but the habitat-quality hypothesis would be better supported if mangrovenesting redwings tended to have smaller clutch sizes. We did not find this to be the case, but it is possible that birds in poor habitat may compensate by having larger territories and low levels of polygyny. We could find no significant correlations between nest success and nest height or vegetation cover over the nest, despite evidence for such patterns in other studies (Brown and Goertz 1978, Goddard and Board 1967). We did not test whether the depth of water under the nest affected predation rates as reported by Searcy and Yakusawa (1995). However, nesting success was much higher for redwings in the Florida Keys, where many nests were found on small keys offshore of the main islands. These keys lack many predators common on the mainline Florida Keys, such as Procyon lotor Linnaeus (raccoon), Rattus rattus. Linnaeus (black rat), Didelphis virginiana Kerr (Virginia opossum), Quiscalus quiscula Linnaeus (Common Grackle), and Coluber constrictor priapus Linnaeus (Southern Black Racer). The breeding distribution of the Columba leucocephala Linnaeus (White-crowned Pigeon), which is highly vulnerable to nest predation, is negatively correlated with the distribution of mammalian predators in the Florida Keys (Strong et al. 1991). Predator abundance may play an important role in determining the breeding success of many species nesting in south Florida. Neither this, nor previous studies, have recorded high rates of cowbird parasitism in Florida redwings (Hoffman and Woolfenden 1986, Prather and Cruz 2002), despite the abundance of this species in south Florida and the regularity with which cowbirds use other populations of redwings as hosts (Ortega and Cruz 1991, Searcy and Yasukawa 1995, Ward et al. 1996, Yasukawa and Searcy 1995). The lack of parasitism did not appear to be related to egg-rejection behavior, as 75% of the artificial eggs that we placed in redwing nests were accepted. We propose three alternative hypotheses: 1) cowbirds are selecting other host species at present, 2) Florida redwings use other behavioral mechanisms, such as aggression responses to deter 2006 J.W. Prather and A. Cruz 553 parasitism (see Prather et al. 1999), or 3) cowbird densities are still so low that parasitism rates are difficult to detect. Testing these alternative hypotheses appears to be warranted. Acknowledgments We thank the National Geographic Society and the US Fish and Wildlife Service for providing funding for this study. We additionally thank the personnel of the National Audubon Society on Plantation Key, and the J.N. Ding Darling National Wildlife Refuge and Sanibel-Captiva Conservation Foundation on Sanibel Island for their assistance during this study. Literature Cited Beletsky, L.D. 1996. The Red-winged Blackbird. The Biology of a Strongly Polygynous Songbird. Academic Press, San Diego, CA. Brown, B.T., and J.W. Goertz. 1978. 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