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Rejection of Cowbird Eggs by Crissal Thrashers

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Rejection of Cowbird Eggs by Crissal Thrashers This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. REJECTION OF COWBIRD EGGS BY CRISSAL THRASHERS DEBORAH M. FINCHl Rocky Mountain Forest and Range Experiment Station, Forestry Sciences Laboratory, Arizona State University, Tempe, Arizona 85281 USA ABSTRACT.-Although the "dwarf" race of the Brown-headed Cowbird (Molothrus ater obscurus) is sympatric with the Crissal Thrasher (Toxostoma dorsale) in the lower Colorado River valley, I observed no parasitism in 15 thrasher nests. To determine whether or not the absence of cowbird eggs was caused by egg rejection, I experimentally parasitized nine thrasher nests. Cowbird eggs were ejected by thrashers at all experimental nests. To test the prediction that ejection or absence of cowbird eggs reduces nesting mortality, I compared thrasher nesting success to that of a sympatric accepter, the Abert's Towhee (Pipilo aberti). Daily nesting success (5) of the Crissal Thrasher was 0.985 during the incubation period, which was significantly higher than clutch 5 of either nonparasitized (5 = 0.955) or parasit­ ized (5 = 0.911) Abert's Towhees. A higher predation rate accounted for the difference (P < 0.05) between clutch 5 of nonparasitized towhees and thrashers, but harassment and ejection of towhee eggs by cowbirds explained the much greater difference (P « 0.001) between clutch 5 of parasitized towhees and thrashers. Ejection behavior by the Crissal Thrasher may either be a preadaptation or an evolved response to repeated cowbird parasitism. Received 31 August 1981, accepted 16 March 1982. AT least 216 avian species have been record­ the "dwarf" race of the Brown-headed Cow­ ed as victims of brood parasitism by the Brown­ bird (M. a. obscurus). The study was conducted headed Cowbird (Molothrus ater) Friedmann on the Colorado River Indian Reservation, et al. 1977). As brood parasitism seriously about 10 km north of Ehrenberg, Arizona. damages the reproductive success of hosts Cowbirds are abundant in this arid region (Mayfield 1977), it would be advantageous for (Anderson and Ohmart 1976), which is domi­ victimized species to evolve defenses against nated by honey mesquite (Prosopis glandulosa), parasitism (Rothstein 1975a). Egg ejection is one but there were thought to be only three poten­ of the most efficient methods of defense; yet, tially suitable hosts: the Black-tailed Gnat­ based on experimental tests of 43 species, only catcher (Polioptila melanura), which is heavily 7 species are known to eject cowbird eggs reg­ parasitized (Kathleen Conine pers. comm.); the ularly (Rothstein 1975a, 1976a,b, 1977). Because Abert's Towhee (Pipilo aberti), which is mod­ many cowbird eggs may be ejected by hosts erately parasitized (Finch 1981); and the Crissal before observers detect them, several cases of Thrasher, for which I had observed no para­ parasitism probably go unnoticed. Species re­ sitism in 15 nests. To determine whether or not ported to be rarely or never parasitized may be the absence of cowbird eggs may be caused by either (1) rejecters or (2) accepters that are sel­ ejection of cowbird eggs by Crissal Thrashers, dom parasitized. Species shown experimental­ I experimented with nine thrasher nests. Be­ ly to be rejecters may be subject naturally to cause instances of acceptance are rarely docu­ cowbird parasitism that ranges from none to mented for ejecter species, I compared thrasher moderately high. nesting success to that of an accepter, the In this paper, I report on the response of the Abert's Towhee, to test the prediction that Crissal Thrasher (Toxostoma dorsale) to eggs of ejection behavior reduces nesting mortality. The honey mesquite habitat of the lower Colorado River contains six other common passerine species (Anderson and Ohm art 1976) 1 Present address: Rocky Mountain Forest and that are not suitable hosts because they (1) usu­ Range Experiment Station, Forest, Range and Wa­ ally reject cowbird eggs [Northern Oriole (Ic­ tershed Laboratory, 222 South 22nd Street, Laramie, terus galbula bullockii), Rothstein (1977)1, or (2) Wyoming 82070 USA. nest before cowbirds begin breeding in May 719 The Auk 99: 719-724. October 1982 720 DEBORAH M. FINCH [Auk, Vo!' 99 [Phainopepla (Phainopepla nitens), Anderson (Johnson 1979). Daily mortality rate (DMR), proba­ and Ohmart (1978)], or (3) build inaccessible bility of survival for one day (s), and probability of domed nests [Cactus Wren (Campylorhynchus survival of broods through incubation and nestling brunneicapillus), and Verdin (Auriparus flavi­ phases of length t days (Sl) were estimated. (Esti­ ceps)], or (4) nest in cavities [Ash-throated Fly­ mates of s are symbolized as 5.) To obtain a large­ sample estimate of the variance (V) of 5, I used John­ catcher (Myiarchus cinerascens) and Lucy's son's (1979) estimator V(S) = (EXP - losses) X lossesl Warbler (Vermivora luciae)). Of the last four EXp3. The square root of this quantity is the esti­ species, only Verdins and Lucy's warblers have mated standard error of s. To test for variation be­ been recorded as occasionally serving as hosts tween daily mortality rates (e.g. incubation and nest­ (Friedmann 1963, Friedmann et a1. 1977) ling phases), I used Johnson's (1979) statistic, herein denoted as 15 2 - 5,1 [V(5,) + V(S2)]', where the sub­ METHODS script specifies each group (note that DMR, - DMR2 = 52 - 5,). If Johnson's statistic is greater than Experimental procedures were similar to those of Za/2' then the null hypothesis that 5, = 52 is rejected. Rothstein (1975a) except that I parasitized nests with real rather than artificial cowbird eggs and did not remove any thrasher eggs because I wanted clutches RESULTS AND DISCUSSION intact for computing nesting success. During the Cowbird eggs were rejected at all nine dwarf cowbird's breeding season (mid-April through thrasher nests (12 rejections), regardless of July), I parasitized 3 nests during the thrasher egg­ laying period, 4 nests during the first 3 days after nesting stage or presence or absence of thrash­ clutch completion, and 3 nests late in the incubation ers at the time of parasitism. On 23 June 1980, stage. One of the these nests was experimentally par­ I observed a Crissal Thrasher ejecting the cow­ asitized twice during the incubation stage, and bird egg I had deposited at 1010 after flushing another nest was parasitized twice during the egg­ the incubating parent (sex unknown). At 1011 laying stage and once during the incubation stage. the thrasher returned, perched on the nest rim, Thus, a total of 12 experiments was conducted at nine and lifted the cowbird egg up in its bill. The nests. Each nest was revisited 30--60 min after para­ egg either slipped or was dropped over the side sitism except for 2 nests that were observed from 100 of the nest. The thrasher immediately retrieved m away to determine the form of rejection. At the the cracked egg and carried it out of sight. At time of experimental parasitism, thrashers were flushed from 5 nests and were absent at 4 nests. Some 1017 it returned to the nest and resumed in­ nests were parasitized more than once to determine cubation. Ejection was also observed at another whether or not parental responses to cowbird eggs nest. Ejection is the most common form of re­ differed for different nesting stages. jection behavior by parasitized species (Roth­ Because captive female cowbirds prefer to lay in stein 1975a). nests that contain host eggs smaller than their own My observations indicate that thrashers eject (King 1979), I measured the length (L) and breadth eggs as soon as they detect them, so I had little (B) of eggs with vernier calipers. One newly laid opportunity to verify natural parasitism. Host­ egg from each of three Crissal Thrasher and 16 Abert's cowbird interactions are apparently of recent Towhee clutches was weighed on an Ohaus scale ac­ origin (Friedmann 1963), however, and the best curate to 0.01 g. Ten fresh cowbird eggs were weighed and treated as independent samples of the popula­ current strategy for the cowbird may be to par­ tion. Eggs that are clutchmates do not represent in­ asitize all nests found, including those of poor dependent samples, so I calculated mean egg weight, hosts (Rothstein 1976b). The Crissal Thrasher length, and breadth for each thrasher and towhee may not represent a favorable host choice, even clutch before estimating population means. Esti­ if it were to accept cowbird eggs, because its mates of egg weight provide an index of weight ad­ own eggs are 135% heavier than cowbird eggs vantages at hatching. Initial weights of eggs were (Table 1); hence, cowbird hatchlings would be estimated from Hoyt's (1979) equation: Weight = Kw· at a competitive disadvantage because of their LB'. The weight coefficient (Kw) is a species-specific smaller size. Another ground-foraging species constant that I calculated from the relationship Kw = WILB2 (Hoyt 1979), where W is equal to measured that shares many features (e.g. distribution, fresh weight. seasonal status, habitat, body coloration, dis­ Nesting success was calculated using Mayfield's persion, and mating system) with the Crissal method based on nest-days of exposure (Mayfield Thrasher is the Abert's Towhee. Although the 1961, 1975). The number of nest-days of exposure Abert's Towhee lays eggs 92% heavier than (EXP) is the period when a known nest is at risk cowbird eggs (Table 1), 22 of 69 towhee nests October 1982] Thrasher Rejection of Cowbird Eggs 721 TABLE 1. Initial egg weights of Crissal Thrashers, Abert's Towhees, and Brown-headed Cowbirds." Initial egg Species Kw (mg)b Egg length (mm) Egg width (mm) weight (g) Crissal Thrasher 0.531 26.53 ± 0.425 19.57 ± 0.138 5.45 ± 0.112 (3:3) (8:21) (8:21) (8:21) Abert's Towhee 0.537 24.65 ± 0.228 18.30 ± 0.113 4.45 ± 0.076 (16:16) (37:98) (37:98) (37:98) Brown-headed Cowbird 0.554 19.27 ± 0.109 14.72 ± 0.214 2.32 ± 0.052 (10:10) (15:15) (15:15) (15:15) a Numbers in parenthes~s are number of clutches:number of eggs.
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