HOODED WARBLER NESTING SUCCESS ADJACENT TO GROUP- SELECTION AND CLEARCUT EDGES IN A SOUTHEASTERN BOTTOMLAND FOREST
Ah.rtruct. During the 1996, 1997, and 199X breeding seasons, WC located and moni- tored Hooded Warbler (Wilsoniu citrino) nests in a bottomland forest and examined the effects of edge proximity, edge type, and nest-site vegetation on nesting success. SW- cessful Hooded Warbler nests were more concealed from below and were located in nest patches with a greater abundance of >O.S-m-tall switchcane (Arundinrrrict giguntc~tr) stems than unsuccessful nests. Daily nest survival rates, clutch size, and number of tledglings per successful nest did not differ between nests near edges of selcction- harvest openings within the bottomland and nests near edges of clearcuts adjacent to the bottomland. Daily survival rate, clutch ske, and number of fledglings per successiul nest did not differ among nests O-SO m, 5 I-100 m, and > 100 m from the nearest edge, and probability of nest survival was not related to proximity to either edge type. How- ever, probability of parasitism by Brown-headed Cowbirds (Molorhrus u/cr) was higher near clearcut edges, and parasitism reduced clutch size and numbers of fledglings per successful nest. The combined effects of edge, especially edge created by the relatively small (c;O.S ha) group-selection openings, on Hooded Warbler nesting success were minimal. However, our study was conducted in a primarily forested landscape, so cow- bird abundance or negative edge effects may have been low relative to agricultural landscapes in the South.
l&to de Anidaci6q de Wiisoniu citrincl en Sitios Adyacentes a Bordes de Claros Formados por Extraccicin de Arboles Seleccionados y por Tala Rasa en Bosques Ribereiios de1 Sureste
Rr.sum~n. Durante las Cpocas reproductivas de 1996, 1997 y 199X, uhicamos y mo- nitoreamos nidos de Wilsonia citrina en un bosque de riberefio y evaluamos 10s el’ectos de la proximidad al borde, el tipo de borde y la vegetaci6n de1 sitio de anidacicin sobre el Cxito reproductive. 1.0s nidos cxitosos estuvicron mris escondidos dcsde abajo y se ubicaron en parches de bosque con una mayor abundancia de tallos de Arundirzurirr giguntccc de r&s dc 0.5 m de alto que 10s nidos no exitosos. Las tnsas de supervivencia diaria de 10s nidos, cl tamafio de la nidada y el nfimcro de polluelos cmplumados por nido exitoso no difirieron entrc nidos ubicados ccrca dc bordes de aperturas de cosecba selectiva dentro de1 valle riherefio y nidos cerca de bordes dc sitios completamente talados adyacentcs al valle. La tasa de supervivencia diaria, cl tamatio de la nidada y el nlimero de polluclos cmplumados por nido exitoso no difiri6 entre nidos ubicados a O--SO m. 5 l-1 00 m y > 100 III de1 borde m6s cercano. v la nrobabilidad de suaervivencia de 10s nidos no esiuvo relacionada con la proximi&d ‘a ningdn tipo dc’borde. Sin embargo, la probabilidad dc parasitismo par Molothrus oter fue mayor cerca de hordes de tala rasa, y cl parasitism0 redujo el tamaiio de la nidada y el nt’miero de polluclos emplumados por nido exitoso. Los efecros combinados de horde sobre el Cxito de ani- dacicin de W. citriucr fucron minimos, especialmente aquellos de 10s bordcs creados pot 10s claros relativamente pequefios (-cO.S ha) formados tras extraer grupos de rirbolcs seleccionados. Sin embargo, nuestro estudio fue realizado en UII paisaje principalmente forestal, dc modo clue la abundancia de M. crter- o 10s efectos de horde negativos puedcn haher sido menores en relaci6n a paisajcs agricolas dcl sur.
Manuscript rcceivcd 33 April 2001 ; accepted I I January 2001. ’ l’rcscnt addrcsh: Department of Forestry, 130x X003, North C’arolina State linivcrsity, Raleigh, NC 7-7695 8003. E-mail: chris-liroorma~~~ncsu.edu INTRODUCTION al of the forest overstory renders the stand tem- porarily unsuitable for canopy-dependent bird Although edges may support increased avian species (Pashley and Barrow 1993). Group se- species richness and abundance (Strelkc and lection is a possible alternative to clearculling as Dickson 19X0), they may also have negative cf- a way to harvest and regenerate commercially fects. Several studies, especially those in agri- valuable trees in southern bottomland forests. cultural landscapes, have documented highet Group selection, in which groups of trees cov- nest predation rates near edges (Gates and Gyscl ering no more than 0.5 ha are cleared, r&dins a 1978, Andren and Angelstam 1988). Yet, other portion of the forest canopy and allows suff- studies failed IO document increased predation cient sunlight for some regeneration of shade- rates nearer to edges, especially in forested land- intolerant. cotnmercially valuable tree species scapes (Yahner and Wright 1985, Ratti and Reese (Kellison and Young 1997, Meadows and Stan- 1988, Hanski et al. 1996). Vegetation complexity turf 1997). The openings created during group at edges may reduce the efficiency of nest pred- selection tnay simulate naturaily occurring, gap- ators (Yahner 198X) and increase concealment phase disturbances (Pashley and Barrow 1993), frotn nest parasites (Burhans 1997). and provide habitat for some early successional Few studies have investigated the effects of birds while retaining most of the overstory for edge on natural bird nests in bottomland forests canopy nesting species (Moorman and Guynn or elsewhere in the soulheastern United States. 2001). However, the many stnall openings crc- Studies of edge effects using artificial nesls have ated during group selection tnaximize the ratio been conducted in the South (Keyser et al. 1998, of edge to opening area and could cause in- Sargent et al. 1998, Saracco and Collazo 1999) creased levels of nest predation and brood par- and throughout the northern hemisphere (Major asitism relative to even-aged forest management and Kendall 1996), but the results of this re- practices such as clearcutting (Thompson 1993). search tnusl be interpreted with caution (Major There is a dearth of information on the effects and Kendall 1996). Lack of parental defense of uneven-aged forest management, like group (King et al. 1999), exclusion of certain predators selection, on Brown-headed Cowbird (Molo- (Haskell 1995, Buler and Hamillon 2000), and rhrus liter) parasilistn rates and predation rates lack of parental and nestling activity (Wilson et of forest-interior bird nests, even though the use al. 1998) arc reasons artificial nests may not ac- of the method is increasing (Thompson 1993). curatcly mimic natural nests. Biases also may Most studies of Ihe effect of edge on avian re- result when extrapolating findings from other t-e- productive success have focused on edges be- gions of the country to heavily forested portions tween different habitat types or between a clear- of the southeastern United States. Inconsistent cut and mature forest. Edges created by finer- results among research projects designed lo in- scale disturbance within discrete habitat units vestigate the effects of edge on avian reproduc- (e.g., natural treefall gaps and selection-harvest tive success may be due to landscape-level or openings) have been studied less. Edges of small regional variations in vegetation structure, prcd- forest openings tnay bc less visible in the land- ator comtnunitics, and bird communities (Rob- scape and less likely to scrvc as visual cues to inson et al. 1993, Donovan et al. I997), war- nest predators and parasites (but see Rich et al. ranting regional and species-specific studies 1994). Therefore, aviati rcproduclivc success (Hoover and Brittinghatn 1993, Woodwnrd et al. tnay be lower along clearcut edges than at the 200 1 ). edges of group-selection openings. Between 2000 and 2030, timber removals in Using natural nests, we investigated the ef- southern bot~omland hardwood forests arc pro- fects of group-sclcction and clearcut edges on jected to increase by IX%, and concurrently, Ihe nesting success of the Hooded Warbler ( Wil- acreage of bottomland forests is expected to dc- so77ia c,itriuu) in a southeastern bottomland for- cline by 9% (USDA Forest Service 198X). This est. The Hooded Warbler was chosen as the fh- increased harvesting will increase amounts of cal species because (1) it breeds at relatively edge in bottomland forests. Clearcutting is the high densities on the study site (Norris 196.1); most proven and popular method used lo rcgcn- (2) it nests in the forest undcrstory, making nests crate bottomland oaks in the South (Clatlcrhuck relatively easy lo locate and monilor; (3) it is an and Meadows 1993). However, cotnplcte remov- area-sensitive, Neotropical migranl but often 308 CHRISTOPHER E. MOORMAN ix- AI
lhragcs or nests in small gaps (Evans Odgen and mately 6 m during the study. All regeneration in Stutchbury 1994, Bisson and Stutchbury 2000). the group-selection openings was natural from Although Hooded Warbler densities have been either seedlings or stump sprouts. Vegetation documented to be higher in selection-harvest composition in the group-selection openings sites than in other areas (Annand and Thompson varied with soil moisture and possibly harvest 1997, Robinson and Robinson 1999), little is disturbance, and canopy height averaged less known of the effects selection harvests have on than 2 m during the study (Moorman and Guynn its reproductive success. Our objectives were to 2001). Only the group-selection openings on the ( I ) determine the effects of group-selection and drier microsites contained significant pine regen- clearcut edges on Hooded Warbler nest survival eration. Vegetation in most of the openings con- and productivity (clutch size, number of fledg- sisted of blackberry (R~thus spp.), grapevines lings per successful nest) and Brown-headed (Vitis spp.), and a diversity of hardwoods, in- Cowbird parasitism of Hooded Warbler nests; cluding sweetgum, red maple (Acer ruhrum), and (2) quantify relationships between Hooded American sycamore (P1atunu.s occidentalis), and Warbler nesting success, edge proximity, and oaks. vegetation structure. NEST SEARCHING AND MONITORING METHODS Hooded Warbler nests were located between I May and 15 July in 1996, 1997, and 199X using STUDY AREA systematic foot searches throughout the site and WC conducted our research within a 362-ha bot- intensive foot searches near singing males. Most tomland forest on the Savannah River Site in nests (78%) were located during the incubation Barnwell County, South Carolina (33”09’N, period. We were unable to determine whether 8 I “40’W). The forest was composed of 70-I OO- liests were first or second attempts because nest- year-old bottomiand hardwoods. Bottomland ing activities of most Hooded Warbler females oaks (Qurrcus spp.) and sweetgum (Liquir/cl/n- were not monitored continuously. Once located, bar .styrm?flutr) dominated the canopy, which the status of each nest was recorded every 2-4 averaged approximately 35 m high (Moorman days. The number of young fledged at successful and Guynn 200 1). Loblolly pine (Pinus taech) nests was assumed to equal the number of nest- occurred more commonly on the drier microsi- lings observed on the last nest visit. tes. The midstory was poorly developed, and the At the end of the breeding season, universal undcrstory was patchy with smnc dense areas of transverse mercator (UTM) coordinates of all dwarf palmetto (SmDal minor) and switchcanc Hooded Warbler nests were determined using a (Arurdirztrriu gigcrntecl). Within the forest, ex- global positioning system (GPS). WC compiled perimental group-selection openings were har- UTM coordinates for each nest into a coverage vested in December 1994. Four sizes (0.06, 0.13, of nest locations using the ARC/INFO geo- graphic information system (GIS; Environmen- 0.26, and 0.5 ha) of cuts were replicated six times on 6.5 ha of the bottomland, with approx- tal Systems Research Institute 1987). We also imately 9% of the 65ha area harvested. Protocol created GIS coverages of group-selection and clcarcut edges using a GPS in the field and the employed to create the 24 openings followed ARC/INFO software package. The coverages harvest practices used in commercial group-se- were overlain and distances to the nearest edge lection cuts, and skidder trails connected all of each edge type were calculated for each nest. openings to two upland logging decks. Eight We grouped all nests into three distance-to-near- clearcuts were harvested in xcas adjacent to the est-edge (two types combined) classes (O-SO m, bottomland stand. The clcarcuts ranged in size 5 I-100 m. and > 100 m from edge). Although crorn 25 to 32 ha and ranged in age fi-om 10 to 25-m distance intervals are preferred, cspecialiy 12 years old at the beginning of the study. The within 100 m of edge (Paton 1994), WC used SO- clearcuts tither had rcgeneratcd naturally (n = m increments to maintain adequate sample sizes 7) or had been row-planted to lohlolly pint (n within groups. =r I). Although vcgctation in ail eight clearcuts was predominantly loblolly pine, hardwoods oc- VEGETATION SAMPLING curred sporadically throughout each stand. Can- We measured vegetation at Hooded Warbler opy height in the clearcuts averaged approxi- nests from early to mici-July in 1996, 1997, and H00DISI~ WARBLER NEST SUCCESS NEAR EDGES 369
1998. We made measurements at the nest sub- for the incubation, nestling, and cntirc nesting strate and in the nest patch, defined as the S-m- periods. A survival-rate estitnate was calculated radius circle centered on the nest substrate (Mar- for each of the incubation and nestling periods tin and Roper 1988, Kilgo et al. 1996). Mea- by raising the daily survival rate for that period surements taken at the nest included the species to the power of the associated number of expo- used as the substrate, nest height, substrate sure days (i.e., 12-day incubation period and S- height, and nest concealment. We calculated an day nestling period; Evans Ogden and Stutch- index of concealment by viewing the nest from bury 1994). Survival rate for the entire nesting a distance of I m above, below, and at nest level period was determined by multiplying incuba- from each of the cardinal directions and esti- tion and nestling period survival rates (Hensler mating percent of the nest obstructed by vege- and Nichols 198 1). tation (O-4: 0 = 0% concealed, 1 = l-25% con- We compared daily survival rates between cealed, 2 = 26-50% concealed, etc.) from each nests 5100 m from group-selection edges and of the six angles. We averaged the concealment nests 2 100 m from clearcut edges using the pro- estimate for the four side measures (Kilgo et al. gram CONTRAST (Sauer and Williatns 1989). 1996). When nests were < 1 m above ground, Five nests were within 100 m of both group- concealment from below was estimated from the selection and clearcut edges and were excluded ground. from comparisons between the two edge types. At the nest patch, we measilred canopy cover, We also used CONTRAST to test for differences stem density of potential nest substrates (<3 m in daily survival rates among nests in the three tail), stem density of dwarf palmetto, stem den- distance-to-nearest-edge (two edge types sity of trees (3 m tall), percent ground cover, pooled) classes, among nests in each of the three and vegetation profile. Canopy cover was esti- years of the study, and between parasitized and mated by reading a spherical densiometer 2 m unparasitized nests. Because daily survival rates from the nest substrate in each of the four car- of nests did not differ (x2? = 0.27, P = 0.87) dinal directions. Potential substrate densities among the three years of the study (mean +- SE: were tallied within five l-m2 quadrats located 0.956 i 0.012, 0.958 Z!I 0.010, and 0.964 t randomly along the four cardinal directions. Po- 0.01 1, respectively), we pooled nests across tential substrates were classified as switchcane years. We used ANOVA to test for differences SO.5 m tall, switchcanc >0.5 m tall, other in clutch size and number of fledglings per suc- woody species 0.3-l m tall, and other woody cessful nest atnong the three distance-to-nearest- species 1-3 m tall (Kilgo et al. 1996). We tallied edge classes. We compared clutch sizes and all live trees and snags 3 m tall within the 5- numbers of fledglings per successful nest be- m radius patch. Percent ground cover of herba- tween parasitized and unparasitized nests, and ceous vegetation was estimated (O-5: 0 = 0% between nests ~~100 m from the nearest group- concealed, 1 = l-20% concealed, 2 = 2 l-40% selection edge and nests 5 100 m from the near- concealed, etc.) in each of the five l-m? quad- est clearcut edge using Student’s f-tests. rats. We measured understory structure in the We tested for correlation between nest-site nest patch by estimating the percent obstruction vegetation variables using Pearson correlation (using the same six categories O-S) for each 0.5 coefficients, and retained only the most easily m interval of a 3-m vegetation profile board lo- measured variable of a correlated pair (u > cated 5 m from the nest substrate in each of the 0.5). Three of the nest-site vegetation vari- cardinal directions (Kilgo et al. 1996). We re- ables (nest substrate height, number of sub- duced the number of profile board variables to strates l-3 m tall other than switchcane, and the percent obstruction of the height interval tncm profile of all 05-m profile board inter- corresponding to the height of the nest and the vals between 0 and 3 m) were not included in average percent obstruction 01’ the six height in- analyses because they were correlated with tervals. other variables. For the remaining nest-site vegetation variables, we used Wilcoxon rank- STATISTICAI, ANALYSES sum tests to analyze vegetation differences be- Using the Mayfield method (Mayfield 1975) as tween successful and unsuccessful nests and modified by Hcnsler and Nichols (198 1), we cal- between parasitized and unparasitized nests culated daily nest survival rates and variances (Kilgo et al. 1996). We used Kruskal-Wallis CHRISTOPHt:R I?. MOORMAN ii’1 AI
not included in the final 11~xk1s. Values reported n Total are means i- SE. For all analyses, statistical sig- nificance was accepted when I-’ 5 0.05 (SAS In-
FII Parasitized stitutc 1996). RESULTS Cl Successful We located 33, 35, and 3 1 Hooded Warbler nests in 1996, 1997, and 1998, respectively. Of the 99 nests, I? were parasitized and 58 Hedged Hood- ed Warblers. Of the 41 failed nests, 4 were aban- doned, 3 wcrc lost because of severe weather, and 34 were depredated. Of the 34 depredated nests, 5 were destroyed or displaced from their substrate, 1 contained eggshell fragments and a
51.IIN) 101-150 IS I -x0 >200 partially eaten acorn, and the remaining 2X were undisturbed but empty. Mayfield survival rates Distance to nearest edge (m) 01‘ all nests located during the study were 0.63 FIGURE 1 Number 01’ total Hoodctf Warbler ncsls, during the incubation period, 0.70 during the number parasitized. and nurnbcr st~~essful within five dislance-to-nearest-ecige intervals in a South Carolina nestling period, and 0.44 during the entire nest- bottomland forest, 19% --I c)%. ing period. For unparasitized nests, the average clutch size was 3.2 t 0.1 and the average num- ber of young fledged per successful nest was 2.9 -i- 0.1. tests to analyze vcgelation differences among Hooded Warbler nests were clumped near the three distance-to-nearest-edge categories. edges of both types with approximately twice as Nonparametric statistics were used because many nests O-SO III from edge than in any of most vegetation variable data were not nor- the other intervals and very few nests >200 m mally distributed according to a Shapiro-Wilk from edge (Fig. 1). Number of tall switchcane test. stems, the most common nest substrate used by We employed multiple logistic regression to Hooded Warblers, was greatest within SO m of determine which factors may be important pre- edge (Table I). dictors of nest outcome (success = 1 or failure = 0) and parasitism (parasitized = 1 or unpar- NEST SIJRVIVAL AND PRODUCTIVlTY asitized = 0). For both nest outcome and para- Concealment rankings from below were higher sitism, we tested the fit of a model containing at successful nests (0.9 t- 0.1) than at unsuc- distance to group-selection edge, distance to cessl‘ul nests (0.7 +- 0. I; % = - 1.96, I-’ = O.(E), clearcut edge, year, and any habitat variable and successful nests ( 10.0 t- 0.X) were surround- found to be significant in the univariate analyses. ed by a greater number of tall (X1.5 m) switch- Many warbler nests wcrc located far (>47S m) cane stems than unsuccessful nests (7.6 i- 0.X; from the group-selection openings but all nests Z = -2.05, I-’ = 0.04). No other vegetation var- were within 475 m of a clearcut cdgc. Thercforc, iablcs differed significantly between successful we fit a piecewise linear model with the rcla- and unsuccessful nests. Daily survival rates, tionship between probability of nest success and clutch sizes, and numhcr of fledglings per suc- distance to a group-selection opening being lin- cessful nest did not differ between nests ~100 ear up to 475 m and beyond 475 having no rc- m from the nearest group-selection edge and lationship (Ncter et al. 10%). Because all but nests 5 100 m from the nearest clcarcut edge one of the parasitized nests (92%) were 5 100 m (Table 2). Daily nest survival rata, clutch sizes from edge (Fig. l), we rcclassificd distance to and number of llcdglings per successful nest did group-selection and clearcut edge as 0 if 5 100 not differ anmng the three distancc-to-nearest- m and I if >I00 m for the parasitism model. edge classes (Table 3). In the multiple logistic Because the interaction hetwcen distance to a regression model, probability of nest success group-selection edge and distance to a clcarcut was not significantly associated with proximity edge was nonsignilicant in both models, it was to groupselection edges (Wald xzr =:= 1.42, P = TABLE I. Mean i SE nest-site and nest-patch vegetation characteristics l’or Hooded Warhlcr nests in three distl\nce-ro-nearesl-edge classes in a South Carolina bottomland t’orest, I%-I YYX.
Distance ftom c&e (m) Vegetation variable o--so (I7 = 4 I) 51-100 (II = 21) > IO0 (II = 37) ,x
Nest height (m) 0.‘) t 0.0 0.0 -‘- 0.0 0.15 Concealment mow (O-4)” 0.‘) i 0. I 0.8 + 0. I 0.8 i 0.1 0.57 Above (O-4) 3.0 i 0.2 3.4 i- 0.2 2.8 t 0.2 0.08 Side (O--4) 1.x “- 0.1 I .x t 0.2 I .7 -i- 0. I 0.77 Canopy cover (R ) 94.0 + 0.0 94.0 i 0.0 Y4.0 + 0.0 0.63 Number cane stems (50.5 in) 4.x -t 0.5 5.7 i- I .o 5.6 i 0.7 0.65 Number cane stems (>O.S m) 10.7 i I .o x.7 i 1.4 7.2 -t 0.7 0.05 Number other substrates 0.3 i 0.1 0.2 +- 0.0 0.7 t 0.2 0.76 Ground cover (O-S)c 1.2 i 0.1 I.1 ir 0.1 1.2 + 0.1 0.57 Number palmetto stems IS.4 i 2.1 16.2 t 3.4 I I .4 +- I .4 0.56 Number trees (~3.0 m) IS.7 i- 2.7 10.0 i 0.4, 10.2 -i- 0.7 0. I 3 Number suags (Z3.0 tn) 0.5 i 0. I 0.6 -t 0.2 0.5 2 0. I 0.76 Profile r\t nest level (0-S)’ 3.5 i 0.2 3.6 2 0.2 3.5 + 0.2 0.73
iI Kruskall-Wrrllis test. h III&X of the percent of the nest concealed when vicwcd from 1 m: 0 = 08, 1 := l-2596, 2 = 26-S()9$, 3 = Sl-7SR, 4 = 76100%. ’ h&x of percent obstruction: (I :- 096, 1 = l-20%,, 2 = 2l-4()%, 3 =- 4l-ho%, 4 = (,1-X()%, S = X1-1()()%.
0.40), proximity to clearcut edges (Wald x’, = hctween parasitized and unparasitized nests (Ta- 0.66, P = 0.42), or number of tall switchcane ble 4). Clutch size of parasitized nests was lower stems (Wald xl, = 2.86, P = 0.09). Probability than clutch size o-l‘ unparasitized nests by 0.8 of nest success increased as nest concealment eggs, and parasitism reduced the number of from below increased (Wald x’, = 3.83, P = fedglings per successful nest by 0.9 young (Ta- 0.05). ble 4). Of’ the I20 Hooded Warbler eggs laid in nondepredated, unparasitized nests, 1 I (9%) did PARASITISM not hatch. 01‘ the 1 I Hooded Warbler eggs laid Rankings of ground cover were lower at para- in parasitized nests that were not depredated, sitized nests (0.0 t 0.1) than at unparasitized four (36%) failed to batch. In nondepredated, nests (1 .2 +- 0.0; % = -2.38, P = 0.02). Daily parasitized nests, all cowbird eggs hatched, and survival rates during the incubation period, nes- all warbler eggs that did hatch yielded fledg- tling period, and nesting period did not differ lings. In the multiple logistic regression model,
TABLE 2. Survival rates and productivity oi‘ Hooded Warbler IICSIS within 100 m of group-selection edges and clearcut edges in a South Carolina botloniland hardwood forest, lYY6-I 098.
Daily survival rate Incubation period Ncsrling period Nesting eriod Clutch sim’ P Produclivilyh~’
ii P-values i’or survival rate comparisons arc li-oni lhc pxpa1~1 CONTKASI~, /‘-values l’or other comparisons are from Sludent’s /-tesls. h Parasitized nesls no1 included in analyses. c Number of Ilcdglinps per successi’ul nest. 372 CHRISTOPHER E. MOORMAN CT AI
TABLE 3. Survival ratcs and productivity of Hooded Warbler nests in three distance-to-nearest-edge classes in a South Carolina bottomland forest, 1996-l 998.
Distance to nearest edge (m)
mean i- SE I, mean i SE II meit, t SE II p"
Daily survival rate Incubation periocl 0.971 2 0.012 32 0.957 i- 0.019 I6 0.957 t 0.014 29 0.70 Nestling period 0.957 + 0.014 34 0.949 i 0.022 16 0.958 i 0.015 2x 0.93 Nesting criocl 0.964 t 0.009 41 0.953 i 0.014 21 0.958 i 0.010 37 0.79 Clutch size g 3.2 i 0.1 28 3.3 +- 0.2 13 3.1 2 0.1 27 0.43 Productivityb~” 2.x t 0.2 20 2.9 i- 0.3 II 2.9 i 0.2 20 0.91
a I-‘-values for survival rate comparisons arc li-om the program CONTRAST. P-values for other comparisons are from ANOVA. t’ Parasitiad nests not included in analyses. c Number of fledglings per successf’ul nest.
probability of parasitism was greater nearer to varying distances from the nearest edge of either clearcut edges (Mi’ald x2, = 4.13, P = 0.04) and type. Other studies have documented either low- at nest sites with less ground cover (Wald x2, = er nesting success along external, abrupt edges 4.43, P = 0.04). The proximity of nests to than along edges of group-selection cuts (Suarez group-selection edges was not significantly rc- et al. 1997), or reduced nesting success along lated to parasitism probability (Wald XI, = 0.24, edges of both group-selection cuts and clearcuts P = 0.63). when compared to the adjacent, interior forest (King et al. 1998). Differences among these DISCUSSION studies most likely are the result of variations in NEST SURVIVAL AND PRODUCTIVITY landscape, habitat, and species-level factors WC failed to document any effect of edge on the (Donovan et al. 1997, Woodward et al. 2001). nest survival or productivity of Hooded War- Predation accounted for a large percentage blers. Most Hooded Warbler nests were located (83%) of nest failures, which is common for O-SO m from an edge, where dense patches of species of relatively small body size (Ricklefs tall switchcane were most common. Despite the 1969). Evidence suggests the primary nest pred- apparent crowding of nests near edges, these ar- ators on the bottomland site likely were snakes. cas did not act as ecological traps (Gates and Corvids, such as Blue Jays (C~yorwcitta cristotu) Gysel 1978). Survival rates and productivity es- and American Crows (Corvus hrcrclzyrl~?~nc~ho.s), timates were similar for nests near group-sclec- were present at low densities in the bottomland tion and clearcut edges and for nests located at (Moorman 1999). Gray rat snakes (Eluphe oO-
TABLE 4. Survival rates and productivity of parasitized and tutparasitized Hoocled Wnrhler nests in a South Carolina bottomland ibrcst. 1996-l 998.
Parasitized Unparasitiaed mean 2 SE mean + SE ,1 p;, Daily survival rate Incubation period Nestling period Nesting period Clutch size Productivityh
ii I-'-values f’or survival rate comparisons are from the program CONTRAST. P-values li)r other comparisons arc from Sludcnt’s t-tests. t’ Number of host fledglings per successl‘ul nest. HOODED WARBLER NEST SlJCCESS NEAR EDGES 373
~sol~to .spiloicks) were observed moving along PARASITISM the tops of switchcane stems, apparently in The probability of parasitism of Hooded Warbler search of nests, on two occasions (CEM, pers. nests increased nearer to clearcut edges. The Sa- obs.). Gray rat snakes and black racers (Coluhrr vannah River Site is a forested area of about constrictor) were present at relatively high den- 78 000 ha, but our bottomland study site was sities (Gibbons and Semlitsch 1991), and the within 3.5 km of the Savannah River Site two snake species did not occur more frequently boundary, where agricultural fields and pastures near group-selection edges than in the interior of are common. Brown-headed Cowbirds travel up the bottomland stand (Cromer 1999). Because to 7 km between foraging and roosting or breed- snakes were not more common along edges, nest ing locations (Thompson 1994, Coker and Ca- success would not be expected to vary with edge pen 199S), so our study site was well within the proximity. Uniform distribution of nest preda- range of individual cowbirds foraging outside of tion by snakes previously has been reported the Savannah River Site. Parasitism rates are (Best 1978), but nest predation by mammals, es- higher in heavily fragmented landscapes (Rob- pecially medium-sized mammals, is more likely inson et al. 1995), and landscapes with high to be concentrated at edges (Best 1978, Keyser numbers of cowbird foraging sites contain high- et al. 1998). er densities of cowbirds, which also may lcad to Successful nests were located in denser patch- higher rates of parasitism (Hoover and Britting- es of tall cane (X.5 m) and were more con- ham 1993, Robinson et al. 1991, Stutchbury cealed from below than unsuccessful nests. Oth- 1997). Howcvcr, most of tbe arca within a 7-km er studies of Hooded Warblers have documented radius around our research site was forested, so few or no relationships between nest-site char- cowbird numbers may be low relative to other acteristics and nest success (Howlett and Stutch- more fragmented landscapes in the region, and bury 1996, Kilgo et al. 1996, Bisson and Stutch- our estimates of cowbird parasitism may not be bury 2000). In a sample smaller than used in this representative of agricultural landscapes in the study, Kilgo et al. (1996) detected no effect of South. the number of potential substrates in the nest Relationships between Brown-headed Cow- patch or of nest concealment on Hooded Warble] bird parasitism and edge proximity vary region- nest predation. Howlett and Stutchbury (1996) ally and with landscape context within a region experimentally manipulated nest concealment (Rothstein and Robinson 1994, Hahn and Hat- and recorded no differences in success between field 1995, Donovan et al. 1997). In the North- 15 nests with surrounding vegetation removed east, Hahn and Hatfield (1995) found no effect and 1.5 control nests. However, Howlett and of edge proximity on rates of parasitism, and Stutchbury (1996) did not manipulate the num- they documented higher nest parasitism rates for ber of substrate stems surrounding the nest, birds nesting in the forest interior than for field which may be an eyually important predictor of or edge-nesting species. Donovan et al. (1997) nest success according to our research. In envi- determined that cowbird abundance was highest ronments with many predator species, the high in the most fragmented habitats but did not vary incidence of nest predation and the diversity of between core and edge habitat in highly frag- nest-searching tactics used by those predators mented, moderately fragmented, or unfragmen- may preclude the existence of predictably safe ted landscapes in the midwestern United States. nest sites (Filliater et al. 1994). However, on our However, on our study site cowbirds apparently study site, snakes appeared to be the predomi- concentrated their nest-searching efforts near nant nest predator. Snakes, specifically gray rat edges of harvest openings X.26 ha. During snakes, may be less successful in locating a 1997, Brown-headed Cowbirds were detected bird’s nest in structurally complex habitats (Mu- more frequently in and adjacent to 0.5ha gaps llin et al. 1998). By placing nests in switchcane, than in unbarvested areas, 0.06-ha gaps, 0.13-ha a common substrate, and particularly in a gaps, or 0.26-ha gaps (Moorman and Guynn switchcane stem closely surrounded by high 2001). The low rates of parasitism (2.7%) for all densities of switchcane, Hooded Warblers may nests > 100 m from edge suggest the effects of have reduced the likelihood of nest predation edge did not extend beyond 100 m on our study (Martin and Roper 1988). site. 373 CHIIIS’l‘OPt~l’II t5. MOOKMAN I:,’ A,
We located 60 nests built by species other group selection creates more edge per unit area than the Hooded Warbler and none were para- of forest harvested (Franklin and Forman 1987). sitized (Moorman 1999). Hooded Warblers may and negative edge effects in selection-harvest have hecn parasitized prcfcrentially hccausc slands may be equal to or greater than in edges they bred at high densities on the site or because adjacent to clcarcuts (Thompson 1993, King ct they built relatively conspicuous nests. Most al. 1998). However, we failed to document rc- vegetation characteristics of parasitized and un- duccd nest survival or productivity at the edges parasitized Hooded Warbler nest sites, however, of clearcuts or group cuts. The proportion of were similar on our study site with the exception Hooded Warbler nests depredated during this that patches around unparasitized nests had study was similar to rates reported in other re- greater ground cover. Because female cowbirds gions (Evans Ogden and Stutchbury l994), and often search for nests from the ground (1,owther slightly lower than documented by Sargent et al. 1993), less ground cover potentially made the (I 996) in a study conducted on the Savannah nests more visible. River Site in bottomland hardwood forests. Ag- Contrary to other studies (Martin 1992), par- ricultural landscapes of the soulheastern linitcd asitism by cowbirds did not reduce the survival States likely contain a more diverse guild of nest rate of Hooded Warbler nests. Nest predators on predators than was present on our study site, the study site may no1 have cued on the presence which IJJZIY result in edge effects different than of cowbird nestlings. Only one Hooded Warbler those documented in this study. Management i’or pair deserted a parasitized nest and that was late dense patcbcs of switchcane along silvicultural in the nestling period and in a nest in which no edges may help prevent high predation rates 01 Hooded Warbler eggs hatched. The presence of Hooded Warbler nests in fragmented landscapes, warbler nestlings in parasitized nests may have while concurrently benefiting other species, such prevented abandonment by the adults. Clutches as Swainson’s Warbler (I,irnrwth/ypis .swtrimo- of Hooded Warbler eggs were smaller in para- nii), that nest or forage in canebrakes. Although sitized nests because female Brown-headed parasitism rates were much lower than reported Cowbirds removed eggs (Ortega 1998). Addi- for Hooded Warblers in other regions (Evans tionally, evidence of a single incident of egg re- Ogden and Stutchbury 1994), I I of the 13 par- moval by a cowbird nestling was observed asitized nests were < 100 m from edges of open- (CEM, pers. ohs.). Although Brown-headed ings 20.26 ha (Moorman 1999). Because Hood- Cowbird parasitism reduced the productivity of ed Warblers are a common host species 01 individual warbler nests, all but one nondcpre- Brown-headed Cowbirds, they should receive dated parasitized nest fledged Hooded Warbler special concern in arcas with high cowbird den- chicks. Even though nests located nearer to sities. clearcut edges were more likely to be parasit- Full assessment of management implications ized, the productivity of thcsc nests was not de- would require evaluating the impacts of group- pressed because overall parasitism rates were selection and clearcut harvesting on the rcpro- relatively low. Therefore, the overall effects ol duction, survival, and abundance of Hooded Brown-headed Cowbird parasitism on Hooded Warblers as well as other species in the brecd- Warbler nesting success were minimal relative ing bird community. Hooded Warbler abun- to other regions within the bird’s range (Evans dance in and around the group-selection open- Ogden and Stutchbury 1994). ings was similar to abundance in tbc uncut por- tions of the bottomland stand (Moorman and MANAGEMENT IMI’l,ICAT1ONS Guynn 2001 ), and warbler abundance along Conservation of forcst-interior species, such as clcarcut edges was relatively high, as indicated the Hooded Warbler, partially depends on iden- by the number of nests placed within SO m 01 tifying and preserving the habitat features that edge. However, prc- and post-harvest density affect breeding productivity (Martin 1992). estimates may be necessary to accurately de- Clearcutting may reduce the success of nests in terminc the effects of cutting OII the Hooded the adjacent forest interior (Rudnicky and Hunt- Warbler population. Because edge effects may er 1993, King ct al. 1996, Flaspohler et al. vary with species, habitat, Inndscapc: context, 2001), so selection harvesting has been cited as and region, further research in the southeastern a possible managcmcnt alternative. However, IJnitcd Statcs is warranted. ACKNOWLEDGMENTS IiVANS ()(ilxiN. I>. J.. ANI) H. J. S,I.~J,I.~.III~~~I~Y. 1094. Hooded Warbler ( Wihnicr c~/u’MI). In A. Poole Funciing for the project ~2~s provided by the I>epart- and F Gill II:I)s.], The birds of North America, mcnt of Energy and the USI1A Forest Service-Savan- No. I IO. The Academy of Natural Sciences. Phil- nab River through the USI)A Forest Service Southern adelphia, PA, and The American Ornithologists H;u-dwoods I,ah under Cooperative Agreement SRS Union, Washington. I>C. .70-CA-96-01X. Experimental manipulations used in ~ll.l.lAXli, T S.. R. I31wrwrsc‘~r, AND I? M. NEAIXN, our study were conducted with support from USDA 1994. Predation on Northern Cardinal nests: does CSREEs Nationnl Research initia&ve Competitive choice of nest site matter’? Condor 96:76 I-768. Grant 93-37101-8662. S. A. Gauthreaux. I? 13. Hamel. FI.ASIQIII.IX, 11. J., S. A. TI:MPIX, ANI) R. N. ROSEN- J. n. Lanham, and L. E. Nix ol‘fcrcd constructive con- f:llil.l~. 2001 Species-specific edge effects on nest men& on earlier versions 01’ the manuscript, and I>. S. success and hrceding bird density in ik forested D&kin, I? A. Porneluzi itnd one anonymous rcvicwcr landscape. Ecological Applications I I :32-46. helped with further improvements. M. I,. Gumpertz as- FIGXNKLIN, J. E. AND R. T. FOKMAN. 1987. Creating sisted with statistical analyses, and W. M. Ford, W. P. landscape patterns by lo,,~~~Giig: ecological conse- Smith, and J. I. Blake helped during the gcncsis of tile quences and principles. I,andscape Ecologp I :S- project. We thank Clemson linivcrsity for travel sup- IX. port and the personnel at the IJSI>A Forest Scrvice- GA.I.I~s. J. 15, ANI) L. W. GYSI~I,. 1978. Avian nest dis- $avtinn& River for spontaneous onsite assistance. Ii. persion and lledging success in field-lhrest cco- Katz, I). Moss, S. Welch, C. Collins, M. MacKinnon, lOllCS. Ecology 59:X7 l-88.3. and J. Sircely provided invaluable assistance in the <;IBBONS, J. W., AND R. I>. St:Mi.rI‘s(~if. 19%) I Guide lo field. the rcptilcs and amphihinns 01‘ the Savannah River Site. University 01‘ Gcorgin Press, Athens, GA. LITERATURE CITED IHAI~N, I>. C., AND J. S. H.~~I:II:I.D. 1005. Parasitism ;II ANI)KIIN, H., ANI) I? A!GEI.S’I‘AM. 1988. Elevated prc- the landscape SC&: cowbirds prefer forests. Con- &lion rates as an edge dfcct in habitat islands: servation Biology 9: 1415-1424. experimental evidence. Ecology 69:544-547. HASKEI.I., D. 19%. Forest fl-agni~nlation and nest pre- ANNAND, E. M., AND F;. Ii. TIIOMPSON. 1997. Forest hid dation: arc experiments with Japanese Quail eggs response to regeneration practices in central hard- misleading? 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