VOLUME 113 AUGUST 2011 NUMBER 3 CONDOR THE CONTENTS RESEARCH PAPERS Using Stable-Isotope Analysis as a Technique for Determining Consumption of Supplementary Foods by Individual Birds VOLUME 113 NUMBER 3 AUGUST 2011 GILLI an N. ROBB , ROBBIE A. MCDO na LD , RI C H A RD IN GER , S. JA MES REY N OLDS , JA SO N NEWTO N , RO na A. R. MCGILL , Dan E. CH A MBERL A I N , TIMOTHY J. E. HA RRISO N , an D STU A RT BE A RHOP . 475 Effects of Postmortem Interval and Preservation Method on RNA Isolated from Field-Preserved Avian Tissues Zac H A RY A. CHE V IRO N , MA TTHEW D. CA RLI N G , an D ROBB T. BRUMFIELD . 483 Actual or Perceived Abundance? Interpreting Annual Survey Data in the Face of Changing Phenologies CHRISTOPHER J. W. MCCLURE , NA TH an D. BURKETT -CA DE na , RUSSELL A. LIGO N , an D GEOFFREY E. HILL . 490 One-Sided Edge Responses in Forest Birds Following Restoration Treatments JA MES BA TTI N an D THOM A S D. SISK . 501 Recent Trends in First-Year Survival for Black Brant Breeding in Southwestern Alaska V JA MES S. SEDI N GER an D CHRISTOPHER A. NI C OL A I . 511 OLUME A New Species of Shearwater (Puffinus) Recorded from Midway Atoll, Northwestern Hawaiian Islands PETER PYLE , AN DRE anna J. WEL C H , an D ROBERT C. FLEIS C HER . 518 113, Egg-Size Variation in the Imperial Cormorant: On the Importance of Individual Effects A LTER va GEL J an D L av IO UI N T ana W S. S F Q . 528 N Vagrant Western Red-shouldered Hawks: Origins, Natal Dispersal Patterns, and Survival UMBER PETER H. BLOOM , J. MI C H A EL SC OTT , JOSEPH M. PA PP, SC OTT E. THOM A S , an D JEFF W. KIDD . 538 Reproduction versus Recruitment of Swainson’s Hawks in the Butte Valley, California 3, CHRISTOPHER W. BRIGGS , MI C H A EL W. COLLOPY , an D BRI an WOODBRIDGE . 547 PP Correlates of Deuterium (δD) Enrichment in the Feathers of Adult American Kestrels of Known Origin 475–708 . JE nn IFER L. GREE N WOOD an D RUSSELL D. DA WSO N . 555 Nesting Density is an Important Factor Affecting Chick Growth and Survival in the Herring Gull MA TTHEW S. Sav O ca , Dav ID N. BO N TER , BE nja MI N ZU C KERBERG , Jan IS L. DI C KI N SO N , an D JULIE C. ELLIS . 565 An Experimental Study of the Use of Social Information by Prospecting Nocturnal Burrow-Nesting Seabirds HE A THER L. Maj OR an D Ian L. JO N ES . 572 The Hybrid Zone between Northern and California Spotted Owls in the Cascade–Sierran Suture Zone GEORGE F. BA RROW C LOUGH , R. J. GUTIÉRRE Z , JEFF G. GROTH , JO na S E. LA I , an D DE nn IS F. RO C K . 581 Effects of Sunlight Exposure on Carotenoid-Based and Structural Coloration of the Blue-tailed Bee-eater ADRI an SURM ac KI , LY nn SIEFFERM an , an D HSI A O -WEI YU an . 590 Passerines Use Nocturnal Flights for Landscape-Scale Movements during Migration Stopover ALEX an DER M. MILLS , BETH an Y G. THURBER , STU A RT A. Mac KE nz IE , an D PHILIP D. TA YLOR . 597 Winter Distribution of Willow Flycatcher Subspecies EBE N H. PA XTO N , PHILIP UN ITT , MA RK K. SOGGE , MA RY WHITFIELD , an D PA UL KEIM . 608 UGUST Male and Female Reproductive Success in a Threatened Polygynous Species: The Strange-tailed Tyrant, Alectrurus risora ADRI an S. DI GI ac OMO , ALE jan DRO G. DI GI ac OMO , an D JU an C. REBORED A . 619 2011 (Continued on inside back cover) A PUBLISHED BY THE COOPER OR N ITHOLOGI ca L SO C IETY Volume 113, Number 3 Issued 8 September 2011 The Condor 113(3):619–628 The Cooper Ornithological Society 2011 male and Female reproductive success in A THreatened PolYGYnous species: THE StranGE-tailed TYrant, ALEctrURUS RISOra ADRI A N S. DI GI A COMO 1, ALEJ A NDRO G. DI GI A COMO 2, A ND JU A N C. REBORED A 1,3 1Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón II Ciudad Universitaria, C1428EGA Buenos Aires, Argentina 2Departamento de Conservación, Aves Argentinas/Asociación Ornitológica del Plata, Matheu 1246, C1249AAB Buenos Aires, Argentina Abstract. The Strange-tailed Tyrant (Alectrurus risora) is an endangered obligate grassland bird that inhabits savannas, wet grasslands and marshes in southern Paraguay and northeastern Argentina. We evaluated the extent of social polygyny, main measures of reproduction (clutch size, hatching success, and chick survival), and fac- tors that influence nest success in this species. We also estimated the reproductive success of females and males by measuring the number and fate of nesting attempts by banded females and the number of females per a male’s territory. More than 80% of the males were polygynous. Males defended contiguous territories of 2–2.5 ha that included the territories of up to four females. Females built the nest, incubated the eggs, and brooded and fed the chicks. On average, successful nests fledged 2.3 chicks. Nest survival over the entire cycle was 0.23 and decreased with nest age and time of breeding. Most females made two or three nesting attempts per breeding season and bred in the same area for 2 or 3 consecutive years. In contrast, males rarely were seen in the same area more than 1 year, suggesting sexual differences in mortality. As a result of this, the reproductive succcess of females and males was similar. Our findings indicate that although males are highly polygynous and nest success is low, the high turnover of males in successive breeding seasons and the high probability of females’ renesting within and in successive breeding seasons reduce the variance in reproductive success of both sexes. Key words: Alectrurus risora, MARK, mating systems, nest survival, reproductive success, Strange-tailed Tyrant. Éxito Reproductivo de Machos y Hembras en una Especie Poligínica Amenazada: Alectrurus risora Resumen. Alectrurus risora es un ave amenazada de pastizal que habita sabanas, pastizales húmedos y baña- dos en el sur de Paraguay y noreste de Argentina. Evaluamos el grado de poliginia social, los principales paráme- tros reproductivos (tamaño de puesta, éxito de eclosión y supervivencia de pichones) y los factores que influencian el éxito de anidación en esta especie. También estimamos el éxito de anidación de hembras y machos midiendo el número y destino de los intentos de anidación de hembras marcadas y del número de hembras en los territorios de los machos. Más del 80% de los machos fueron poligínicos. Los machos defendieron territorios contiguos de 2–2.5 ha que incluyeron los territorios de hasta cuatro hembras. Las hembras construyeron el nido, incubaron los huevos y empollaron y alimentaron a los pichones. En promedio, los nidos exitosos produjeron 2.3 volantones. La supervi- vencia de los nidos a lo largo del ciclo de anidación fue de 0.23 y disminuyó con la edad del nido y la fecha de la temporada reproductiva. La mayoría de las hembras tuvo 2–3 intentos de anidación por temporada reproductiva y anidó en la misma área 2–3 años consecutivos. Por el contrario, los machos raramente fueron vistos en la misma área más de un año, lo que sugiere diferencias sexuales en la mortalidad. Como resultado de esto, el éxito repro- ductivo durante el período de estudio fue similar para hembras y machos. Nuestos resultados indican que si bien en esta especie los machos son altamente poligínicos y las hembras tienen un bajo éxito de anidación, el alto recambio de machos entre temporadas reproductivas y la alta probabilidad de anidación repetida de hembras dentro y entre temporadas reproductivas reducen la varianza en el éxito reproductivo de machos y hembras. from some of the species of the North Temperate Zone and Introduction tropics; information on representatives of the subtropics or The tyrant flycatchers (Tyrannidae) are endemic to the New South Temperate Zone is scarcer. World, reach their highest diversity in the Neotropical Re- Within Tyrannidae the subfamily Fluvicolinae includes, gion, and constitute one of the world’s most diverse bird fami- among others, the genera Fluvicola, Alectrurus, Knipolegus, lies, with 429 species and 104 genera recognized (Fitzpatrick Hymenops, Muscisaxicola, Xolmis, and Agriornis (Ohlson et 2004). The breeding biology of this family is known mainly al. 2008, Tello et al. 2009). These flycatchers evolved in open Manuscript received 2 April 2010; accepted 13 December 2010. 3E-mail: [email protected] The Condor, Vol. 113, Number 3, pages 619–628. ISSN 0010-5422, electronic ISSN 1938-5422. 2011 by The Cooper Ornithological Society. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/ reprintInfo.asp. DOI: 10.1525/cond.2011.100067 619 620 Adrian S. DI Giacomo ET A L . habitats and occupy grasslands, steppes, and wetlands along extinction (Birdlife International 2008). For example, polyg- the Andes, in Patagonia, and on the Pampas at the west and yny is expected to produce variance in male reproductive suc- south of South America (Ohlson et al. 2008). They share some cess higher than that of alternative mating systems (Stiver et characteristics such as large bodies, sexual dimorphism, com- al. 2008), and in populations of constant size an increase in plex aerial displays and foraging techniques (Traylor and Fitz- the variance in reproductive success of either sex will reduce patrick 1982, Fitzpatrick 1985, 2004).