Elevational differences in migration phenology of Lazuli Buntings do not support selection-based hypotheses for protandry

Ornithology, 2023, 140, 1–11 https://doi.org/10.1093/ornithology/ukad030 Advance access publication 22 June 2023 Research Article Elevational differences in migration phenology of Lazuli Buntings do not support selection-based hypotheses for protandry Kim Savides,1,*, and Clark S. Rushing2, Department of Wildland Resources and the Ecology Center, Utah State University, Logan, Utah, USA University of Georgia, Warnell School of Forestry and Natural Resources, Athens, GA, USA 1 2 ABSTRACT Documenting and understanding sex-specific variation in migratory phenology is important for predicting avian population dynamics. In spring, males often arrive on the breeding grounds before females (protandry), though whether these patterns result from fitness benefits vs. sexspecific constraints on arrival timing remains poorly understood. Sex-specific variation in the timing of fall migration is less well-documented than in spring, in part because documenting fall departures is often limited by cryptic behaviors, lower vocalization rates, and shifting territory boundaries during this time of year. We used 2 years of high-resolution encounter data from radio-frequency identification (RFID)-equipped bird feeders to monitor the daily presence of male and female Lazuli Buntings (Passerina amoena) throughout the breeding season at a high and a low-elevation site in Cache County, Utah, USA. These encounter data were used to estimate daily arrival and departure probabilities and to investigate possible differences in migration timing in relation to sex and elevation. At low elevation, male arrival (n = 15) preceded female arrival (n = 16) by ~1 week, consistent with previous research that has documented protandry in other migratory songbirds. At high elevation, however, no significant differences were found between male (n = 19) and female arrival (n = 6). In fall, we found little difference in departure dates between elevation or sex, or between years. Our observations are most consistent with constraint-based hypotheses explaining protandry, possibly relating to sex-specific constraints operating during the nonbreeding period. We additionally emphasize the need for quantifying uncertainty in phenological estimates and importance of addressing potential differences across demographic groups. Keywords: Bayesian point-process model, elevational gradient, Lazuli Bunting, mark–recapture, migratory phenology, protandry, RFID How to Cite Savides, K., and C. S. Rushing (2023). Elevational differences in migration phenology of Lazuli Buntings do not support selection-based hypotheses for protandry. Ornithology 140:ukad030. LAY SUMMARY • Documenting arrivals and departures of migratory birds to and from the breeding grounds is important for understanding whether and how individuals time these events to coincide with conditions that maximize survival and reproduction. But measuring arrivals and departure dates is challenging because birds may be difficult to detect during these periods. • We used bird feeders that automatically record visits by individual marked birds to model arrival and departure dates of male and female Lazuli Buntings at low and high-elevation breeding sites in northern Utah, USA. • We found evidence that males arrive in spring before females (protandry) at low-elevation breeding sites, but at high-elevation sites the timing of male and female arrivals did not differ. • We found no difference in fall departure timing between the sexes or elevations. • Male birds are typically thought to arrive before females to acquire better breeding locations and engage in more breeding opportunities. But in the similarity between male and female arrival times at high elevations suggest that protandry might be more related to constraints on the timing of female arrival, rather than fitness benefits of early male arrival. Las diferencias de elevación en la fenología de migración de Passerina amoena no respaldan las hipótesis basadas en la selección para la protandria RESUMEN Documentar y comprender la variación específica de cada sexo en la fenología migratoria es importante para predecir las dinámicas de las poblaciones de aves. En primavera, los machos a menudo llegan a los sitios de reproducción antes que las hembras (protandria), aunque aún no se comprende bien si estos patrones son el resultado de beneficios derivados de la aptitud biológica versus limitaciones específicas del sexo en cuanto al momento de llegada. La variación específica de cada sexo en cuanto al momento de la migración de otoño está menos Submission Date: October 31, 2022. Editorial Acceptance Date: May 29, 2023 Copyright © American Ornithological Society 2023. All rights reserved. For permissions, e-mail: . Corresponding author: * 2 Lazuli Bunting migratory phenology K. Savides and C. S. Rushing documentada que para la primavera, en parte porque documentar las partidas de otoño a menudo se ve limitado por comportamientos crípticos, tasas de vocalización más bajas y cambios en los límites del territorio durante esta época del año. Utilizamos 2 años de datos de encuentros de alta resolución en comederos para aves equipados con identificación por radiofrecuencia para monitorear la presencia diaria de individuos machos y hembras de Passerina amoena durante toda la temporada de reproducción en un sitio de elevación alta y otro de elevación baja en el Condado de Cache, Utah, EEUU. Estos datos de encuentros fueron utilizados para estimar las probabilidades de llegada y partida diarias e investigar posibles diferencias en el momento de la migración en relación con el sexo y la elevación. En elevaciones bajas, la llegada de machos (n = 15) precedió a la llegada de las hembras (n = 16) por aproximadamente 1 semana, lo cual es consistente con investigaciones previas que han documentado la protandria en otras aves migratorias cantoras. Sin embargo, en elevaciones altas, no se encontraron diferencias significativas entre la llegada de machos (n = 19) y hembras (n = 6). En otoño, encontramos poca diferencia en las fechas de partida entre la elevación o el sexo, o entre años. Nuestras observaciones son más consistentes con las hipótesis basadas en limitaciones que explican la protandria, posiblemente relacionadas con limitaciones específicas del sexo que operan durante el período no reproductivo. Además, enfatizamos la necesidad de cuantificar la incertidumbre en las estimaciones fenológicas y la importancia de abordar las posibles diferencias entre grupos demográficos. Palabras clave: fenología migratoria, gradiente altitudinal, identificación por radiofrecuencia, marca-recaptura, modelo bayesiano de proceso puntual, Passerina amoena, protandria All organisms must time key biological events, including reproduction, molt, and migration, to coincide with the availability of sufficient resources (Alerstam et al. 2003, Bauer and Hoye 2014). The alignment, or misalignment, of life history events with resource peaks can h (...truncated)