Maria E. Pereyra

Interspecific variation in photo-induced GnRH plasticity among nomadic cardueline finches.

Changes in response to photoperiod are of fundamental importance to seasonal cycles in avian reproductive development. In this study we present data on photo-induced changes in gonadotropin-releasing hormone (GnRH) immunoreactivity associated with the development of photorefractoriness in males of 3 closely related species of cardueline finch: common redpoll (Carduelis flammea), pine siskin (C. pinus) and white-winged crossbill (Loxia leucoptera). All are nomadic, mid- to high-latitude breeders with varying levels of reproductive flexibility: redpolls are typically seasonal, pine siskins more flexibly seasonal and white-winged crossbills temporally opportunistic. Males were exposed to either long (20L:4D) or short days (5L:19D), beginning in January, and GnRH was assessed via immunocytochemistry 4.5 months later. Plasma LH, testis size, and molt score were also measured. Significant reductions in the number of irGnRH cells detected, cross-sectional cell area and optical density of these cells occurred in both redpolls and siskins on long, as compared with short, days. These decreases in irGnRH, coupled with spontaneous testicular regression and onset of molt were consistent with the development of absolute photorefractoriness on constant long days. In contrast, the tendency for numbers of GnRH immunopositive cells and mean measures of GnRH immunoreactivity (cell area, optical density and axonal fiber density) to decrease were much more moderate in white-winged crossbills held on long versus short days. Although none of the four measures of GnRH immunoreactivity differed significantly between treatments in crossbills, significant shifts in the frequency distribution of irGnRH neurons to include greater numbers of less immunoreactive cells suggested that antigenic GnRH content was reduced in many cells. These photo-induced shifts in optical densities of GnRH cells in long-day, as compared with short-day crossbills followed, to a lesser extent, the pattern in pine siskins and common redpolls. For all three species, patterns of decrease in GnRH immunoreactivity, gonadal regression and molt suggest that some form of refractoriness to photostimulation develops on constant long days. However, more moderate tendencies for GnRH immunoreactivity to decrease in white-winged crossbills, coupled with limited gonadal regression and molt on long days suggest that absolute photorefractoriness might develop more slowly in this taxon than in others, or alternatively, a condition arises that is more similar to relative photorefractoriness. These data indicate that relatively small differences in the response of the neuroendocrine system to stimulatory cues such as day length may lead to remarkably different capabilities for reproductive flexibility even within a closely related family of birds.

REPRODUCTIVE EFFORT AND RETURN RATES IN THE MOUNTAIN WHITE-CROWNED SPARROW

Abstract We analyzed return rates of high-altitude-breeding Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) in relation to five components of their previous seasons reproductive effort: number of fledglings produced, double brooding, number of nesting attempts (first nests plus renests), total number of eggs laid, and fledging date. No relationship of return rate to reproductive effort occurred except in the case of fledging date. Fledging dates spanned a 2-month period from mid-June to mid-August. Returns of females to the study area held steady no matter when their chicks fledged in the previous breeding season, but male return rates increased significantly when their chicks fledged after 20 July. Coincident with that time frame, they entered molt and often withdrew parental care. We hypothesized that males in this montane environment, where thermoregulatory costs are high, traded off reproductive effort (parental care) with survival (return rates). This fits well with models of life-history evolution; however, the corresponding prediction that return rates of females with late-season broods should decrease due to their assumption of greater parental care was not supported. Apparently, the cost of reproduction shifted to the young: late-season nestlings grew more slowly, fledged at a smaller mass, and exhibited a fourfold increase in brood reductions. Their recruitment as breeders in the following season was also greatly reduced. Thus, a cost of reproduction was expressed in two forms, one as changes in survival rates of breeding males, the other as changes in quality of offspring. Esfuerzo Reproductivo y Tasas de Retorno en Zonotrichia leucophrys oriantha Resumen. Analizamos las tasas de retorno en las poblaciones reproductivas de alta montaña de Zonotrichia leucophrys oriantha con relación a cinco componentes de su esfuerzo reproductivo de la estación anterior: número de volantones producidos, eventos de dobles nidadas, número de intentos de nidificación (primer nido más re-nidificaciones), número total de huevos puestos y fecha de emplumamiento. No encontramos una relación entre la tasa de retorno y el esfuerzo reproductivo, excepto en el caso de la fecha de emplumamiento. Las fechas de emplumamiento se extendieron por un período de dos meses, desde mediados de junio hasta mediados de agosto. El regreso de las hembras al área de estudio se mantuvo constante, sin importar cuándo los pichones abandonaron el nido en la estación reproductiva anterior, pero las tasas de retorno de los machos incrementaron significativamente cuando sus pichones dejaron el nido luego del 20 de julio. En coincidencia con este momento, los machos comenzaron la muda y frecuentemente dejaron de cuidar a de los pichones. Hipotetizamos que los machos en este ambiente de montaña, donde los costos de termorregulación son altos, canjearon esfuerzo reproductivo (cuidado parental) por supervivencia (tasas de retorno). Esto se ajusta adecuadamente con los modelos de evolución de historias de vida. Sin embargo, la predicción correspondiente de que las tasas de retorno de las hembras con nidadas tardías deberían disminuir debido a la suposición de que brindarían mayor cuidado parental, no fue respaldada. Aparentemente, el costo reproductivo sería trasladado a las crías: los pichones de finales de la estación crecieron más despacio, dejaron el nido con menor masa corporal y exhibieron un incremento de cuatro órdenes de magnitud en la reducción de la nidada. Su reclutamiento como individuos reproductivos en la siguiente estación también se redujo enormemente. De este modo, el costo reproductivo fue expresado de dos formas, una como cambios en las tasas de supervivencia de los machos reproductivos y la otra como cambios en la calidad de la progenie.

Autumnal Migration Departure Schedules in Mountain White-Crowned Sparrows

During seven autumn seasons, Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) departed on migration from a subalpine meadow in September and October. On average, juveniles migrated 3.2 days earlier than adults and there was no difference in schedules of males and females within these two classes. Data from frequently trapped individuals showed that they departed as soon as they became fat. Peaks in departure dates did occur but only once did one occur just before a storm. Mean departure dates among years varied by 14 days in juveniles and eight days in adults. This interannual variation was related to reproductive schedule; departure of the population being delayed by about one day for every two days of delay in nesting (the latter being caused by latelying snows). Reasons for a decrease in the fledging-migration interlude in juveniles in heavy snow years could have involved an overlapping of postjuvenal molt with premigratory fattening and/or telescoping of molt. Late-born young did not migrate later than those born earlier nor were they leaner at the time of departure. Temporal compensation in adults during heavy snow years was accomplished primarily by overlapping postnuptial molt with parental care and sometimes with premigratory fattening because molt duration did not decrease with season. Premigratory fattening occurred in about nine days in all birds with the average gain per day being 2.3 to 2.6% of initial body mass for the four sex-age groups. Propensity to return to the study area was not related to the body mass at departure in the previous autumn or to its date. It was hypothesized that the attainment of peak fat stores usually served as the proximate stimulus for migration.

Egg laying in Dusky Flycatchers and white-crowned sparrows

Many birds lay one egg per day, often in the early morning. The time interval between eggs is usually close to 24 hrs, although longer and more irregular periodicities have been noted in a number of species (Skutch 1952, Schifferli 1979, Astheimer 1985). Despite the interspecific variation in laying times and intervals, most of the speculation regarding adaptive significance has centered on the hour of laying (Schifferli 1979, Feare et al. 1982, Weatherhead et al. 1991, Watson et al. 1993). The purpose of this note is twofold: first, to present data on the laying patterns observed in two passerine species inhabiting roughly the same environment and second, to stimulate new discussion of some of the factors that might influence laying patterns in general.

EFFECTS OF SNOW-RELATED ENVIRONMENTAL VARIATION ON BREEDING SCHEDULES AND PRODUCTIVITY OF A HIGH-ALTITUDE POPULATION OF DUSKY FLYCATCHERS (EMPIDONAX OBERHOLSERI)

ABSTRACT. Reproductive schedules of Dusky Flycatchers (Empidonax oberholseri) at Tioga Pass, in the central Sierra Nevada of California, varied considerably among years. Dates of earliest laying ranged from 29 May to 1 July and varied with snow depth, melt dates, and other factors associated with snowpack. A small apparent advance in laying date over the course of the study (0.52 days year-1 over 15 years) was associated with extremes in winter snowpack and spring melt that were related to wet—dry cycles in the Sierra Nevada. Spring snowpack acted on reproductive timing through direct and indirect effects of temperature, melt schedule, and phenological changes in habitat availability. The influence of these variables on laying dates was greatest on the first females to lay (most in early June) and declined through mid-July, when breeding terminated. Clutch size and fledgling production varied between years of heavy and light snowpack as a direct consequence of delayed egg laying in years of heavy snow. In years of light snowpack, females that laid eggs by mid-June produced larger clutches, fledged more young per nest, and were more likely to renest if nest failure occurred. Only two females attempted two broods in a single season, and both were among the first to breed. Seasonal declines in sensitivity to local environmental conditions may constrain the ability of Dusky Flycatchers to adjust breeding schedules to match phenological delays produced by changes in winter temperature, precipitation, and snowpack, and may limit range expansion to areas with arid winters and earlier spring melt.

MINING PATTERNS OF THE ASPEN LEAF MINER, PHYLLOCNISTIS POPULIELLA, ON ITS HOST PLANT, POPULUS TREMULOIDES

ABSTRACT. Feeding behavior of aspen leaf miner (Phyllocnistis populiella) larvae on quaking aspen (Populus tremuloides) leaves was examined, by using image-analysis software, to determine the pattern and distance of the larvaes mining trails and measure the surface area of infested leaves and consumed leaf tissue. Mining pathways between hatching and pupation sites were serpentine and generally followed predictable, sequential changes in orientation. This pattern was most obvious in smaller leaves, those with a surface area of about 700 mm2 or less. In leaves larger than 700 mm2, plasticity in mining behavior increased. The serpentine pattern was initiated but became truncated when larvae reached full growth without having to consume all available leaf material. The final phase of the behavioral program was still executed, however, and larvae entered pupation at the usual leaf-edge location. We hypothesize that the serpentine pattern is genetically controlled and that it has been selected for because it promotes efficient consumption of a fixed food supply and ensures a favorable location for pupation, especially when reproductive females oviposit on small leaves.

Development of Incubation Temperature and Behavior in Thrushes Nesting at High Altitude

Abstract Onset of incubation was studied in three Hermit Thrushes (Catharus guttatus) and one American Robin (Turdus migratorius), all with four-egg clutches, at a high altitude site in the Sierra Nevada, California, USA. Behavior of laying females at the nest was measured from continuous recordings of internal egg temperatures of first-laid eggs. Full nocturnal nest attentiveness began immediately with the first egg. Daytime attentiveness increased steadily during laying as foraging time away from the nest decreased. On-off bouts by tending females in daytime increased in frequency and decreased in duration until the last egg was laid. Time on the nest could not be directly equated to occurrence of incubation because eggs were not uniformly warmed to exceed the temperature threshold required for embryonic development (physiological zero). Incubation began, both day and night, after laying of the second egg. It increased steadily thereafter with percentage of daytime devoted to incubation lagging well behind that of nighttime. Nest attentiveness and incubation temperatures reached maxima about the time of clutch completion and were continued during following days.

Song exposure during juvenile dispersal in Mountain White-crowned Sparrows

ABSTRACT The degree to which local song structure reflects the singers population of origin is a long-standing and contentious issue. Young songbirds that settle to breed outside their natal song-dialect area may learn to produce nonnatal dialect by hearing and memorizing these songs during juvenile dispersal. We quantified adult singing rates in a population of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) in the Sierra Nevada in 2 yr, and compared them with the arrival schedule of juveniles not hatched on the study area. Our results show that opportunities for juveniles to learn new (nonnatal) song types become highly limited during the premigratory dispersal phase, because adult singing rates are very low by the time nonlocal juveniles begin to arrive. Thus, if individuals learn nonnatal songs during dispersal, they must do so hearing relatively few repetitions of the unfamiliar dialect.