Noel F. R. Snyder

Mate desertion in the snail kite

Mate desertion during the breeding cycle was documented at 28 of 36 (78%) snail kite, Rostrhamus sociabilis nests in Florida between 1979 and 1983. Offspring mortality occurred at only one deserted nest, however. Parents that were deserted by their mates continued to care for their young until independence (3–5 additional weeks) and provided snails at a rate similar to that of both parents combined before desertion. Males and females deserted with nearly equal frequency, except in 1982 when more females deserted. No desertion occurred during drought years, whereas desertion occurred at nearly every nest during favourable conditions. The occurrence of mate desertion was generally related to indirect measures of snail abundance: foraging range, snail delivery rates to the young and growth rates. Small broods were deserted more frequently by females than by males and tended to be deserted earlier than large ones. After desertion, deserters had the opportunity to re-mate and nest again since breeding seasons were commonly lengthy, but whether they did so was impossible to determine conclusively in most cases. The deserted bird sometimes incurred increased energetic costs and lost breeding opportunities during periods of monoparental care.

Biology and Conservation of the California Condor

On Easter Sunday of 1987, the last California Condor (Gymnogyps californianus) known to exist in the wild was trapped for captive breeding, joining 26 others of his species at the San Diego and Los Angeles Zoos. A young male adult, he was a bird whose life had been followed closely for a number of years. His movements and interactions with other condors, his molting patterns and changes in coloration with age, as well as his pairing with an old female in late 1985, and his first breeding attempts in 1986 had all been documented in considerable detail. Like every other bird in the remnant population, he was known and had been studied as an individual. With his capture, an era of intensive investigations of condor natural history and ecology had come to a close.

Primary molt of California condors

Primary molt of the California Condor (Gymnogyps californianus) was studied intensively from 1982 through 1985, using repeated flight photographs of the remaining individuals in the wild population as a basis for most analyses. On the average, wild condors replaced 4.4 of the 8 emarginated primaries on each wing each year. The specific primaries molted were generally the ones missed in the previous year and were usually well-distributed among the eight possibilities, with a tendency for low-numbered primaries to molt earlier than high-numbered primaries. Within individuals, molt of one wing was commonly very different from that of the other wing. Primary molt of captive juveniles was similar to that of wild juveniles. The interval from loss to full replacement of individual primary feathers was normally 3/2 to 4 months, with the primaries closest to the leading edge of the wing growing most slowly. Most primaries were shed between 1 February and 1 September. Primaries lost in late fall and early winter were not replaced until the following summer, indicating interrupted molt over the winter. In general, primary molt of the condor differs from that of smaller cathartids in being highly seasonal, highly variable in sequence, highly asymmetric between wings, and in following a roughly 2-year cycle. Molt of the condor shows many similarities to that of the White Stork (Ciconia ciconia) and to that of large accipitrid vultures.

Nest-site biology of the California condor

A study of 72 historical and recent nests of the California Condor (Gymnogyps californianus) has revealed considerable variability in nest-site characteristics. This paper primarily summarizes the data on nest elevations and dimensions, entrance orientations, nest longevity and re-use, vulnerability of sites to natural enemies, and use of sites by other species. Although all known nests have been natural cavities, some have been little more than overhung ledges on cliffs, while others have been deep, dark caves with nest chambers completely concealed from the outside. Two sites have been cavities in giant sequoias (Sequoiadendron giganteum). Contrary to previous assumptions, condors do modify the characteristics of their nest sites significantly and commonly construct substrates of coarse gravel on which to rest their eggs. Many nests have been completely accessible to terrestrial predators, many have been poorly protected from avian predators, and some have had structural flaws leading directly to nesting failure. The use of suboptimal sites has not been clearly related to a scarcity of better quality sites.

WATER LEVELS AFFECT NEST SUCCESS OF THE SNAIL KITE IN FLORIDA: AIC AND THE OMISSION OF RELEVANT CANDIDATE MODELS

Abstract Dreitz et al. (2001) analyzed the factors affecting nest success of the Snail Kite (Rostrhamus sociabilis) in Florida. They concluded that success was unrelated to water levels because Akaikes Information Criterion rated models with water-level terms as poor compared to other models. Their suite of candidate models, however, did not include models with area-specific differences in the way that water levels affect nest success. We believe that such differences should have been included among the a priori models examined, and that their best model is neither ecologically informative nor useful for management. Using the same statistical methods, we reanalyze Dreitz et al.s data on nesting success from the five areas with sufficient years and nests for analysis (comprising 89% of their 1542 nests) and show that, when spatial effects of water levels are included, water levels have an important influence on nest success over the entire range of water levels, not just during low water conditions. Furthermore, Dreitz et al.s definition of nesting attempts excluded nests found during the 10–21 day pre-laying period, when many nests fail. Thus, they overestimated nest success and underestimated the amount of nesting activity under low water conditions. Low water conditions occur relatively frequently throughout much of the kites range, and under these conditions few kites nest and even fewer fledge young. The effects of low water extend well beyond nest success, causing many kites to forgo nesting altogether, shortening the breeding season, and decreasing the opportunity for multiple brooding. Los Niveles de Agua Afectan el Éxito de Nidos de Rostrhamus sociabilis en Florida: Criterio de Información de Akaike y la Omisión de Modelos Potenciales Relevantes Resumen. Dreitz et al. (2001) analizaron los factores que afectan el éxito de anidación de Rostrhamus sociabilis en Florida. Ellos concluyeron que el éxito no está relacionado con los niveles de agua porque según el Criterio de Información de Akaike, los modelos que incluían términos relacionados con el nivel de agua eran de poco valor en comparación con otros modelos. Sin embargo, entre los modelos evaluados no incluyeron aquellos con diferencias específicas de área en la manera en que los niveles de agua afectan el éxito de los nidos. Nosotros creemos que dichas diferencias han debido ser incluidas en los modelos evaluados a priori y que su mejor modelo no es ecológicamente informativo ni útil en términos de manejo. Utilizando los mismos métodos estadísticos, reanalizamos los datos de éxito de anidación de Dreitz et al. de las cinco áreas que tenían suficientes años y nidos para analizar (comprendiendo el 89% de sus 1542 nidos). Demostramos que cuando se incluyen los efectos espaciales de los niveles de agua, éstos tienen una influencia importante en el éxito de los nidos en todo el rango de niveles de agua (no sólo en condiciones de aguas bajas). Más aún, la definición de intentos de anidación empleada por Dreitz et al. excluyó a aquellos nidos encontrados durante el período de 10 a 21 días pre-postura, cuando muchos nidos fracasan. Por lo tanto, ellos sobreestimaron el éxito de los nidos y subestimaron la cantidad de actividad de anidación en condiciones de aguas bajas. Las condiciones de aguas bajas se presentan con relativa frecuencia en gran parte del rango de distribución de R. sociabilis. Bajo esas condiciones, pocos individuos anidan y aún menos crían polluelos exitosamente. Los efectos de aguas bajas se extienden más allá del éxito de los nidos, causando que muchos individuos totalmente renuncien a anidar, acortando la época reproductiva y reduciendo las oportunidades de tener múltiples nidadas.

FAUNAL REMAINS IN CALIFORNIA CONDOR NEST CAVES

Abstract Studies of faunal remains in California Condor (Gymnogyps californianus) nests in the 1980s yielded bones and hair of a variety of small, medium-sized, and large mammals, and a near absence of avian and reptilian materials. A prevalence of small to medium-sized species may reflect ease of penetration of hides of such carrion and a relative abundance of ingestible bone from such species. Remains also included metal, plastic, and glass artifacts, likely mistaken for bone materials by condors. Size distributions of bone materials and percentage artifacts among hard remains suggest an overall absence of severe calcium-supply problems for condors.

Comment on “Terrestrial Scavenging of Marine Mammals: Cross-Ecosystem Contaminant Transfer and Potential Risks to Endangered California Condors (Gymnogyps californianus)”

Ecosystem Contaminant Transfer and Potential Risks to Endangered California Condors (Gymnogyps californianus)” C condor (Gymnogyps californianus) recovery has made substantial progress since the species comprised fewer than 30 individuals in the 1980s. However, substantial obstacles to full recovery persist. Kurle et al., in discussing contaminant issues for coastal California condors, confirmed potentially detrimental exposure of these birds to a variety of pollutants in marine food chains. Yet in targeting DDE for primary consideration, the authors may be focusing on the wrong culprit. An earlier paper by Snyder and Meretsky, evidently overlooked by Kurle et al., provides cautionary data regarding DDE impacts on condor reproduction and encourages both greater insistence on a “smoking gun” for DDE impacts and broader investigation of the unfortunately large number of other contaminants to which condors are exposed. Kurle et al. reported high levels of DDE in blood plasma samples from coastal free-flying California condors feeding on contaminated marine mammals and noted that ongoing reproductive problems in coastal condor populations could trace to this contaminant. However, earlier studies with condors and DDE, largely focused on potential effects on eggshell thickness, did not provide consistent or persuasive support for such a link. Kiff et al. reported a strong negative correlation of DDE with eggshell thickness in samples from just 7 inland nests of the 1960s and 1970s−a population not known to feed on marine mammals from 1930 through final capture in 1987. But Snyder and Meretsky, working with the same population and a considerably larger sample (46 nests from the 1960s, 1970s, and 1980s) were unable to confirm this correlation. More recently, Burnett et al. reported coastal condor eggs with thin, highly permeable shells that apparently resulted in increased failure to hatch, through dehydration. These authors also suggested that DDE was the primary cause of difficulties, but they did not present correlations of DDE levels with shell characteristics or other reproductive parameters, nor did they cite other studies correlating increased egg permeability (as opposed to fragility) with elevated DDE. Burnett et al. noted that the egg permeability problems they identified did not match previously reported problems associated with DDE in other raptors. Thus, the present hatching problems may well have resulted, at least in part, from causes other than DDE contamination. The tendency to suspect DDE when shell abnormalities are found stems from many historical raptor studies. However, such suggestions with respect to condors neglect important evidence from the wild condor population of the 1980s presented by Snyder and Meretsky. This population produced many eggs with high DDE levels in their lipid layers (>100 ppm), yet showed no clear evidence of the reproductive problems often tracing to DDE contamination in other bird species. Fertility, hatchability, nestling survival, and nesting success were all evidently normal in those years and were crucial in the establishment of today’s vigorous captive population. Similarly, data from the 1960s for the same wild population indicated apparently normal nest success similar to that in preDDT years. Some eggs from the 1960s−1980s did exhibit abnormal shell layering, but this condition was not persuasively associated with either high levels of DDE or egg failure. The main stress to the population was very high adult and juvenile mortality, now confirmed as tracing mainly to lead poisoning (except at very high tissue concentrations, DDE contamination does not normally produce adult or juvenile mortality problems in birds, and its primary documented detrimental avian effects have been reproductive). In overlooking these contaminant and reproductive data from the original wild population of condors, Kurle et al. missed evidence that condors may well be among the species that are relatively resistant to DDE effects. Bird species vary widely in their sensitivity to this contaminant. And while we do not assert that DDE is without effects on condors, the failure to confirm any strong DDE influence on condor eggshell thickness and the absence of frequent egg breakage (other than occasional egg predation by common ravens) or any other pervasive reproductive problems in the highly contaminated condor population of the 1980s together suggest at most limited effects. The hatchability problems noted by Burnett et al. were clearly not characteristic of condors of the 1980s, nor is there credible evidence for their existence in the 1960s and 1970s. All these populations and time periods yielded shell samples with similarly high levels of DDE, so involvement of DDE in hatchability problems seems unlikely unless produced by synergisms with other contaminants varying among periods and populations. Given the worrisome egg hatchability problems in recent coastal condors, additional research on effects of various environmental contaminants, singly or in combination, is needed for the species. Tubbs similarly recommended additional studies and noted the possibility that mixtures of contaminants might be responsible for eggshell issues in condors. With lead poisoning mortality now well understood, if unsolved, reproductive problems become an important research area for condors. In addition to broad-based contaminants work, noncontaminant factors might also be considered. For example, Snyder and Meretsky presented preliminary evidence that some eggshell abnormalities in wild condors might be related to nutritional condition of females. Similar results have been reported in other birds. Unfortunately, field studies of effects of contaminants and noncontaminant factors are often difficult to control rigorously, Correspondence/Rebuttal