Christopher W. Briggs

Correlates of survival in Swainson's hawks breeding in northern California†

ABSTRACT We used a 30-year study of breeding Swainsons hawks (Buteo swainsoni) in northern California to examine correlates of adult apparent survival using multistate models in Program MARK. Specifically, we examined age-related patterns in adult apparent survival and how adult survival was correlated with average annual nest productivity, annual reproductive output, western juniper (Juniperus occidentalis) density around nest sites, distance to agriculture, and amount of agriculture within a territory. Annual estimates of adult survival varied from 0.85 to 0.9 (SE = 0.02). There were no indications of senescence or other patterns of age-related changes in adult apparent survival. Adult survival was inversely correlated with average reproductive output, with individuals producing >2 offspring having decreased survival, reflecting a possible trade-off between reproduction and survival. Conversely, reproduction in any year was positively correlated with survival, providing evidence of individual quality influencing adult survival. The distance an individual had to travel to agriculture, where most individuals forage, was negatively related to survival. Primary productivity within the average Swainsons hawk territory was positively correlated with adult survival. Our results indicate that individuals may have higher survival and fitness in areas with high proportions of irrigated agriculture that provides high prey densities, particularly alfalfa.

Plumage polymorphism and fitness in Swainson's hawks

We examine the maintenance of a plumage polymorphism, variation in plumages among the same age and sex class within a population, in a population of Swainson’s Hawks. We take advantage of 32 years of data to examine two prevalent hypotheses used to explain the persistence of morphs: apostatic selection and heterozygous advantage. We investigate differences in fitness among three morph classes of a melanistic trait in Swainson’s Hawks: light (7% of the local breeding population), intermediate (57%) and dark (36%). Specifically, we examined morph differences in adult apparent survival, breeding success, annual number of fledglings produced, probability of offspring recruitment into the breeding population and lifetime reproductive success (LRS). If apostatic selection were a factor in maintaining morphs, we would expect that individuals with the least frequent morph would perform best in one or more of these fitness categories. Alternatively, if heterozygous advantage played a role in the maintenance of this polymorphism, we would expect heterozygotes (i.e. intermediate morphs) to have one or more increased rates in these categories. We found no difference in adult apparent survival between morph classes. Similarly, there were no differences in breeding success, nest productivity, LRS or probability of recruitment of offspring between parental morph. We conclude that neither apostatic selection nor heterozygous advantage appear to play a role in maintaining morphs in this population.

Temporal Morph Invariance of Swainson's Hawks

Persistent genetic polymorphisms (e.g., different plumage morphs irrespective of age and sex classes) have long fascinated biologists, are found in approximately 3.5% of avian populations (Roulin 2004), and have prompted many hypotheses about how plumage morphs arise and are maintained (Lank 2002). In birds, many hypotheses have been proposed to explain intrapopulation differences of plumages, such as the handicap principle (Zahavi 1975) and different mating strategies (Lank et al. 1995). Raptors (Falconiformes and Strigiformes) in particular have a high prevalence of persistent genetic polymorphisms occurring in approximately 30% of species (Fowlie and Krüger 2003). In raptors, this prevalence of polymorphism has been related to prey type (Paulson 1973), wing load and population size (Fowlie and Krüger 2003), and heterozygous advantage (Krüger et al. 2001). For buteonine hawks, it has often been assumed that plumage morph is invariant over time (i.e., plumage pattern does not change as an individual ages and molts), and this has been used to identify individuals across years in some species (e.g., Krüger and Lindström 2001). In fact, similar polymorphisms have been associated with variations in the melanocortin-1 receptor (MC1R) gene across a wide range of avian taxa (reviewed in Mundy 2005). These results indicate that morph in many avian species may be invariant over time. For example, in Ruffs (Philomachus pugnax) plumage polymorphisms remain unchanged over the course of an adult male’s life (Lank et al. 1995). However, no studies have examined the persistence of morph within individuals over time in raptors. We examined the assumption that individual morph is invariant over time in a population of Swainson’s Hawks (Buteo

Inheritance Patterns of Plumage Morph in Swainson's Hawks

PATRONES DE HERENCIA DEL MORFOTIPO DE PLUMAJE EN BUTEO SWAINSONI La herencia de los polimorfismos basados en melanina es poco conocida a pesar de ser relativamente comun entre las rapaces. Examinamos la hipotesis de que el plumaje oscuro se hereda mediante un sistema de un locus con dos alelos en Buteo swainsoni. Presumimos que los individuos de plumaje oscuro y de plumaje claro eran homocigotos, mientras que los heterocigotos presentaban alguna forma intermedia de plumaje. Examinamos el morfotipo de 31 adultos de B. swainsoni y los comparamos con los fenotipos parentales. Todos los individuos se ajustaron a nuestra hipotesis de herencia mediante un locus con dos alelos. No existio una diferencia significativa entre lo que se esperaria de acuerdo a ese sistema y lo que nosotros observamos, lo que representa evidencia adicional de que el morfotipo en B. swainsoni esta asociado con un patron de herencia de un locus con dos alelos.

Correlation Between Feather Isotopes and Body Condition for Swainson's Hawks, and Implications for Migration Studies

Abstract The study of individual movement patterns using stable hydrogen isotopes (δ 2H) as a natural marker has grown; however, recent studies have suggested that measurement of δ 2H in feathers (δ 2Hf) may prove unreliable as a means for identifying region of origin of migrating or wintering birds, particularly raptors. In this study, we examine whether differences in body condition could explain some of the variability in δ 2H in feathers. We analyzed growing feathers of 21 Swainsons Hawks breeding in northern CA for δ 2H, nitrogen (δ 15N), and carbon (δ 13C) stable isotopes in relation to body condition. We found that δ 2H was variable (range = 40‰), and that variability was significantly associated with body condition. Raptors derive most or all of their moisture from prey. Therefore, we suggest that individuals in poor condition have an enriched pool of body water relative to individuals in good condition, due to fractionation of body water stores during respiratory water loss and metabolic processes. Body condition was also negatively correlated with δ 15Nf. However, δ 2Hf, δ 15Nf, and δ 13Cf were not correlated, suggesting that the relationship between δ 2Hf and body condition is a result of physiological processes rather than differences in dietary δ 2H. We used an isotopic basemap of δ 2Hf values to assess individual origin as if they were encountered naively on the migration or wintering grounds, and all individuals fell within the 95% confidence interval of our study area. Conversely, the 95% confidence interval of δ 2Hf values obtained encompassed almost the entire breeding range of this species, indicating little ability to differentiate origins of this species.

Reproduction Versus Recruitment of Swainson's Hawks in the Butte Valley, California

Abstract. Measures of reproductive success have long been assumed to be reasonable surrogates of fitness. We examined reproductive success and recruitment at the individual and population levels in Swainsons Hawks breeding in northern California. At the population level, we investigated whether the number of individuals subsequently recruited could be predicted by the number of offspring produced in any given year, finding no evidence of differences in probability of recruitment by cohort or year. Instead, age of recruits was the best predictor of probability of their being recruited into the local breeding population. At the individual level, we used generalized linear models to examine the correlation between lifetime reproductive success of individual females and the number of their offspring recruited. For individuals, the number of fledglings produced was significantly correlated with both the number of those offspring and of grandchildren recruited into the breeding population. We also examined the relationship between the average annual reproduction of an individual and the number of offspring recruited, finding these significantly correlated. The relationship was not linear: individuals producing moderate numbers of fledglings, on average, yielded the highest numbers of recruits, reflecting a trade-off between adult survival and reproduction. These results provide evidence that monitoring of reproductive success of individuals or a population can give reasonable indices of future recruitment if the study is continued for a long period. But individuals with the highest average rate of reproduction may not be the most fit.