I conducted a two-year study of offspring sex ratios and tested the effects of disturbance and food availability on breeding American Kestrels (Falco sparverius) near Boise, Idaho.
Fisher’s (1930) equilibrium hypothesis predicts that sexually-size-dimorphic birds rear more offspring of the gender that is energetically cheaper to produce. Male kestrels were probably cheaper because males and females fledged at about the same age and male fledglings were 9% smaller. There was no evidence suggesting that allocation of nutrition by gender equalized production costs.
Exactly half of the 298 fledglings recorded were male, so fledgling sex ratio was not biased toward the cheaper sex (male) as predicted by the equilibrium hypothesis. However, fledging sex ratios may not have reflected sex ratios at termination of parental care, because females may have had higher mortality soon after fledging.
Primary sex ratio did not differ from unity. Ninety-two of 181 nestlings (50.8%) in complete broods were male. Secondary sex ratios probably reflected primary ratios because nestling mortality was low.
Myers’ (1978) hypothesis predicts that birds with relatively abundant food supplies rear greater proportions of chicks of the gender that is energetically more expensive to produce. Supplemental feeding did not affect fledging sex ratios. Also, broods that hatched early in the breeding season when food may have been relatively abundant had similar sex ratios as broods later in the year. Therefore, my data did not support Myers’ hypothesis.
Male bias increased with hatching sequence for 125 nestlings in this study. This bias trend may facilitate alteration of sex ratio by brood reduction when food is scarce; however, nestling mortality was low during the study.
Randomly selected pairs were frequently disturbed and received supplemental feeding, some were frequently disturbed but not fed, and others were not frequently disturbed or fed. Eight of 15 reproductive variables were significantly influenced by frequent investigator disturbance, supplemental feeding, or yearly differences. Food availability may have declined between 1992 and 1993 resulting in later nesting, smaller clutches, and lower productivity the second year. Weather conditions may have affected food availability. Food supplementation reduced the effects of disturbance and year on egg laying, median hatching date, and clutch size. Based on these results, I suggest that food availability during the breeding season may be a proximate factor for egg laying and clutch size. Alternatively, timing of breeding may have influenced clutch size, because timing and clutch size were inversely correlated. Interactions between food availability and disturbance for whether or not eggs were laid, median hatching date, clutch size, and egg abandonment indicated that food-stressed kestrels were more sensitive to human disturbance than better-fed birds.
Breeding kestrels were trapped with mist nets and a live Great Horned Owl (Bubo virginianus) lure. We trapped 13 males and seven females during 23 trapping attempts at 19 nest boxes in 1993. Trapping success was unrelated to age of nestlings, time of day, or whether tape recorded owl vocalizations were used. Mist nets with owl lures seem to be a safe and effective technique to capture adult kestrels during the breeding season.