Field Sparrow Reproductive Success and Nesting Ecology

-Field Sparrow reproductive success and nesting ecology were studied in central Illinois on a tract composed of grassland, shrub-grassland, and shrub-woodland. Egg and nest successes were 11.2% and 10.2% respectively. Major causes of nest failure included: predation, primarily by snakes (76% of all active nests), nest desertion following brood parasitism by the Brown-headed Cowbird (7%), and nest desertion from unknown causes (7%). Eleven percent of the nests were parasitized; most (63%) were deserted as a result. Nests with young received heavier predation than nests with eggs. Early nests were placed predominantly in standing grass litter, but as the breeding season advanced forbs, trees, and shrubs became the most frequently used nesting substrates. Mean nest height increased as the season advanced. The frequency of desertion (from unknown causes), cowbird parasitism, snake predation, and successful fledging was not significantly different among nests placed in the major vegetation types, although mammalian predation did differ significantly. Nest height did not influence nesting outcome significantly. Nest vulnerability to snake predation was independent of relative nest concealment. Desertion (nonparasitized nests) and snake predation were distributed uniformly among nests throughout the study area, but nests preyed upon by mammals were somewhat clumped, and cowbirds parasitized nests within or near the shrub-woodland. The occurrence of snake predation was proportional to the number of nests. Predation intensity was not related to brood size. Nesting success increased slightly as the season advanced. The incidence of snake predation was negatively correlated with maximum daily relative humidity, but unrelated to other weather measurements. -Department of Ecology, Ethology, and Evolution, University of Illinois, Champaign, Illinois 61820. Present address: Department of Animal Ecology, Iowa State University, Ames, Iowa 50011. Accepted 28 June 1976. DESPITE the proliferation of statistics on reproductive success in ornithological literature, few studies attempt in-depth analyses of factors influencing the nesting outcome. The Field Sparrow (Spizella pusilla), a species common to old fields in eastern United States, provides such an example. The breeding biology of this species has been well documented (Walkinshaw 1936, 1939, 1945, 1968; Crooks 1948; Crooks and Hendrickson 1953; Sutton 1960; Best 1977; and others), but the factors influencing its nesting success have yet to be scrutinized. The persistent renesting of the Field Sparrow over a long breeding season, concomitant with the intense predation pressure observed in the present study, afforded an excellent opportunity to quantify and evaluate variables influencing nesting outcome. STUDY AREA AND METHODS The 23.6-ha study area was in Allerton Park, near Monticello, Illinois. Approximately 8.5 ha of shrub-woodland occupied its periphery (Fig. 1), with tree heights reaching 15 m. The central region consisted of 9.0 ha of shrub-grassland (tree and shrub heights all under 8 m) and 6.1 ha of grassland. The major plant species in the three vegetation types are described in Best (1974b). In 1971 I visited the study area on 72 days between 25 April and 24 August, more frequently after May; I made 133 visits in 1972 from 9 April to 15 September. During each visit I recorded the status of each nest. Time spent at active nests was minimized to avoid attracting predators. Most nests were located by watching females during nest building. Additional nests were found while walking through territories or by closely watching pairs known to be actively nesting. Nest locations were recorded on a grid map of the study area. Adult and nestling Field Sparrows were individually marked with leg bands. The following were recorded after nest abandonment: nest height, vegetation providing nest support and cover, relative nest concealment at and below nest level, and degree of disturbance to the nest and surrounding vegetation after nest predation. Statistical significance was set at P c 0.05. 9 The Auk 95: 9-22. January 1978 This content downloaded from 157.55. 39.208 on Fri, 29 Jul 2016 05:37:24 UTC All use subject to http://about.jstor. org/terms 10 Louis B. BEST [Auk, Vol. 95 __=X} ~ ~~ =) K . t4