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Blood samples from birds are used to address a wide variety of questions and to assess individual attributes such as gender, condition, and physiological status. As the use of blood samples expands, researchers have begun to investigate whether sampling has negative impacts on the individuals being sampled. The majority of studies to date have examined survival probability, mostly finding no detectable effect on survival or recapture rate. Significantly fewer studies have looked for effects of collecting a blood sample on measures of reproductive success. Of those that have, none found a detectable effect unless both parents were bled prior to incubation. Since fitness is determined by both survival and reproductive success, we used data from a population of Field Sparrows (Spizella pusilla) in northeastern Pennsylvania for which no negative effect of blood sampling on apparent survival was found and tested whether taking a blood sample affected measures of reproductive success. We found no evidence that clutch size, egg volume, number of offspring fledged, and nest success/failure differed between bled and unbled birds, suggesting minimal impact of blood collection in this population. [ABSTRACT FROM AUTHOR]