I will be collecting an invasive bird species, the House Sparrow, from farms located off of UMBS property. I will examine two historically important hypotheses of avian clutch size patterns, 1) the food availability hypothesis and 2) the adult mortality hypothesis. I will test these hypotheses in a single species, the House Sparrow, at three sites that differ in clutch size and latitude. The House Sparrow is a suitable model organism for studying avian clutch size patterns because House Sparrow clutch sizes have changed as House Sparrows have spread across North and Central America. This change in range and clutch size has resulted in a latitudinal clutch size pattern that parallels the latitudinal gradient seen in other bird species. Moreover, this change is relatively recent, occurring only in the last 150 generations. I also will examine potential proximate mechanisms controlling avian clutch size. I will study two hormones, 1) plasma prolactin and 2) the hypothalamic neuropeptide vasoactive intestinal polypeptide, which are known to control incubation onset and cessation of laying in turkeys. I will characterize how these hormones vary across the laying cycle, and use this information to make informed pharmacological manipulations in House Sparrows and look for a concomitant change in clutch size. It is only by understanding the proximate and ultimate mechanisms of clutch size differences in birds that researchers can develop and test hypotheses to solve a long unanswered question in life history research and uncover how and why birds vary reproductive effort across latitude.