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The breeding grounds of migrant generation monarch butterflies in eastern North America are well known. In stark contrast the location of natal grounds of western migrants has not been delineated. We show that 55% of the area within seven western states was potential breeding range based on: (1) the occurrence of milkweed host plant species with phenology making them available during late-summer and (2) regional thermal conditions supportive of adult reproductive activity and development of immature stages. We next used a series of spatially explicit "bottom-up" regression models to test this first-approximation natal origins distribution. We tested for associations between variation in moisture availability at putative natal habitat and inter-annual variation in monarch abundance at western wintering sites for a 10 year period (1998-2007). Variation in moisture availability, as measured by Palmer’s drought severity index (PDSI), across the western region predicted monarch abundance patterns. In contrast and as expected, PDSI across known eastern breeding grounds did not predict variation in western monarch migrant abundance. The pattern of moisture availability was not uniform between states or within states and permitted similar tests of association at a finer geographical level. PDSI for California, Idaho, Nevada, and Oregon (but not Arizona, Utah, or Washington) were each significantly associated with monarch wintering abundance patterns with California exhibiting the strongest relationship. At a more focused spatial scale we tested the local recruitment hypothesis. This is the notion that western coastal wintering monarch populations derive only from nearby coastal breeding habitat and that monarchs do not migrate from more distant natal grounds. Variation in moisture availability within a block of three contiguous central California climate divisions (Sacramento Drainage, San Joaquin Drainage, and Southeast Desert Basin) significantly predicted inter-annual abundance of migrant generation monarchs. In contrast PDSI patterns of three coastal California climate divisions, i.e., ones local to wintering sites, as well as that of climate divisions in western Nevada and Arizona did not predict variation in monarch abundance at this more focused spatial resolution. Our findings suggest that moisture regimes act as a strong bottom-up driver of monarch abundance pattern via resource availability in western USA.