Birds can display large spatio-temporal daily and seasonal variation in body mass. For example, birds generally have higher mass in winter when food supply is unpredictable or when there is less time for feeding. In contrast, opportunities for self-feeding should be reduced during breeding, when food needs to be provided to chicks, yet many adult birds decrease body mass. This could reflect a cost of parental care or a strategy to decrease costs of hard work through lowering wing loading. While “adaptive” modulation of body mass is well documented, two fundamental questions remain: a) how do birds assess their own body mass, and b) what mechanisms may birds use to rapidly adjust body mass? Here we describe a proof-of-concept study using captive zebra finches (Taeniopygia guttata) to test how rapidly birds can strategically adjust mass in response to an externally mounted weight and if behavior is a key correlate of this response. Individuals (n = 40) were separated based on sex and divided between four cages (n = 10 per cage). After two weeks of habituation (day 0) individuals were weighed and assigned to the control group or assigned weighted backpacks based on initial mass (e.g., 13.0 g bird = 1.30 g weight) to ensure backpacks were all approximately 10% body mass. Every other day (day 0–22), individuals were weighed at 09:30, returned to their home cage, and filmed for 1 h beginning at 12:00. Weighted backpacks were removed on day 14 but continued to be weighed and filmed every other day until day 22. Data suggest zebra finches rapidly lost mass (between days 0–2), but only partially compensated for the additional weight, before re-establishing constant, lower mass. They then re-established initial mass upon removal of weighted backpacks.