Most of the information we have come across is that birds acclimate to devices such as balloons, tape, and plastic owls in a fairly short time (not specifics on that). Noise seems to be one consistently reliable way to keep birds away—but we will not tell your neighbor. However, if we may add this counsel, perhaps you should talk with your neighbor and try to better understand their concern? You may, over time, show them that feeding the birds can help to keep them out of their property and in yours.
Most of the information we have come across is that birds acclimate to devices such as balloons, tape, and plastic owls in a fairly short time (not specifics on that). Noise seems to be one consistently reliable way to keep birds away—but we will not tell your neighbor. However, if we may add this counsel, perhaps you should talk with your neighbor and try to better understand their concern? You may, over time, show them that feeding the birds can help to keep them out of their property and in yours.