## 1 Introduction  
Produce contaminated with fecal-derived pathogens is a recognized food-safety concern. Current practice is to visually survey fields prior to harvest for signs of fecal contamination, and to restrict harvesting of identified problem areas. The difficulty of detecting bird droppings in fields is a concern. To address this concern, this study examined optical parameters and methods that could be used to develop optical survey instruments to detect bird droppings in produce fields. Fresh produce represents a particular food-safety risk as it is commonly consumed fresh, without an intervening kill step such as cooking to reduce any potential pathogen load. [PERSON] et al. [PERSON] et al. (2016) reported that produce is the most common source of food borne illnesses, and incidences of produce-related food-safety events have been increasing in recent years [PERSON] et al. (2016). Most incidents of non-viral foodborneillness associated with consumption of produce can be traced to contamination of the produce in fields prior to harvest [3; 4]. Thus, any action taken prior to or at harvest that can reduce the probability of contaminated produce entering the food chain will have the greatest potential impact on food safety. Steps taken in fields to alleviate contamination events include restricting access to fields, cleaning harvest equipment prior to and during harvest, and human hygiene requirements [5; 6; 7]. A critical step is the surveying of fields for fecal contamination. Field surveys at multiple production stages, including just prior to harvest, are required [5; 6; 7]. Generally, trained farm personnel walk through fields looking for indications of possible fecal contamination such as signs of animal intrusion including tracks, rooting, or crop damage. If a potential contamination site is identified, the site is flagged and the surrounding area is not harvested. If the signs of animal intrusion are extensive, food-safety personnel may flag the area on the assumption that the area is likely to contain fecal material. The problem with birds is that they can visit a field, leave their droppings, and depart without leaving any readily identifiable indications that they have ever been in the field. Birds, and particularly gulls, are known carriers of human pathogens [8; 9; 10], and have been implicated in the contamination of local waters [11; 12; 13; 10]. The goal is to develop practical methods to allow fields to be surveyed for bird droppings. It would be impractical to ask personnel to visually examine each plant to fully survey the produce fields. Survey methods that can be automated or used without requiring personnel to enter fields are needed; it would be an added benefit if these methods could be used to detect fecal material from all types of animals. Our laboratory has developed numerous optical methods to detect fecal matters [14; 15; 16; 17; 18; 19; 20; 21; 22]. For field use, using fluorescence [14; 15; 16; 17] or reflectance [18; 22] imaging in the visible wavelengths are the most promising methods. Methods based on measuring fluorescence responses to UV or purple illumination are probably the most sensitive. Fluorescence methods have two principal drawbacks--it is very difficult or costly to detect fluorescence responses under ambient daylight conditions, and an illumination source is needed to elicit a fluorescence response. Acquiring reflectance images at two appropriate wavelengths can be used to detect cow feces; however, false positives are a problem [18]. It was hypothesized that the color of bird droppings might facilitate detection of bird droppings. Birds use only a single orifice for excretions, the cloaca; as a result, their droppings usually include a whitish component made of uric acid, a form of nitrogenous waste [23]. This white waste component may represent a target characteristic that could facilitate the detection of bird excrements. We recently demonstrated that dairy manure could be reliably detected using fluorescence imaging at dusk or at night [17].