Abstract
T he North Carolina Zoological Park maintains a varied bird collection. In recent years we have included a collection of native North American Wood Warblers. Little is known about the long term husbandry of these birds. Since the science of captive warblers is still in its infancy, there are many informational gaps that must be filled.
One important area that deserves concentration is body weight. Aside from helping to detect illness, compiling monthly weight data serves to establish a set of baseline information. This data record can be used for a variety of purposes and to answer a variety of questions. Gaining as much information as possible from the captive warbler population will aid in developing appropriate husbandry protocols to ensure long term captive propagation in zoological collections. A desire to
contribute to the husbandry of captive warblers stimulated the North Carolina Zoo to develop a remote weighing device specifically for very small birds. Primarily the scale was developed to record as many weight values as we wanted without handling the delicate specimens.
By researching available products we were able to purchase, assemble and modify a system that is reliable, accurate. and easy to use. This scale will work for all small perching birds not just the Wood Warblers we initially designed it for. The zoo also obtained components for a larger range scale which has not been assembled. Total cost for the perch scale is moderate. Equipment prices were near six hundred dollars. However, the benefits outweigh the price tag. We can reliably and accurately weigh birds without having to catch, touch, or otherwise disturb
them. Using this equipment we have been consistently monitoring the weight of: American Redstart (Setophaga ruticilla), Common Yellowthroat (Geothlypis trichas), Prothonotary Warbler (Protonotaria citrea), Northern Parula Warbler (Parula americana), and Louisiana Waterthrush (Seiurus motacilla). In a few cases we have weight records spanning three years without having handled the bird solely for obtaining a current weight.
All the required parts were bought from a local
scale distributor and
hardware store. It is
possible that a different
combination of prod-
ucts from other manu-
facturers would work
just as well and may
cost less.
Minimally, the
apparatus consists of three components: a miniature beam sensor, a display, and a perch. Before a scale dealership is contacted, attempt to learn the technical jargon. A stick or dowel becomes the actual perch the bird will use. The mini sensor is a very specific type called a Planar Beam Sensor, which falls under the category of a load cell. When the perch is attached to the miniature beam sensor a "Load Cell Based Weight Platform" is created. This means that you have provided a place for the bird to
land mounted on a load cell.
Load cells are contained in virtually all electronic scale systems and there are many types. Everything from cars and trucks to the ingredients at a cookie factory are measured with load cells. Basically, the Planar Beam Sensor is a type of force transducer that uses a strain gauge to change weight (downward force) into an
electrical signal. When a strain gauge becomes stressed it experiences a measurable change in electrical resistance. Gluing the strain gauge to the strain element, in our case the little metal bar, forms the load cell.
The last component is the display or digital indicator. An indicator deciphers the change in electrical resistance experienced by the load cell due to a stress such as a bird perching. It determines when a stress occurs by constantly monitoring the electrical signal to and from
the platform. Changes in the electrical signal are proportional to the amount of stress and are converted into a number that is displayed as the weight.
The sensor we used is a Rice Lake Weighing Systems Planar Beam Sensor [Model # RLS1101 2 lb.]. The sensor has a maximum load rating of 2 lbs. and is accurate to +/- 0.10
grams. [There is also a sensor available that has a maximum load rating of 5 lbs. with an accuracy of+/- 0.20 grams] The beam sensor is a small strip of metal approximately 3/8" wide by 3" long. Each end of the strip has a small hole drilled through it. There is a delicate lead wire attached to one end. Under all of the solder at this end is the strain gauge. The lead wire is connected to a standard 25 ft. cable which serves to attach the weighing platform to the digital indicator. Longer cables can
be ordered at $3.00 per extra foot.
By far the hardest part of setting up the system is finding the appropriate fastening hardware. An assortment of very small fasteners will be needed. We used stainless steel hardware to prevent rusting. After assembling the entire remote weighing device, we encountered one last obstacle. Convening the digital indicator from its lb./oz. setting to a metric display proved challenging. The instruction manuals were too difficult for a lay person to understand. To their credit the manufacturer does provide a helpful technical support line. Be sure to have read and attempted to comprehend the instruction manual before calling.
Pennsylvania Scale Company is the manufacturer of the digital indicator we purchased
[Model #7400M]. The zoo bought a very basic indicator but it has a several features that we may never utilize. The ability to connect the indicator directly to a printer or a computer are two features we have yet to benefit from. O'Haus Scale Company also sells a digital indicator [model #lSSJ which fulfills the same purpose.