A comparison of age, size, and health of House Finches captured with two trapping methods

Abstract

House Finches are being increasingly studied because of their variation in male plumage, evolving migration, and susceptibility to mycoplasmal conjunctivitis. Researchers traditionally use two methods for capturing House Finches: a walk-in hardware mesh cage around a bird feeder and mist nets placed next to lure feeders. However, because one method relies on birds' willingness to enter a foreign structure to obtain food, while the other captures them while flying, these methods may not sample the same subset of the House Finch population. This possibility could have serious consequences for studies of plumage coloration, where sex and age ratios are important; for studies of migration, where measures of wing lengths reflect migratory ability; and for studies of mycoplasmal infections, where disease state and measures of health are important. I used data from a long-term monitoring project of House Finches that employs both capture techniques to test if any attributes of House Finches differed between mist-netted and cage-trapped birds. Of 1173 House Finch captures over 3 yr, I found no trap-related difference in the proportion of birds with conjunctivitis, nor in the proportions of males or in the ratios of molting versus non-molting birds. There was also no difference in House Finch tarsus lengths or weights between capture methods. However, a larger proportion of young birds were captured in cage traps, and wing lengths were greater in birds captured with mist nets. I conclude that in general, cage traps and mist nets sample similar subsets of House Finches, but that researchers should view their trapping data with these inherent age and size biases in mind.