Technique of Raptor Food Habits Study

The field observation method has been used in Wisconsin to advantage on Redtails and Marsh Hawks.For obvious reasons it is not so easy to apply to nocturnal predators.Nest studies.Raptor nest studies, like field observations, are possessed of potentialities for quantitative data, but must be handled with discretion.Quantitative data of unquestionable merit as to the diets of the young are obtainable by keeping nests under constant observation, as from a blind ; but this method has the weakness of being so time-consuming that too few nests can be watched in a season.Data from but one or two or three nests, regardless of how complete for those nests, may or may not be representative even of the local food habits of the species studied.' Every once in a while raptors of non-typical food habits are encountered, the proper allowance for which can not be made unless data are had treating with large numbers of the species.Nest visits, conducted at intervals of a day or two, have been most profitable the first few days after hatching out of the young, when the nestlings were unable to eat all of the food brought in.The nests yielded less and less as the nestlings became large enough and hungry enough to clean up the prey, though valuable data were still to be derived from young hawks by examination of their gullet contents.Feather material persisted for varying periods of time, while only the barest traces of unknown numbers of small mammalian prey remained evident in the majority of nests.Thus the proportion of bird to small mammal prey recorded from ordinary nest visits (both for hawks and owls) tended to exceed the proportions determined by tried quantitative methods.This has been repeatedly checked.Mice, for example, occurred commonly in the pellets of nestling Horned Owls and in the gullets of young Marsh Hawks, yet seldom was corresponding evidence of mice to be seen in the nests.Despite the general disadvantages of the method-to, which should be added the personal risk and labor of heavy routine climbing, even with accessories as ropes and climbing irons-it was found desirable to examine the nest litter of Horned Owls and Red-tails about once a week.Observations indicated that feathers of quail and ruffed grouse, especially primaries and tail feathers, working deep into, the interstices of stick nests, would remain there indefinitely, unless deliberately removed by hand.In this way an investigator, assigned to a p' articular game bird species, may be enabled to employ another check (not claimed utterly infallible) as to ,whether that species is brought into a given nest.Stick nests, that may be used on successive years or perhaps in the same season by Red-tails, Horned Owls, etc. (a late nesting raptor will sometimes take over a recently vacated nest of another), should be given a careful cleaning soon after the contained eggs hatch, to eliminate possibility of avian prey from a previous occupant being diagnosed as the prey of the wrong predator.For that matter, where feasible, a careful clean-up should also be made at each visit.Stomach examinations.Drawbacks to the exclusive use of stomach data have already received space in literature.Stomach data at best show only what individual animals in a given locality have eaten at a given time.At worst they show only part of what individual animals have eaten some place, some time, especially where differentially resistant contents are in advanced stages of digestion.It may be stated, preliminary to discussion later on in this article, that owl stomachs furnish much more reliable data as a rule than hawk stomachs, for the reason that bo,ny material suffers less from the digestive processes of owls than from hawks.On the other hand, feathers and fur appear td survive digestion better in hawk stomachs than in those of owls.These generalizations are based upon feeding experiments with 6 Horned Owls, 1 Barred Owl, 1 Long-eared Owl, 3 Red-tailed 2