Effects of flushing nesting greater prairie-chickens in Illinois

Important reasons for evaluating researcher impact in nest studies include the possibility of increased abandonment, depredation, embryonic mortality, or bias of other parameters. From a 29-year data set of 1101 Greater Prairie-Chicken (Tympanuchus cupido pinnatus) nests in Illinois, we assessed the performance of 115 nests that were visited by investigators during egg laying or incubation. The minimum return rate for 107 hens flushed up to 5 times was 88% and apparently decreased (G = 9.76, df = 4, P = 0.042) with increased frequency of flushing caused by investigators. The success of 105 nests from which hens were flushed (68%) was higher (G = 7.55, df = 1, P < 0.01) than that of 840 undisturbed nests (53%) whose fates were determined prior to discovery. No desertions could be attributed to investigators activities. Embryonic mortality in disturbed nests appeared 3.7X higher (Z = 4.59, P = 0.001) than in undisturbed nests, but some egg removal by predators prior to our inspections may have biased results. Neither embryonic loss nor nest success was related to number of flushes during incubation. Nearly half of embryonic deaths that we could age in disturbed nests occurred before hen flushes, and most other losses, including whole clutches, were not clearly caused by investigator disturbance. Received 30 June 1997, accepted 27 Oct. 1997. Information about researcher impact on nesting success is vital in studies of nesting birds (Gotmark 1992). Precautions against negative impacts are especially important when studies involve endangered species, such as the Greater Prairie-chicken (Tympanuchus cupido pinnatus) in Illinois (Illinois Endangered Species Protection Act of 1972, amended 1977). Some investigations suggest researcher disturbance of nesting prairiechickens might increase rates of abandonment (Davison 1940, Lehmann 1941, Yeatter 1943, Riley et al. 1992), depredation (Silvy 1968, Bowen 1971, Bowen et al. 1976), or egg breakage (Baker 1953). Other studies, however, report little or no impact with various nest-searching methods (Drobney 1973, Toepfer 1988), including cable-chain drags (Svedarsky 1983, 1988; J. R. Keir, pers. comm.). Search techniques in these studies also included use of dogs, rope drags, flushing bars, landowner interviews, observing departures and returns by nesting hens to morning and evening feeding sites, and most currently, radio telemetry. Thus, reasons for differences in researcher impact on clutch performance are not well known. Events during two phases of Illinois' 29year study (1963-1991) of 1 101 Greater Prairie-Chicken nests suggested possible researcher-caused bias of egg success. During 19831987, egg success was no better in prairiechicken nests from which parasitic Ringnecked Pheasant (Phasianus colchicus) eggs had been removed than for those from which pheasant eggs were not removed (Westemeier 1988). In 1990-1991, during efforts to increase genetic variability by exchanging eggs in two isolated remnant populations, embryonic mortality appeared exceptionally high, though seemingly unattributable to researcher activities (Westemeier et al. 1991). These findings prompted an expanded study. Our objectives were to determine return rates of Greater Prairie-Chicken hens to their nests after investigator-induced disturbance, compare fates of investigator-disturbed with undisturbed nests, compare embryonic mortality between investigator-disturbed and undisturbed successful nests, and to determine if effects of investigator disturbance varied with stage in nesting cycle, or the number of hen flushes.