Complete Sanitation of Dead Nestlings by Parent Birds May Bias Nest Depredation Rates

1. Bruce Taubert Bruce Taubert Figure 1. An empty, intact red-faced warbler nest - depredated or sanitized? Figure 4. Daily precipitation recorded from 10 June to 12 July 2006at Mt. Lemmon and Marshall Gulch weather stations, Santa Catalina Mountains, AZ (Pima County 2006). Stations were located <1 km from study plots. Figure 5. Temperature and precipitation recorded on 4 July 2006at the Mt. Lemmon weather station, Santa Catalina Mountains, AZ (Pima County 2006). The first storm began at approximately 1230. Figure 3. Still image from video footage showing female yellow-eyed junco removing dead nestling from nest on 5 July 2006 in the Santa Catalina Mountains, AZ YEJU nest sanitized RFWA nest sanitized Complete Sanitation of Dead Nestlings by Parent Birds May Bias Nest Depredation Rates Chris Kirkpatrick1, Courtney J. Conway2, and Moez H. Ali1 1 University of Arizona; 2 USGS Arizona Cooperative Fish and Wildlife Research Unit • RESULTS (CONTINUED) • Daily nest survival (DNS) was lower for all RFWA and YEJU nests (n = 17) monitored during the 5-day period before the storms compared to the 5-day period after the storms. • DNS pre-storm = 1.0 (SE = 0) Z = 1.8 • DNS post-storm = 0.938 (SE = 0.036) P = 0.07 • Sixty-seven percent of nests that failed after the storms were incorrectly identified by field personnel as having been depredated (video footage revealed these nests failed due to weather and were later sanitized by parent birds). ABSTRACT Estimates of avian nesting success typically assume that nests found to be empty and intact (i.e., nest cup and lining undisturbed; Fig. 1) prior to fledging are likely to have been depredated. We provide evidence that an empty nest may not always indicate a depredated nest, but instead may result from complete sanitation of dead nestlings by parent birds. In 2006, we used time-lapse video cameras to monitor nests of red-faced warblers (Cardellina rubrifrons; Fig. 2) and yellow-eyed juncos (Junco phaeonotus; Fig. 2) in high-elevation, mixed-conifer forests of the Santa Catalina Mountains, Arizona. Following rain and hail storms on 4 & 5 July 2006, video footage revealed that parent yellow-eyed juncos and red-faced warblers removed all nestlings from 2 nests (the nestlings apparently died after the storms). Our results indicate that complete sanitation of dead nestlings by parent birds may be a relatively common behavior, suggesting that researchers use caution when assigning fates to empty nests during future nest-monitoring studies. Figure 2. Yellow-eyed junco (YEJU; left) and red-faced warbler (RFWA; right) - 2 ground-nesting birds that breed in close association in southeastern AZ. • DISCUSSION • We provide evidence that an empty, intact nest may not always indicate a nest depredation, but instead may result from complete sanitation of dead nestlings by parent birds following • inclement weather. • Inclement weather can lead to complete brood mortality (Stewart 1972) and young (1-8 day old) nestlings are especially susceptible because of their inability to maintain constant body temperature (Dunn 1976). • Preliminary results suggest that complete sanitation of dead nestlings by parent birds may be a relatively common behavior in red-faced warblers and yellow-eyed juncos. • The frequency with which this behavior may occur in other species is unknown. However, complete brood mortality and subsequent sanitation of dead nestlings is likely to occur: • 1) following strong storms • 2) at nests that are prone to flooding • 3) when nestlings are young and brood sizes • are small (Dunn 1976) • Researchers should use caution when assigning fates to empty, intact nests (especially after inclement weather) given that the incorrect assignment of nests fates has the potential to bias nest depredation rates. • RESULTS • Two nests (1 RFWA and 1 YEJU) that were monitored continuously by video cameras and checked periodically by field personnel failed in early July 2006 (Table 1). • Both nests contained young nestlings (1-4 days old) prior to failure. • Field observers assumed that the RFWA nest had been depredated (the nest was empty and intact) and the YEJU nest had been partially depredated (the nest had 1 infertile egg and was intact). • Video footage showed parent birds sanitizing nests by removing the entire brood of dead nestlings (Fig. 3). • Precipitation and temperature data from 2 local weather stations (Figs. 4 & 5; Pima County 2006) indicated that strong storms occurred on 4 & 5 July 2006, 1-2 days before the failure of both nests. • STUDY AREA • 5 16-20 ha plots located in high-elevation (2,300-2,800 m), mixed-conifer forest of the Santa Catalina Mountains, Arizona. • Mean annual precipitation in the Santa Catalina Mountains is 26.8 inches (Brown 1994) with most precipitation falling during a brief season of strong thunderstorms that begins in early July. • METHODS • From April-July 2006, we monitored a total of 148 red-faced warbler and yellow-eyed junco nests using standard nest-monitoring protocols (Martin and Geupel 1993). • We used time-lapse video cameras to continuously monitor a subset of 18 nests during the incubation and nestling periods. • We compared estimates of daily nest survival (Mayfield 1961, 1975) during a 5-day period before and during a 5-day period after strong thunderstorms on 4 & 5 July 2006. ACKNOWLEDGMENTS We thank M. Eastwood, N. Nardello, and T. Selvidge for assistance locating nests and operating video cameras and S. Sferra (Bureau of Reclamation) for the loan of the video cameras. Funding was provided by the U.S. Fish and Wildlife Service and the U.S. Geological Survey. LITERATURE CITED Brown, D. E. 1994. Biotic Communities Southwestern United States and Northwestern Mexico. University of Utah Press, Salt Lake City, Utah. Dunn, E. H. 1976. The relationship between brood size and effective homeothermy in nestling house wrens. Wilson Bull. 88:478-482. Martin, T. E., and G. R. Geupel. 1993. Nest-monitoring plots: methods for locating nests and monitoring success. Journal of Field Ornithology 64:507-519. Mayfield, H. 1961. Nesting success calculated from exposure. Wilson Bull. 73:255-261. Mayfield, H. 1975. Suggestions for calculating nest success. Wilson Bull. 87:456-466. Pima County. 2006. Pima County ALERT System. http://rfcd.pima.gov/alertsys/ Stewart, R. M. 1972. Nestling mortality in swallows due to inclement weather. Ca. Birds 3:69-70.