Pamela J. Pietz

Identifying predators and fates of grassland passerine nests using miniature video cameras.

Nest fates, canses of nest failure, and idenfities of nest predators are difficult to determine for crassland passerines. We developed a miniature video-cainera system for use in grasslands and deployed it at (a) needs of 10 passerine species in North Dakota during 1996-97. Abandonment rates were higher at nests 1 day after camera deplov ment (23%) than 1 day or night (22-116 hr) at 6 nests 5 of which were depredaled by ground squirrels or tuice. For nests without cameras, estimated predation rates were lower for fround nests than aboveground nests (P = 0.055), but did not differ between open and covered nests P -74. Open and covered nests differed, however, when prodation risk (estimated by initial-predation rate was examined separately for day and night using camera-monitored nests; the frequency of initial predations that recurred during the day was higher for open uests than covered nests (P = 0.015). Thus, vulnerability of some test types may depend on the relative importance of nocturnal and dinrual predators. Predation risk increased with testhing age from 0 to S days (P = 0.07). Up to 15% of fates assigued to camera-monitored nests were strong when based solely on evidence that would have been available from periodic nest visits. There was ne evidence of disturbance at nearly half the depredated nests, including all 5 depredated by large mannuals. Oterlap to types of sign left by different predator species, and variability of sign within species, suggests that evidence at nests is unreliable for identifying predators of grassland passerines.

TIME-SPECIFIC VARIATION IN PASSERINE NEST SURVIVAL: NEW INSIGHTS INTO OLD QUESTIONS

Abstract Understanding nest survival is critical to bird conservation and to studies of avian life history. Nest survival likely varies with nest age and date, but until recently researchers had only limited tools to efficiently address those sources of variability. Beginning with Mayfield (1961), many researchers have averaged survival rates within time-specific categories (e.g. egg and nestling stages; early and late nesting dates). However, Mayfield’s estimator assumes constant survival within categories, and violations of that assumption can lead to biased estimates. We used the logistic-exposure method to examine nest survival as a function of nest age and date in Clay-colored Sparrows (Spizella pallida) and Vesper Sparrows (Pooecetes gramineus) breeding in north-central North Dakota. Daily survival rates increased during egg laying, decreased during incubation to a low shortly after hatch, and then increased during brood rearing in both species. Variation in survival with nest age suggests that traditional categorical averaging using Mayfield’s or similar methods would have been inappropriate for this study; similar variation may bias results of other studies. Nest survival also varied with date. For both species, survival was high during the peak of nest initiations in late May and early June and declined throughout the remainder of the nesting season. On the basis of our results, we encourage researchers to consider models of nest survival that involve continuous time-specific explanatory variables (e.g. nest age or date). We also encourage researchers to document nest age as precisely as possible (e.g. by candling eggs) to facilitate age-specific analyses. Models of nest survival that incorporate time-specific information may provide insights that are unavailable from averaged data. Determining time-specific patterns in nest survival may improve our understanding of predator-prey interactions, evolution of avian life histories, and aspects of population dynamics that are critical to bird conservation.

Effects of harness transmitters on behavior and reproduction of wild mallards

Radio telemetry has been an important research tool in waterfowl studies for >20 years, yet little effort has been made to evaluate potential effects of transmitters on the birds that carry them. As part of a 4-year mallard (Arras platyrhynchos) study in the prairie pothole region of North Dakota and Minnesota, we compared radio-marked and unmarked female mallards in terms of percent time observed feeding, resting, and preening; nest initiation date; and clutch size and egg volume. Radio-marked females carried a 23-g back-mounted transmitter attached with a 2-loop harness (Dwyer 1972). On average, radio-marked females tended to feed less, rest and preen more, initiate nests later, and lay smaller clutches and eggs than unmarked females

Nest Survival of Clay-Colored and Vesper Sparrows in Relation to Woodland Edge in Mixed-Grass Prairies

Abstract The quantity and quality of northern mixed-grass prairie continues to decline because of conversion to agriculture, invasion of woody and exotic plants, and disruption of important ecological processes that shape grasslands. Declines in grassland bird populations in North Dakota, USA, have coincided with these largely anthropogenic alterations to prairie habitat. In grasslands of north-central and northwestern North Dakota, woody plants have increased due primarily to fire suppression, extirpation of bison (Bos bison), and widescale planting of tree shelter belts. In northern grasslands, effects of woody vegetation on survival of grassland birds are poorly understood, and conclusions are based mainly on studies conducted outside the region. We examined nest survival of clay-colored sparrows (Spizella pallida) and vesper sparrows (Pooecetes gramineus) relative to the distance nests were located from aspen (Populus tremuloides) woodland edges and relative to other habitat features near the nest. Clay-colored and vesper sparrow nest survival was higher for nests located near woodland edges, nests with greater cover of Kentucky bluegrass (Poa pratensis), and nests more concealed by vegetation. Vesper sparrow nest survival increased as the percent cover of tall shrubs near the nest increased. Based on video-camera data, the 13-lined ground squirrel (Spermophilus tridecemlineatus) was the most common predator of sparrow eggs and young. Thirteen-lined ground squirrels were more common far from woodland edges than near, and this pattern may, in part, explain clay-colored and vesper sparrow nest survival in relation to woodland edges. In contrast to our results, studies conducted in other grassland systems generally report lower nest survival for grassland birds nesting near trees and shrubs. This disparity in results demonstrates the need to identify specific nest predators and their distributions with respect to important habitat features because these data can be important in explaining—and perhaps predicting—patterns of nest predation.

Frequency of Egg and Nestling Destruction by Female Brown-headed Cowbirds at Grassland Nests

Abstract Researchers have suggested that Brown-headed Cowbirds (Molothrus ater) destroy nest contents of potential hosts to induce renesting and thus enhance future opportunities for parasitism. Although cowbird destruction of passerine nests has been witnessed and surmised, few data are available on frequency of those events. We used miniature video-cameras at nests of grassland passerines and documented partial or complete destruction of eggs or nestlings by cowbirds at 7 of 132 nests monitored with cameras. At least three of the seven cases appeared to be attempts to totally destroy the nest contents; those cowbirds did not appear to be motivated by food or an intent to parasitize the nest. Three cases probably were associated with parasitism, but two involved egg removal late in incubation and the third was unusually destructive. Cowbirds were responsible for 24% of egg losses and 5% of nestling losses caused by predators. The importance of cowbirds as an agent of egg and nestling loss undoubtedly varies among sites and years, but it should not be overlooked.

Factors affecting gadwall brood and duckling survival in prairie pothole landscapes

Waterfowl biologists need reliable predictors of brood and duckling survival to accurately estimate recruitment rates. We examined 30-day survival rates of gadwall (Anas strepera) broods (1992-1994) and ducklings (1990-1994) in eastern North Dakota, USA, during years when water conditions ranged from extremely dry to extremely wet. We evaluated effects of several variables on brood survival: (1) percent of seasonal wetland basins containing water, (2) occurrence of rain on the current or 2 previous exposure days, (3) minimum ambient temperature averaged over the current and 2 previous exposure days, (4) hatch date, (5) duckling age, and (6) brood size. Only 9 of 58 radiomarked females lost their entire broods; Kaplan-Meier 30-day survival rate for broods was 0.84. Brood size (adjusted for hatch date) was a better brood-survival predictor than were any of the environmental variables. Risk of tool brood loss decreased by 24% for each additional duckling in the brood. We monitored fates of 212 radiomarked ducklings from 94 broods. Daily risk of death for ducklings was twice as high when seasonal ponds were scarce as when ponds were abundant. Duckling survival rate was lower during the first 7 days of life whether it rained or not; survival rate was greater for 8- to 30-day-old ducklings, but was reduced by rain. Thirty-day duckling survival was greatest when minimum daily temperatures exceeded 10°C and no rain occurred. We attributed 86% of 87 deaths of radiomarked ducklings to predation; American mink (Mustela vison) accounted for ≥68% of the 40 deaths for which predator type could be ascertained. Despite apparent resilience of gadwall populations during drought, our study documented a positive effect of seasonal wetland availability on gadwall duckling survival. Management efforts to improve recruitment will be more effective in years when most seasonal basins contain water.

Mallard Brood Movements, Wetland Use, and Duckling Survival During and Following a Prairie Drought

Abstract We used radiotelemetry to study mallard (Anas platyrhynchos) brood movements, wetland use, and duckling survival during a major drought (1988–1992) and during the first 2 years of the subsequent wet period (1993–1994) at 4 51-km2 sites in prairie pothole landscapes in eastern North Dakota, USA. About two-thirds of 69 radiomarked mallard broods initiated moves from the nest to water before noon, and all left the nest during daylight. On average, broods used fewer wetlands, but moved greater distances during the dry period than the wet period. Broods of all ages were more likely to make inter-wetland moves during the wet period and probabilities of inter-wetland moves decreased as duckling age increased, especially during the dry period. Brood use of seasonal wetlands nearly doubled from 22% to 43% and use of semi-permanent wetlands declined from 73% to 50% from the dry to the wet period. Eighty-one of 150 radiomarked ducklings died during 1,604 exposure days. We evaluated survival models containing variables related to water conditions, weather, duckling age, and hatch date. Model-averaged risk ratios indicated that, on any given date, radiomarked ducklings were 1.5 (95% CI = 0.8–2.8) times more likely to die when the percentage of seasonal basins containing water (WETSEAS) was ≤18% than when WETSEAS was >40%. An interaction between duckling age and occurrence of rain on the current or 2 previous days indicated that rain effects were pronounced when ducklings were 0–7 days old but negligible when they were 8–30 days old. The TMIN (mean daily minimum temperature on the current and 2 previous days) effects generally were consistent between duckling age classes, and the risk of duckling death increased 9.3% for each 1°C decrease in TMIN across both age classes. Overall, the 30-day survival rate of ducklings equipped with radiotransmitters was about 0.23 lower than the survival rate of those without radiotransmitters. Unmarked ducklings were 7.6 (95% CI = 2.7–21.3) times more likely to die on any given day when WETSEAS was ≤18% than when WETSEAS was >40%. Higher duckling survival and increased use of seasonal wetlands during the wet period suggest that mallard production will benefit from programs that conserve and restore seasonal wetland habitat. Given adverse effects of low temperatures on duckling survival, managers may want to include this stochastic variable in models used to predict annual production of mallards in the Prairie Pothole Region.

DOES PRESENCE OF PERMANENT FRESH WATER AFFECT RECRUITMENT IN PRAIRIE-NESTING DABBLING DUCKS?

Abstract In the Prairie Pothole Region (PPR) of North Dakota, USA, American mink (Mustela vison) are a major predator of ducklings. Mink populations plummet during severe droughts, but some mink survive where permanent fresh water is available. In 1992–1993, we evaluated whether development of a permanent water body, the 125-km McClusky Canal (MC), had affected survival of gadwall (Anas strepera) and mallard (A. platyrhynchos) broods and ducklings in surrounding wetland complexes. Twelve of 25 radiomarked gadwall and mallard hens experienced total brood loss, and 148 of 199 radiomarked ducklings from 58 broods died by day 30. Gadwall broods (n = 18 radiomarked hens) survived to 30 days at a lower rate (0.52) than predicted for similar areas in the region with limited permanent fresh water (0.85; P = 0.009). Observed (n = 162 radiomarked ducklings from 48 broods) survival rates also were lower than predicted for gadwall ducklings 0–7 days old (0.42 vs. 0.60; P < 0.001) and 8–30 days old (0.41 vs. 0.80; P < 0.001). We attempted to include mallards in models constructed to predict brood and duckling survival rates in the Koenig Study Area (KSA), but data were too sparse. Rates of survival to 30 days for gadwall and mallard ducklings declined from an estimated 0.83 and 0.68 in 1976–1981 (Lokemoen et al. 1990), when the MC was first filling with water, to 0.36 and 0.31 (adjusted for radiotransmitter effects) in 1992–1993 after the MC had become a permanent freshwater body. Estimated gadwall recruitment rate (females fledged per hen) during 1992–1993 was 0.5, <50% of the estimated recruitment rate in 1976–1981. Of 130 radiomarked ducklings (both species) for which we determined cause of death, 114 mortalities were attributed to predation; at least 65% of 62 deaths in which the predator type could be discerned were caused by mink. Environmental planners and waterfowl managers should be aware of potential risks to waterfowl production from development of permanent freshwater bodies in prairie pothole landscapes and may wish to refine duck productivity models to consider negative effects of permanent water on duckling survival.

HABITAT SELECTION BY SYMPATRIC SPRUCE AND RUFFED GROUSE IN NORTH CENTRAL MINNESOTA

Fifteen spruce grouse (Canachites canadensis) and 7 ruffed grouse (Bonasa umbellus) were radio-tracked between December 1977 and June 1979 in a 10-km2 area in Minnesota. Additional data were collected from display site and nest surveys and from direct observations. In months of snow cover, all spruce grouse preferred jack pine (Pinus banksiana) upland; from May through September all males and most females selected coniferous lowlands. Strong preferences for display sites in black spruce (Picea mariana) bogs and for nest sites in mixed bogs suggest that such lowlands may be critical for reproduction. Ruffed grouse varied considerably in patterns of habitat use. Logs used for drumming, nests, and broods were most frequently in alder (Alnus spp.) or upland-alder edge, suggesting that alder habitat may be important to the success of these ruffed grouse. Spruce grouse and ruffed grouse overlapped little in habitat use during snow-free periods. During periods of snow cover they both showed increased use of upland conifers. We believe the interspersion of lowland habitats with jack pine uplands allows ruffed and spruce grouse to coexist. J. WILDL. MANAGE. 46(2):391-403 Spruce and ruffed grouse occur sym- patrically throughout portions of their range in North America with the latter being considered a species dependent upon subclimax deciduous forests and the former upon climax coniferous for- ests. In Minnesota, habitat selection by spruce grouse has been studied by An- derson (1973) and Haas (1974) while Eng (1959), Kupa (1966), Godfrey (1975), and Maxson (1978) investigated habitat use by ruffed grouse. Few of these studies were conducted in areas that contained a wide variety of both upland and low- land habitats and none was conducted in an area that contained both species of grouse. Only Robinson (1969) in Michi- gan attempted to evaluate habitat use by both grouse on the same site at the same time. However, his research focused pri-

White-tailed Deer (Odocoileus virginianus) Predation on Grassland Songbird Nestlings

Abstract White-tailed deer (Odocoileus virginianus) were videotaped depredating four songbird nests in grassland habitats in southeastern and northcentral North Dakota, 1996–1999. Deer ate two Savannah sparrow (Passerculus sandwichensis), two grasshopper sparrow (Ammodramus savannarum), one clay-colored sparrow (Spizella pallida), one red-winged blackbird (Agelaius phoeniceus) and three brown-headed cowbird (Molothrus ater) nestlings. Deer removed nestlings quickly (5–19 sec/nest) at night (22:00 to 05:17 Central Daylight Time) and left no evidence of predation. Although probably opportunistic, deer predations clearly were deliberate and likely are more common than generally believed.