| dc.description.abstract | Abundance estimates allow wildlife managers to make informed management decisions, but differential detectability of individuals can lead to biased estimates of abundance. Sandhill cranes (Grus canadensis) are large-bodied and long-lived, rendering them ideal for mark-recapture estimates, but territorial birds have limited movement when compared to non-territorial birds, which may violate the assumption of equal detectability. Our objective was to quantify differential detectability for non-territorial and territorial sandhill cranes on a breeding ground, and to use this information to estimate population size for each group. We hypothesized that territorial sandhill cranes would be detected more often than non-territorial cranes. In 2009, the International Crane Foundation surveyed three routes in Briggsville, Wisconsin, two days per week with six passes per day. Technicians recorded bird locations on an aerial photograph. We created capture histories for banded territorial (n=52) and banded non-territorial cranes (n=23), and used the Huggins closed capture model in program MARK to estimate detection probability and abundance for each group. We identified a priori models that may explain daily crane detection over the sampling period using distance from road, territoriality, sampling event (first or subsequent sighting), and time of season as variables. We used Akaike’s Information Criterion adjusted for small sample size to rank models. The best approximating model included territoriality and sampling event (AICc weight= 0.92). Probability of detection was higher for territorial (𝑝̂= 0.11, CI=0.08-0.14) than for non-territorial (𝑝̂=0.03, CI= 0.01-0.07) birds. In subsequent sampling events (each day was considered a sampling event), detection probability almost doubled to 0.18 (CI= 0.17-0.20) for territorial cranes, and almost tripled to 0.11 (CI= 0.09-0.14) for non-
territorial cranes. Potential reasons for differential detection include differing degrees of movement by birds and/ or an observer effect in which the ability of observers to spot birds increases over time. We also used the N-mixture model to estimate detection probability and abundance for all cranes in the area (banded and unbanded), and obtained similar results for detection probabilities of banded territorial and banded non-territorial cranes. Abundance estimates for unbanded cranes from the N-mixture model were unrealistically high. Our research demonstrates the importance of differential detection when calculating abundance for sandhill cranes on breeding grounds.
In 1941, less than twenty whooping cranes (Grus americana) remained in one wild population wintering at Aransas National Wildlife Refuge, Aransas, Texas (Allen 1952). This population had increased to 257 (95% CI 178-362) in the winter of 2012 (Harrell and Bidwell 2013).The United States Fish and Wildlife Service began reintroducing whooping cranes into Wisconsin in 2001, but this population is not self-sustaining (WCEP 2012) potentially because nest success is low. Several hypotheses have been proposed for low nest success including improper nesting behavior resulting from captive rearing techniques. In addition to typical nesting behaviors, Wisconsin whooping cranes may lack adaptive behaviors for coping with ectoparasites specific to the breeding grounds in Wisconsin. Greater sandhill cranes (G. c. tabida) also breed in Wisconsin, and seem to exhibit adequate nest success. We aimed to identify behaviors that differ between nesting whooping cranes and sandhill cranes to better understand why population growth is limited in the reintroduced population of whooping cranes breeding at Necedah National Wildlife Refuge. From March – June 2014, we observed nesting behavior through trail cameras programmed to take one photo every five minutes. During
nesting, we monitored nests until either eggs hatched or nests were abandoned. Nine whooping crane nests and seven sandhill crane nests were used for analysis. We identified behaviors (incubating, away from nest, manipulating nest platform, etc.) in 16,487 sandhill crane and 25,544 whooping crane photographs. Apparent nest success was 0.56 for whooping cranes and 0.57 for sandhill cranes during the 2014 breeding season. All whooping crane nest failures were abandonments while sandhill crane nests failed for various reasons including predation. We used a two way factorial ANOVA with sub-sampling and found that birds on nests where one egg hatched spent significantly more time incubating eggs for both crane species (F (0.5),1,12 =14.24, P = 0.0027). Additionally, whooping cranes spent more time away from nests than sandhill cranes (F(0.5),1,12 =5.3, P = 0.0395) and cranes of both species that had successful nests spent less time away from the nest (F (0.5),1,12 =11.7, P = 0.0051). Cranes with successful nests spent less time manipulating eggs than those with unsuccessful nests (F (0.5),1,12 = 16.97, P = 0.0014). All other behaviors were not significantly different between cranes with successful and those with unsuccessful nests or between whooping cranes and sandhill cranes. At this time, we do not know if birds abandoned nests because the eggs were not fertile or if eggs did not hatch because adults left the nests. Behavior differences are confounded with species, rearing (captive or wild), and genetic differences. We also caution the interpretation of the data because we only monitored 16 total nests over one nesting season. However, this first year of data collection supports our hypothesis that whooping cranes and sandhill cranes exhibit some behavioral differences. | en_US |
