The objective of this study was to investigate the differences in environmental characteristics, reproductive successes and level of parental care between grassland and forest breeding territories of Mountain Bluebirds (Sialia currucoides). It is hypothesized that the differences in the particular environmental features of grasslands and forests will influence the reproductive behavior of parents and/or their breeding success. This may influence broader processes such as the evolution and the life-history adaptations of Mountain Bluebirds. This study was conducted under a permit from Environment Canada, and the research protocols were approved by the UNBC Animal Care and Use Committee (O’Brien and Dawson 2008).
Mountain Bluebirds are common in Western Canada and the native breeding habitats include prairie lands and the alpine tundra. Mountain Bluebirds begin nest building at beginning of breeding season, which is early April to late May (Calder 1970). Nest boxes are often used as set cavities. The average clutch size is four to five eggs (Rounds and Munro 1982). The incubation period is approximately 13 days long and the young typically spend 21 days in the nest before they leave (Power and Lombardo 1996). Male Mountain Bluebirds are mostly light blue with dark wings. The females are mostly gray-brown with blue in their wings and tail. (Budden and Dickinson 2009). The diet mainly of Mountain Bluebirds consists of insects and berries (Rounds and Munro 1982). Common prey includes grasshoppers and caterpillars (O’Brien and Dawson. 2008).
The study was located southwest of Williams Lake, British Columbia, Canada (52.1417° N, 122.1417° W). The study was conducted between the months of April 2015 to August 2017 (A White, unpubl. data). The territories, either grassland or forest, were categorized by an observer in a heterogeneous area. The forest area was classified by multiple patches of trees while the grassland was classified by open areas. Both sites incorporated 49 paired nest boxes mounted on fence posts. Within the pair of boxes, individual boxes were separated by 5 meters. The adjacent pairs were separated by 200 meters.
The reproductive success of Mountain Bluebirds was investigated in terms of the quantity and quality of the offspring produced. This was determined by the size and growth rates of the offspring.
The rate of offspring survival was determined by recording the number of offspring hatched and the number of offspring fledged. This was measured as it alludes to the population growth rate which can be used to forecast the future population trends of Mountain Bluebirds (Vinuela and Bustamante 1992). To determine the clutch initiation date and size, the nest boxes were investigated every other day. The clutch initiation date and the consistency of clutch size may reveal that there is a hereditary disposition to lay a clutch of fixed size in that particular breeding area and that this may be an adaptation to the different environmental factors (Przybylo et al. 2013). The number of hatched offspring was determined on day 12 after clutch completion due to the incubation period. Each clutch was then examined daily to determine hatching date and the number of nestlings hatched. To determine the rate of survival to fledging, the nestlings were counted on alternative days from the day of hatching to day 22 (Power and Lombarado 1996).
On day 1 of the brood rearing phase, the nestlings were weighed using an electronic scale. Weight measurements were taken to be within 0-01 g. For the purpose of identification and to track the growth rates of the offspring, each individual`s tarsi was marked with varying patterns of color. The number of nestlings fletched were then examined daily from day 3 to day 15. The mass and length of the longest primary feather was recorded in order to determine the body size and condition of the Mountain Bluebirds. These measurements are important as they reveal to the nutritional status of the birds. A sufficient dietary intake results in a larger mass and a greater length of primary feather in Mountain Bluebirds (Christopher 1989). The measurements were taken with the following specifications: mass was measured with a spring balance and was recorded to the nearest T25g. The length of the longest primary feather was measured with a ruler to the nearest 0-5 mm. The values were averaged to represent the entire brood.
Parental care was determined by the feeding rate and the relative quantity of larvae fed to the offspring by the adult bluebirds.
To determine the quantity of larvae fed to the offspring by the adult bluebirds, a ratio of the number of larvae to the number of grasshoppers was created. This allowed for an insight into the bluebird’s diet and the common prey type. The number of feeding visits per hour per nestling was determined in order to analyze how often the nestlings were fed. This was useful in observing the relationship between diet and growth of the nestlings. The feeding visits were then separated by the sex of the parents in order to evaluate the relationship between parental sex and feeding visits. The level of parental commitment is important as this heavily influences the survival of the offspring (Moreno 1987). To determine the number of feeding visits, video recording was used on day 7 of brood rearing to record nestling diet through the use of cameras with infrared and motion detecting abilities (Hawk Eye HD, Birdhouse Spy Cam, West Linn, OR) (Pagani-Nu?n?ez et al. 2017). The cameras were placed inside the nest boxes and were camouflaged with branches and vegetation to minimize disturbance. The per capita feeding rates of the male and female bluebirds was calculated by dividing the recording length and the brood size.
The environmental characteristics that were investigated were the presence of nest parasites, insect abundance and the ambient temperature.
In Mountain Bluebirds, the presence of Protocalliphora spp. pupae have a negative impact on chick growth, hematocrit, metabolic capacity, and survival before and/or after fledging (Clifford and Dahlsten 1983). After fledging, the nests were brought back to lab where they were stored in sealed bags at approximately 20-25 °C for a period of 14 days. After the waiting period, the parasite abundance was determined. To determine the presence of parasites, the number of Protocalliphora spp. pupae in the nests was recorded at day 22 (O’Brien & Dawson 2013).
Temperature was measured as it may result in an increase in metabolic energy in the female Mountain Bluebird, and this may allow them to invest more energy into child rearing (Gardner and Smiseth 2011). For each clutch, the average temperature was recorded hourly at a daily basis through data loggers (iButton DS1921, Maxim Integrated Products, Sunnyvale, CA), for day 1 to day 22 of brood rearing (Perrelin et al. 2016). The results were then averaged.
Determining insect abundance is useful as it provides a rough estimate of the relative abundances and species richness in an area (Skvarala et al. 2014). This can be used to determine the Mountain Bluebird’s movement pattern and it is also an indication of the potential prey and the amount of food availability (Power 1980). In the study, a total of four pitfalls were utilized and each were spaced at 50 m intervals. Each pitfall was located on the breeding territory along a linear transect. Small plastic cups were used to capture the insects. The preservative added to the inner cup included approximately 100 mL of propylene glycol (Skvarala et al. 2014). The pitfall design was established from day 1 of the brood rearing period to day 22. At day 22 the total number of insects in the pitfalls were counted and then averaged over the total number of traps per territory.
Only nestlings that fledged at least one offspring were included in the analysis. To calcuate the mass growth constant, the logistic model of M(x) = a/1 + be-cx was used. In this formula, M(x) was the mass at age x, a is asymptotic mass, b is the inflection point, and c is the growth rate constant. To calculate the growth rate constant of the 8th feather, the Gompertz model was used: T(x) ae- ebc-x. T(x) was the length of the structure at age x, a was asymptotic length, b was the inflection point, and c was the growth rate constant (Dawson and Bidwell 2005).
To determine the significance between breeding territories, an unpaired, type 2, t-test was conducted in excel. A degree of freedom of 47 was used when comparing the variables in the different breeding territories. When comparing the female per capita rates, male per capita rates and the larvae:grasshopper, the degree of freedom was 45 as data was not collected for two days due to poor weather. In all tests, ?=0.05.