Optimizing breeding phenology, an important aspect of fitness, is complex for migratory species as they must make key timing decisions early, and remotely, from breeding sites. We examined the role of weather (locally and cross-seasonally), cavity availability, and competitive exclusion in determining among-year variation in breeding phenology over 17 years for two migratory, cavity-nesting birds: Mountain Bluebirds (Sialia currucoides; n = 462 nests) and Tree Swallows (Tachycineta bicolor; n  = 572) using natural tree cavities in British Columbia, Canada. We assessed weather effects within the winter and migratory range and at our study sites. We quantified competition as the proportion of cavities occupied by European Starlings (Sturnus vulgaris) (for both species) and Mountain Bluebird (for Tree Swallow only) in each year. For 229 bluebird and 177 swallow nests with known fates, we tested whether late years resulted in reduced productivity. Although the effects were small, heavy rainfall and strong diurnal westerly winds during migration were associated with breeding delays for Mountain Bluebirds. However, cavity availability (earlier breeding with increases) had a 5-8X greater effect on timing than migratory conditions. There was no evidence that starling competition delayed bluebirds. In Tree Swallows, greater local daily rainfall was associated with delayed breeding, as was starling abundance (the effect of starlings was 1.4X times smaller than that of rainfall). Neither bluebird abundance nor cavity availability changed swallow phenology. Neither species showed reduced productivity in late breeding years. In both species, individuals that bred late relative to conspecifics within-year had smaller clutches and greater probability of nest failure.  We conclude that breeding ground conditions, particularly cavity limitation and local rainfall (for swallows), are important drivers of breeding phenology for our focal species, but that the productivity cost of late years, at least for Tree Swallows, is minimal. 

Please see associated manuscript for complete methodologies (Drake and Martin 2020)

Winter and migration weather data source: The NCEP/NCAR 40-Year Reanalysis Project: March, 1996 BAM; NOAA National Center for Environmental Prediction Reanalysis I

Breeding site weather data source: Environment and Climate Change Canada weather station Williams Lake A (WMO ID 71104; 52.1800N, 122.0500W; elevation 939.7m; http://climate.weather.gc.ca)

Field data notes:

Breeding data for Mountain Bluebird and Tree Swallow were collected between 1995 and 2011 at two study sites ~38 km apart, “Riske Creek” (52.0025°N, 122.4116°W) and “Knife Creek” (52.0068°N, 121.8619°W), near Williams Lake in south-central British Columbia, Canada. This region is part of the warm and dry Interior Douglas-fir biogeoclimatic zone. In this study, the Riske Creek site consisted of 16 mixed conifer stands (Douglas-fir (Pseudotsuga menziesii var. menziesii ), lodgepole pine (Pinus contorta var. latifolia), and white and Englemann spruce hybrids (Picea glauca x engelmannii )) with trembling aspen (Populus tremuloides) within a grassland-wetland matrix. Knife Creek consisted of 11 mixed conifer stands with some deciduous riparian zone. Stands ranged from 7-32 ha in size. No nest boxes were present at either site and all nesting was done in natural tree cavities. Mountain Bluebird and Tree Swallow were two of a total of 32 cavity-nesting bird species found within the study sites over the monitoring period (Wesolowski and Martin 2018).

During the 1995 to 2011 period, systematic searches were conducted between May 1 and July 31. These surveys were conducted for approximately 6–7 h/stand/week by walking the entirety of each stand and examining previously identified nest-sites and following birds (Aitken and Martin 2007). The number of stands monitored increased between 1995 and 1998, but thereafter survey effort was equivalent. The majority of nests were found in the laying or early incubation stage (Koch et al. 2012).  Active nesting cavities were identified based on adult behaviour (carrying nesting material/food or entering/exiting cavities) or the vocalizations of young (Martin et al. 2004). All cavities at the study site were given unique identifiers and their persistence and use was recorded in each year of the study. Between 1995 and 2004, we accessed active cavities up to 5.2m above the ground using ladders and mirrors to assess the stage of breeding. One hundred and twelve nests (10% of the total dataset) were inaccessible during this 10-year period. These inaccessible cavities were recorded as active based on adult behaviour or begging chicks but could not be assigned a clutch initiation date or hatch date in the field. After 2004, all nests were accessible using a video camera mounted on a pole (TreeTop Peeper; Sandpiper Technologies, Manteca, CA, USA) to identify the stage of breeding in cavities up to 15m above the ground (Edworthy et al. 2012). Nests were checked, on average, every 5 days during their active phase and, when possible, clutch initiation date was determined using observed clutch size (if nests were found during laying) or observed final clutch size and hatch date (if nests were found during incubation), combined with a 1 egg/day laying interval and a 14- or 15-day mean incubation period for Mountain Bluebird and Tree Swallow, respectively (Koch et al. 2012). Nesting attempts found after young had hatched were not assigned a clutch initiation date in the field. Nest activity periods were recorded as the first and last days that each cavity was observed as actively being used (containing fresh nesting material, eggs, or nestlings).

The number of fledglings produced by each nest was recorded when fledging was observed or when chicks were old enough at the last nest check to survive out of the nest and where there was no evidence of predation within the cavity or around the nesting site when the nest was found empty at the penultimate check. The presence of adults feeding chicks in the immediate nest area was also used as an indicator of success where fledging was not directly observed.

Please see associated manuscript for complete methodologies (Drake and Martin 2020)