DOI: 10.5061/dryad.280gb5mmp
title: Rainfall and nest site competition delay Mountain Bluebird and Tree Swallow breeding but do not impact productivity

description: 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. 
description: Please see associated manuscript for complete methodologies
 (Drake and Martin 2020) <strong>Winter and
 migration weather data source:</strong> The NCEP/NCAR 40-Year
 Reanalysis Project: March, 1996 BAM; NOAA National Center for
 Environmental Prediction Reanalysis I
 <strong>Breeding site weather data source:
 </strong>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) <strong>Field data
 notes:</strong> Breeding data for
 Mountain Bluebird<i> </i>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 (<i>Pseudotsuga</i> <i>menziesii</i>
 var. <i>menziesii</i> ), lodgepole pine (<i>Pinus
 contorta </i>var. <i>latifolia</i>), and white and
 Englemann spruce hybrids (<i>Picea glauca</i> x
 <i>engelmannii</i> )) with trembling aspen (<i>Populus
 tremuloides</i>) 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.
description: Please see associated manuscript for complete methodologies
 (Drake and Martin 2020)