1. Corticosterone is a steroid hormone integral to a variety of physiological pathways and is strongly associated with the vertebrate stress–response. In avian species, circulating corticosterone is sequestered into developing feathers and is used as an indicator of energy allocation during feather growth and widely applied in conservation physiology.

2. The northern spotted owl (Strix occidentalis caurina) is a federally threatened old–growth forest obligate of conservation concern endemic to the Pacific Northwest of the United States and Canada. The effects of landscape characteristics and individual variation on early development in spotted owls remain unstudied despite long recognition of this knowledge gap and its potential importance to species conservation.

3. We quantified corticosterone concentrations in 4,720 feathers from 1,056 juvenile spotted owls across seven study areas between 2001 and 2017. We used an information–theoretic approach to examine the environmental and individual factors related to feather corticosterone in juvenile spotted owls as an indicator of challenges during early development.

4. Feather corticosterone was positively related to temperature and precipitation, and negatively related to juvenile mass at banding. We found strong support for an interaction between mass and precipitation, with greater amounts of precipitation being associated with higher levels of feather corticosterone in lighter juveniles. The temperature and precipitation metric with the strongest relationship with feather corticosterone occurred during the fledging period, suggesting that this period presents an energetic challenge for juvenile spotted owls. Greater juvenile mass decreased the effect of precipitation, suggesting that greater mass was important for juveniles to maintain homeostasis during fledgling.

5. Feather corticosterone in juvenile spotted owls provided insights to the challenges faced during early development, adding to our understanding of spotted owl life history and potential for population recovery.

Crews monitored historic spotted owl territories annually between 15 March and 31 August 2001–2017, capturing and banding fledged juveniles for individual identification. During banding observers collected contour feathers from the belly, breast, back, and head of juveniles and stored them in plastic bags at room temperature (19 – 22° C). Sample collection was opportunistic and to minimize harm to owls, the area of the body and number of feathers collected varied by natural feather loss, but 4–12 contour feathers was typical. All feathers were fully grown at the time of collection.

To minimize potential bias, we standardized feather CORT values by total sample mass (picograms CORT per milligram of feather). We used radioimmunoassays to estimate corticosterone in feathers between 5–90 mg total mass. Five percent of the samples were greater than 90 mg and were divided into two or three parts of approximately similar mass. Nearly all feathers were entirely or mostly plumulaceous.

Following the protocol established by Bortolotti et al. (2008), we removed each calamus, measured along the rachis to the nearest mm, and then weighed each feather to the nearest tenth of a milligram. We cut the feather into pieces less than five mm² and placed them in 20 ml test tubes with 7 ml of high-performance liquid chromatography grade methanol (VWR International, Radnor, Pennsylvania). We placed samples in a sonicating water bath at room temperature with a cap to limit evaporation for 30 minutes and then moved them to a shaking water bath at 50° C overnight. We used individual 23 cm disposable glass Pasteur pipettes to transfer each sample extract to 14 ml test tubes and rinsed the original feather sample with an additional 3.0 ml of methanol for two hours and that was added to the sample in the 14 ml test tube. We dried each sample under an evaporator rack with pressurized air in a water bath at 40° C. We reconstituted the dried samples in 250 ml of buffer solution, vortexed the samples, and then refrigerated them overnight. We then aliquoted the samples into duplicate 5 ml tubes and performed radioimmunoassays following manufacturer’s instructions (MP Biomedicals, LLC; Immunochem^TM Double antibody Corticosterone ¹²⁵I RIA Kit, Cat. No. 07-120103), except we used half-volumes. We used a parallelism test of two pool samples sequentially diluted 1:1 (20% binding) to 1:64 (90% binding) to ensure samples fell within the quantitative range of the assay and that our feather mass was adequate to achieve ~ 50% binding. Both samples were parallel to a standard curve and a 1:4 dilution was adopted. Inter-assay coefficient of variation was 9.2% across seven assays and intra-assay coefficient of variation was 1.9%. For samples that were subset and used to estimate extraction variation, we used the mean feather CORT for subsequent analyses.

For the health and safety of some owls, they were released prior to obtaining a mass measurment. These owls were populated with the study area specific means.

Description of Data within NSOfeatherCORT.

ID: Individual ID number that is specific to a different owl in the data set
Collection Year: Hatch year fo rht e owl in which feathers and other data were collected
NN: Nest number that is specific to territory and year to account for auto-correlation of nest mates, but also broods from previos years of the same adults on the same territory
MASS: Mass in grams recorded during banding
mCORT: designated as mean CORT for any owl that had enough feathers collected to subset into multiple samples. Otherwise denotes the corticosterone (picograms/mg feather) extracted from a feather sample.
lCORT: natural lo-g-transformed corticosterone
STUDY: The study area on which the juvenile was reared
rBO: range-wide average annual proportion of historic spotted owl terriories in which a barred owl was detected at least once on territories surveyed three or more times within the breeding season.
xBO: Study area specific annual proportion of historic spotted owl terriories in which a barred owl was detected at least once on territories surveyed three or more times within the breeding season.
PPT 1-7: Annual study area specific mean of daily precipitation (mm) in 8 different 2-week periods: 1: April 10–April 23; 2: April 24–May 7; 3: May 8–May 21; 4: May 22–June 4; 5: June 5–June 18; 6: June 19–July 2; 7: Jul 3–July 17; 8: July 18–July 31
Tmean 1-7: Study area specific mean of daily mean temperatures (°C) in in 8 different 2-week periods: 1: April 10–April 23; 2: April 24–May 7; 3: May 8–May 21; 4: May 22–June 4; 5: June 5–June 18; 6: June 19–July 2; 7: Jul 3–July 17; 8: July 18–July 31
Meas: The number of known years a juvenile’s father had occupied a territory, considered their number of breeding seasons of experience at a site.
Feas: The number of known years a juvenile’s mother had occupied a territory, considered their number of breeding seasons of experience at a site.
NR FOREST: The proportion of 30 x 30 m pixels in a juvenile’s natal territory covered in forest designated as suitable for nesting/roosting in the year they hatched.