Determining how site characteristics influence reproductive success can help guide conservation planning for declining wildlife populations. For colonially-breeding seabirds, nest survival and predation risk can be influenced by both colony and nest site characteristics such as colony size, nest density, and nest location. We evaluated reproductive success of a declining population of Roseate Terns (Sterna dougallii) in the U.S. and British Virgin Islands to identify primary causes of nest failure and investigate the influence of colony and nest site covariates on nest survival. In addition, we measured the influence of colony characteristics on chick provisioning to determine if resource competition in larger colonies affected foraging rates. Monitoring was conducted in 2017–18 via motion-triggered cameras, which enable 24-hour monitoring with minimal nest disturbance. Nest survival was defined by both hatch success and early chick success, defined as a chick surviving from hatching through the 4^th post-hatch day. We monitored 118 nests in 6 colonies over the 2 breeding seasons. We found that predation was the primary cause of nest failure. Both hatch and nest success increased with colony size, and neither nest survival nor predation probability were influenced by individual nest site characteristics. Provisioning rates were comparable among colony sites and did not vary with colony size. Our results suggest that directing management efforts toward enhancing colony size, rather than focusing on nest site characteristics, can be beneficial for tropical seabird conservation.

We monitored 6 tropical Roseate Tern colonies over 2 breeding seasons in 2017-18 in the US and British Virgin Islands to investigate the effects of colony and nest-site characteristics on nest survival, predation risk, and chick provisioning rates. Nests (n = 118) were monitered via remote motion activated nest cameras. Camera data was analyzed manually. Site covariates were measured at each individual nest site. Nest survival, predation, and provisioning data were all derived from camera imagery.

Byerlyetal2020_dailysurvival: dataset formatted for daily nest survival/predation analyses, with one row per individual nest per day of surviving from monitoring initated to either day of nest failure or last day monitored. Data collected in the US and British Virgin Islands, 2017-18.

Survival variables:

• Individual nest ID ("nest")
• Camera site ID ("cam")
• Colony size, in number of breeding pairs ("n")
• Colony site ("island")
• Colony site location ("terr")
• Date of each day/row ("date")
• Date of each day/row in Julian date format ("julian")
• Estimated date first egg per nest was laid ("lay_date")
• Estimated date first egg per nest was laid in Julian date format ("LD")
• Date first egg per nest hatched ("hatch_date")
• Date first egg per nest hatched in Julian date format ("HD")
• Nest age at start of initiation onward, in number of days old ("age")
• Whether or not one egg per nest successfully hatched, as 0 = no, 1 = yes ("hatch")
• Whether or not one chick per nest survived to 4 days post-hatching, as 0 = no, 1 = yes ("chick")
• Whether or not a nest survived the entire period from laying to 4 days post-hatching, as 0 = no, 1 = yes ("survive")
• Whether or not a nest was depredated, as 0 = no, 1 = yes ("pred")
• Nest monitoring interval in days ("t")
• Number of days monitored, starting from 1 (date of monitoring initiation) and ending at last day monitored ("obvs")

Nest site variables:

• Nest type ("type", as covered or bare [exposed])
• Substrate at nest ("ground")
• Vegetation type at nest ("veg")
• Nest placement within colony ("place")
• % cover at nest site ("per_cover")
• Tallest vegetation height at nest site in cm ("str_veg")
• Tallest rock height at nest site in cm ("str_rock")
• Maximum height of cover at nest site in cm ("max_str")
• Density of nests per camera site as number of nests per area of camera view in m² ("dens")
• Distance to nearest neighbor nest in m ("nn")
• Index of breeding Laughing Gull presence at colony ("nest_lagu")
• Index of breeding tern presence at colony ("nest_tern").

Byerlyetal2020_nestdata: contain the same data as Byerlyetal2020_dailysurvival, but formatted as one row per nest, with additional covariates:

Survival variables:

• Date nest monitoring began ("start_date")
• Date monitoring ended, either due to chick suriviving to the 4th day post hatch or the nest failing ("last_date")

Nest site variables:

• Cause of nest failure ("cause") as no failure ("NA"), predation ("P"), non viable egg ("NV"), abandoned ("A"), or unknown ("u")
• Type of predator ("pred_type") as hermit crab ("HC"), Laughing Gull ("LAGU"), roof rat ("RAT"), red-tailed hawk ("RTHA"), yellow-crowned night-heron ("YCNH"), Great Egret ("GREG"), Roseate Tern ("ROST"), or unknown predator ("U")

Byerlyetal2020_NestSurvivalModels: Annotated R code for modeling daily nest survival and predation rates

Byerlyetal2020_provisioningdata1.csv: chick provisioning dataset with one row per day per nest per nest for the first 4 days post chick hatch.

Variables:

• Individual nest ID ("nest")
• Year of monitoring ("year")
• Colony site ("island")
• Colony site location ("terr")
• Colony size, in number of breeding pairs ("n")
• Date of monitoring day ("date")
• Date of monitoring day in Julian date format ("Julian")
• Day of nest survival from 1-4 ("day")
• Period of chick age for days 1-2 ("early") and 3-4 ("late")
• Number of fish delivered to chick per day ("fish_n")
• Number of fish delivered to chick per hours monitored ("hourly")
• Mean estimated length of fish brought to chick in cm ("length")
• Mean estimated width of fish brough to chick estimated from dorsal to ventral sides ("width")
• Number of chicks per nest ("chick_n")

Byerlyetal2020_provisioningdata2.csv: chick provisioning dataset with one row per provisioning event per nest for the first 4 days post chick hatch

Variables:

• Individual nest ID ("nest")
• Camera site ("cam")
• Colony site ("island")
• Colony size, in number of breeding pairs ("n")
• Date of provisioning event ("date")
• number of chicks per nest ("chick_n")
• Day of nest survival from 1-4 ("day")
• Period of chick age for days 1-2 ("early") and 3-4 ("late")
• Time of provisioning event ("time")
• Hour in the day of provisioning event (standardized from 7:00 am ["1"] onward)
• type of prey brought to chick ("type") as fish ("F") or unknown ("NA")
• Category of fish type brought to chicks as dward herring ("DH"), false pilchard ("FP"), or other ("NA")
• Estimated length of fish brought to chicks in cm ("length")
• Estimated width of fish brough to chick estimated from dorsal to ventral sides ("width")
• Estimated volume of fish brought to chick as l x w ("volume")
• Length of fish via a standardized visual index developed by Shealer (1998) and based on the mean length of a roseate tern bill (~35 mm), with prey length then assigned to categories of tiny (≤18 mm), small (18–35 mm), medium (35–52 mm), and large (>52mm) ("shealer_length")
• Estimated wet mass of prey based on regression equation in Shealer (1998) where x is total length in millimeters (which we defined as the mean length in mm from each size category) and y is wet mass in grams: . This translated to index wet mass categories of tiny (≤0.1 g), small (0.1–0.4 g), medium (0.4–1.3 g), and large (>1.3 g).