As species shift their ranges and phenology to cope with climate change, many are left without a ready supply of their preferred food source during critical life stages. Food shortages are often assumed to be driven by reduced total food abundance, but here we propose that climate change may cause short-term food shortages for foraging specialists without affecting overall food availability. We frame this hypothesis around the special case of birds that forage on flying insects for whom effects mediated by their shared food resource have been proposed to cause avian aerial insectivores’ decline worldwide. Flying insects are inactive during cold, wet, or windy conditions, effectively reducing food availability to zero even if insect abundance remains otherwise unchanged. Using long-term monitoring data from a declining population of tree swallows (Tachycineta bicolor), we show that nestlings’ body mass declined substantially from 1977 to 2017. In 2017, nestlings had lower body mass if it rained during the preceding three days, though females increased provisioning rates, potentially in an attempt to compensate. Adult body mass, particularly that of the males, has also declined over the long-term study. Mean rainfall during the nestling period has increased 9.3±0.3 mm/decade, potentially explaining declining nestling body mass and population declines. Therefore, we suggest that reduced food availability, distinct from food abundance, may be an important and previously overlooked consequence of climate change, which could be affecting populations of species that specialize on foraging on flying insects.
Long term nestling body size data
10-16 day old tree swallow nestlings body size measurements from 1977-2017. Column names are as follows: nestlingID = Unique ring Canadian Wildlife Service number identifying individual nestlings, nestID=Unique code for each nest, year= year, age=number of days since hatching (e.g., on the hatch date, age=0) in days, hatchdate=date most of the nestlings hatched (tree swallows typically hatch synchronously) (Julian date), hatchsize=number of hatched nestlings, fledgedate=date most of the nestlings fledged or the nest failed (Julian date),
fledgesize =number of nestlings that fledged, mass=nestling body mas (g), tarsus=nestling tarsus length (mm), ninprim= nestling wing chord (mm), year2= decades since 1977 (the first year nestlings were measured)
Long term adult body size data
Adult body size records from 1983-2017. Column headings are as follows: band=unique ring Canadian Wildlife Service number identifying individual birds, year=year , sex=male (M) or female (F), age=males are listed as AHY (after hatch year) when caught as adults initially. Females may be aged by plumage as SY (second year, a one-year-old bird) or ASY (after second year, and 2+ years old). Birds caught subsequently are listed based on their age as first capture (e.g. A3Y (after third year), 4Y (four years old), mass=adult’s body mass (g), tarsus=adult’s tarsus length (mm), wingChord=adult’s wing chord (mm), nestID=nest box identifier, not unique, laydate=date the first egg was laid (Julian date), incdate=date the last egg was laid, when incubation begins (julian date), hatchdate=date most of the nestlings hatched (tree swallows typically hatch synchronously) (Julian date), fledgedate=date most of the nestlings fledged or the nest failed (Julian date), dateMeas2=date mass, tarsus, and wing chord measurements were taken (Julian date), year2=years since 1975, when adults were first captured, diff=days between when eggs were first laid and when the bird was measured (dateMeas2-laydate), Period=Was the nest in the incubation stage or the nestling stage?
Nestling growth data 2017
Detailed nestling growth records with weather conditions in 2017. Nestlings were measured on day 2, 4, 6, 8, 10 and 12. Column headings are as follows: NestlingID=unique ring Canadian Wildlife Service number identifying individual nestlings, NestID=unique code for each nest, HatchSize=number of hatched nestlings, HatchDate=date most of the nestlings hatched (tree swallows typically hatch synchronously) (Julian date), FledgeDate=date the nest fledged (Julian date), FledgeSuccess=0 if this nestling did not fledge, 1 if it did, whyFail=If the nest failed, lists the cause, either “NESTLINGS DIED” if nestlings were found dead in the nest, or “PREDATION” if no nestlings were found, Renest=lists whether nest was a renest or not. All nests used in this analysis were the first nest of the pair, Mass=nestling body mass (g), Tarsus=nestling tarsus length (mm), Wingchord=nestling wing chord (mm), Age=number of days since hatching (e.g., on the hatch date, age=0), MaxTemp3day=mean value of the maximum daily temperatures during the preceding three days (°C), MeanTemp3day=mean value of the mean daily temperatures during the preceding three days (°C), MeanWindspeed3day=Mean value of the mean windspeed (5:30am-8pm) during the preceding three days (m/s), TotalRainFall3day=total rainfall during the preceding three days (mm), ThermoReg=thermoregulatory strategy employed by nestlings of this age (poikilothermic (0-6 days old), intermediate (7-8 days old), or endothermic (9+ days old))
Provisioning rate data 2017
2017 provisioning rates for tree swallows with weather data. Column headings as follows: boxID=nest box ID (not unique), ffv=female provisioning rate (feeding visits/hr), mfv=male provisioning rate (feeding visits/hr), NestlingsAlive=The number of nestlings in the nest on the date provisioning rate was measured, Date=Date provisioning rate was measured (YYYY-mm-dd), meanwindspeed=mean windspeed from 5:30am-8pm on the day we measured provisioning rate (m/s), MaxTemp=maximum daily temperature on the day we measured provisioning rate (°C),
TotalRain_3day=total rainfall in the three days preceding when we measured provisioning rate (mm), TotalRain_3day2= “Rain” if it had rained more than 3mm in the preceding days, otherwise “None”
Long term nestling body size analysis
R code for the long term nestling body size analysis
Long term adult body size analysis
R code for the long term adult body size analysis
Nestling growth analysis 2017
R code for the nestling growth analysis in 2017
Provisioning rate analysis 2017
R code for the provisioning rate analysis in 2017