DOI: 10.5061/dryad.gb5mkkwr0
title: Proximate and evolutionary sources of variation in offspring energy expenditure in songbirds

description: Aim: Understanding variation in offspring energy expenditure is important
 because energy is critical for growth and development. Weather may exert
 proximate effects on offspring energy expenditure, but in altricial
 species these might be masked by parental care and huddling with siblings.
 Such effects are particularly important to understand given changing
 global weather patterns, yet studies of wild offspring in the presence of
 parental care are lacking. Offspring energy expenditure may also vary
 among species due to evolved responses to environmental selection
 pressures, requiring studies at both proximate and ultimate levels.
 Location: USA, South Africa, Malaysia. Time period: 2016-2019. Major taxa
 studied: Songbirds. Methods: We used the doubly-labeled water technique to
 estimate nestling daily energy expenditure of 54 songbird species across
 three continents. We used Bayesian phylogenetic mixed models to test
 proximate and evolutionary causes of variation in offspring energy
 expenditure while accounting for phylogeny and phylogenetic uncertainty.
 Results: Offspring energy expenditure increased with more rainfall and
 colder air temperatures, but decreased among offspring in broods with more
 siblings. Across species, nestling and adult mortality, but not growth
 rate, were positively associated with offspring energy use. Main
 conclusions: Weather had clear proximate effects on offspring
 energy expenditure and parents were either unable or unwilling to fully
 offset these effects. However, the decrease in offspring energy use when
 huddling with more siblings demonstrated a modulating effect of life
 history traits. For example, high nest predation rates favor reduced
 parental care and can force offspring to spend more energy coping with
 environmental conditions. Furthermore, reduced energy expenditure is
 thought to facilitate increased longevity, which is increasingly realized
 with lower extrinsic mortality rates, providing an explanation for the
 positive association between adult mortality and offspring energy
 expenditure. Ultimately, both proximate and evolutionary influences need
 to be considered to better understand causes of offspring energetics.
description: We used the two-sample doubly-labeled water
 technique administered on nestling songbirds of 54 species across 3
 continents. Nestlings were sampled on pin break, the day the 8th primary
 feathers broke from their sheaths. Blood samples were spun to separate
 plasma, and flame-sealed in the field. At the end of each season, samples
 were microdistilled to pure water for laboratory analyses. Isotope
 concentrations were analyzed using a Picarro L1102-I or a LGR DLT-100
 liquid water isotope analyzer (Picarro Inc., Santa Clara, CA, USA) at the
 University of New Mexico. Data were normalized to the IAEA water standard
 VSMOW. We estimated CO<sub>2</sub> production using the
 equation by Nagy (1983, Eqn. 1). CO<sub>2</sub> production was
 then converted to DEE using conversion factors based 26.3 J/ml
 CO<sub>2</sub> for Arizona, and 26.7 J/ml
 CO<sub>2</sub> for South Africa and Malaysia. Air temperature
 and rainfall were collected in Arizona and South Africa using an Onset
 data logging rain gauge with a tipping bucket and air temperature
 logger. Weather variables in South Africa were provided by Eskom Holdings
 from a meteorological station at the field site. To account for the linear
 decrease in temperature with increasing elevation, we used a predicted
 temperature value at the Malaysia site based on the elevation of each nest
 and the lapse rate of 0.55°C*100 m<sup>-1</sup>. Growth rate
 constant K estimates came from Martin 2015, nest predation rates were
 modeled using the logistic exposure method from Martin 2015. Adult
 mortality estimates came from Martin et al. 2015a and Martin et al. 2017.
 We removed far outliers that were ± 3*IQR for each species following
 Tukey’s boxplot rule for far outliers.
description: Data file shows each data
 point for daily energy expenditure of nestling songbirds. Sites are AZ
 (Arizona), SA (South Africa), and MY (Malaysia). Nest-ID is the unique
 nest identifier. DEE and nestling body mass are
 log<sub>10</sub>-transformed. Brood_Size is the number of
 nestlings in the nest during the 24 or 48-hour sampling interval. Min_Temp
 is the lowest temperature over the 24 or 48 hour sampling period in °C.
 Sum_Rain is the total rainfall over the 24 or 48-hour sampling period in
 mm. nstl_pred is the daily nest predation rate estimated per species.
 k_mass is the growth rate constant K for body mass estimated per species.
 Mort is the annual mortality rate estimated per species. Animal is the
 latin binomial name used for phylogenetic analyses. All variables with an
 underscore_s (e.g., L_Mass_s) have been mean-centered and scaled to units
 of standard deviation.