DOI: 10.5061/dryad.gf1vhhmkb
title: Females compensate for moult-associated male nest desertion in the Hooded Warbler

description: Uniparental offspring desertion occurs in a wide variety of avian taxa and
 usually reflects sexual conflict over parental care. In many species
 desertion yields immediate reproductive benefits for deserters if they can
 remate and breed again during the same nesting season; in such cases
 desertion may be selectively advantageous even if it significantly reduces
 fitness of the current brood. However, in many other species parents
 desert late-season offspring when opportunities to renest are absent. In
 these cases, any reproductive benefits of desertion are delayed, and
 desertion is unlikely to be advantageous unless the deserted parent can
 compensate for the loss of its partner and minimize costs to the current
 brood. We tested this parental compensation hypothesis in Hooded Warblers
 Setophaga citrina, a species in which males regularly desert late-season
 nestlings and fledglings during moult. Females from deserted nests
 effectively doubled their provisioning efforts, and nestlings from
 deserted nests received just as much food, gained mass at the same rate,
 and were no more likely to die from either complete nest predation or
 brood reduction as young from biparental nests. The female provisioning
 response, however, was significantly related to nestling age; females
 undercompensated for male desertion when the nestlings were young, but
 overcompensated as nestlings approached fledging age, probably because of
 time constraints that brooding imposed on females with young nestlings.
 Overall, our results indicate that female Hooded Warblers completely
 compensate for male moult-associated nest desertion, and that deserting
 males pay no reproductive cost for desertion, at least through the point
 of fledging. Along with other studies, our findings support the general
 conclusion that late-season offspring desertion is likely to evolve only
 when parental compensation by the deserted partner can minimize costs to
 the current brood.
description: <b>Study site and general methods</b>
 This study was conducted in northwest Pennsylvania, USA, at
 Hemlock Hill Field Station in Crawford County. A colour-ringed population
 of Hooded Warblers has been under continuous study at Hemlock Hill since
 2010 (Mumme 2014, 2018) and Hemlock Hill has been the site of numerous
 previous studies of this species (Chiver et al. 2011). After spending the
 winter in Central America, Hooded Warblers arrive at Hemlock Hill in early
 May, with the first nests initiated in mid-May and the final nesting
 attempts of the season started in late July. Because breeding adults
 typically begin their annual moult in late July or August, moult can
 overlap extensively with late-season nesting and parental care,
 particularly for males (Mumme 2018). Intensive efforts are made each year
 to capture and colour-ring all breeding adults and to locate and monitor
 all nests. Most of the data reported here were collected from late-season
 nesting attempts in July and August, 2018-2019.
   <b>Provisioning of
 nestlings</b> We collected data on provisioning
 of nestlings from 37 different late-season nests when nestlings were 1-7
 days old; no data were used from hatching day (day 0) or from nests with
 young older than 7 days old, as fledging typically occurs at age 8-9 d. We
 performed 90 individual nest observations (never more than 1 per day) from
 the 37 different nests, including 44 observation bouts conducted from
 hides established near some nests, and 46 observations obtained from
 high-definition video recordings. The mean duration of the 90 observation
 bouts was 96 min (range 22-221 min). For each observation bout we recorded
 the number of feeding visits made by the colour-ringed female and male,
 and converted the counts to total provisioning rates (feedings
 hr<sup>-1</sup>) and nestling per capita feeding rates
 (feedings nestling<sup>-1</sup> hr<sup>-1</sup>).
 Most of the provisioning data were collected July 11 - August 8 in 2018
 (32 observations) and 2019 (56 observations), but data from 2 additional
 late-season video nest recordings made in 2016 were also included in the
 dataset. All observation bouts were conducted during dry weather, with
 most conducted 0600-1000 hrs. We collected data from 27
 undeserted nests where males fed nestlings during all watches, 5
 completely deserted nests where males never fed nestlings during the
 duration of the nestling period, and 5 partially deserted nests, where
 males were present and feeding nestlings at least 1 day during the
 nestling period, but were absent and not provisioning nestlings on other
 days. At 2 of the partially deserted nests, males initially fed nestlings
 but then deserted midway through the nestling period. At the other 3
 partially deserted nests, moulting males did not feed young nestlings but
 fed older nestlings before permanently deserting immediately after the
 young fledged. Collectively, the 90 separate nest observations included 42
 observations where males were not provisioning nestlings, and 48 where
 they were.    <b>Nestling
 growth and survival</b> We investigated the
 effects of male desertion on nestling growth by collecting data on body
 mass of nestlings from late-season nests, July 10 - Aug 9, 2018-2019. We
 weighed nestlings daily used a 20-g spring scale and recorded nestling
 mass to the nearest 0.1 g. Because Hooded Warbler nestlings more than 4
 days old will sometimes fledge prematurely if handled by humans, no
 nestlings older than day 4 were weighed, and we report data for nestling
 mass for days 1-4. Our dataset includes 233 nestling weights collected
 from 34 nests, including 4 completely deserted nests (46 individual
 nestling weights), 5 partially deserted nests (44 nestling weights), and
 25 biparental nests (143 nestling weights). To examine
 the effect of male nest desertion on survival of nestlings from hatching
 to fledging, we used a sample of nests from the years 2013-2019 that were
 initiated after July 10 where at least some eggs hatched and male feeding
 status at the nest was unambiguously determined. A total of 38 nests,
 including 13 completely deserted nests, 5 partially deserted nests, and 20
 undeserted nests, fulfilled these criteria. Eight of these nests were
 destroyed by predators before the nestlings fledged, but the remaining 30
 nests were used in an analysis of brood reduction and survival to
 fledging.    <b>Statistical
 analysis</b> Data files used in the analyses are
 openly available in the repository Dryad (Mumme 2020). All statistical
 analyses were conducted using JMP Pro 12.2 (SAS Institute 2016). Data on
 provisioning rates were analyzed with general linear mixed models using
 the Mixed Model personality of JMP Pro’s Fit Model platform and focused on
 3 different rates: total provisioning rate (total feedings
 hr<sup>-1</sup>), nestling per capita feeding rate (feedings
 nestling<sup>-1</sup> hr<sup>‑1</sup>), and female
 provisioning rate (female feedings hr<sup>-1</sup>). Number of
 nestlings, nestling age (d), and male feeding status that day (male
 present and feeding nestlings or absent and not feeding young) were
 entered into models as fixed effects. To account for multiple observations
 from the same nest, a unique nest identifier was included in all models as
 a random effect. All provisioning rates were log-transformed to equalize
 variance, and models were validated by ensuring that residuals were
 approximately normally distributed, as determined by examination of normal
 quantile plots and application of the Shapiro-Wilk test for
 normality.  Data on nestling body mass were also
 analyzed with a general linear mixed model. The number of nestlings
 present in the nest and nestling age were included as fixed effects, as
 was nest desertion type — no male desertion, partial male desertion, or
 complete male desertion. Because of the very rapid pace of nestling
 growth, the time of day that nestlings were weighed was also included as a
 fixed effect. A unique nest identifier was included as a random effect to
 account for multiple repeated measures from the same nest. The resulting
 model was validated by ensuring that residuals were approximately normally
 distributed, as described above. To examine the effects
 of male desertion on brood reduction and survival of nestlings to
 fledging, the proportion of hatched young that fledged for each nest was
 used as the dependent variable. We compared the proportion fledged for
 biparental, partially deserted, and completely deserted nests with a
 nonparametric Kruskal-Walis test.