Predator-Specific Effects on Incubation Behaviour and Offspring Growth in Great Tits

<div><p>In birds, different types of predators may target adults or offspring differentially and at different times of the reproductive cycle. Hence they may also differentially influence incubation behaviour and thus embryonic development and offspring phenotype. This is poorly understood, and we therefore performed a study to assess the effects of the presence of either a nest predator or a predator targeting adults and offspring after fledging on female incubation behaviour in great tits (<i>Parus major</i>), and the subsequent effects on offspring morphological traits. We manipulated perceived predation risk during incubation using taxidermic models of two predators: the short-tailed weasel posing a risk to incubating females and nestlings, and the sparrowhawk posing a risk to adults and offspring after fledging. To disentangle treatment effects induced during incubation from potential carry-over effects of parental behaviour after hatching, we cross-fostered whole broods from manipulated nests with broods from unmanipulated nests. Both predator treatments lead to a reduced on- and off-bout frequency, to a slower decline in on-bout temperature as incubation advanced and showed a negative effect on nestling body mass gain. At the current state of knowledge on predator-induced variation in incubation patterns alternative hypotheses are feasible, and the findings of this study will be useful for guiding future research.</p></div

Fledging Age.

<p>Fledging age (model estimations of mean ± SE) for offspring of mothers exposed to either control birds (C) or to predatory birds (P) before and during egg-laying, raised either under matching or mismatching conditions. Asterisk represents significant difference (<i>p</i><0.05).</p

Average number of on-and off-bouts (mean ± SE) from a GLMM with Poisson error structure.

<p>(a) Females incubating in increased nest predation risk environments (Pw) had a lower number of off-bouts/active day compared to the control. Females under an increased post-fledging predator risk (Ps) had a tendency for a lower number of off-bouts compared to the control (<i>P</i> = 0.008). (b) Females incubating in increased nest predation risk environments (Pw) had a significantly lower number of on-bouts/active day compared to the control. Females under an increased post-fledging predator risk (Ps) had a slightly but non significantly lower number of on-bouts compared to the control (<i>P</i> = 0.032).</p

Mass growth curves.

<p>Nestling mass on three measurement days (Mixed Effects Model estimations of mean ± SE). The shape of growth curves differed significantly according to the interaction between the treatments. When mothers were exposed to predation risk before or during ovulation, growth depended on offspring environment. When growing with predation risk, i.e. a matching environment, early growth rate increased (steeper slope between days 2–8) compared to mismatching conditions. Under mismatching conditions, the fast mass gain, as well as reaching asymptotic mass, was postponed. C = mothers exposed to control treatment; P = mothers exposed to predator treatment.</p

Summaries for hatching and fledging probability GLMs.

<p>Coefficients are untransformed and stem from a GLM with binomial (hatching probability) and Poisson (number fledged) errors. Laying and hatching are centered for ease of interpretation. Reference level for all models is a nest from the prenatal control-match treatments. Values for non-significant interactions are just before removal from the model. N2 =  number of nestlings on day 2 after first hatch. SE  =  Standard error.</p

ANOVA table for fledging age.

<p>Reference level for treatment coefficients is prenatal maternal control treatment and matching environment. Brood size and hatching date centred.</p

Nestling mass (mean±SE) on three measurement days.

<p>The shape of growth curves differed significantly according to the interaction between the treatments (<i>P</i><0.001). Growth was slower from day 3 to day 8 in the Ps group and from day 8 to day 15 in the Pw group. C = mothers exposed to control treatment; Ps = mothers exposed to the sparrowhawk predator treatment; Pw = mothers exposed to the weasel predator treatment.</p

ANOVA table of LMMs and GLMs for incubation behaviour.

<p>Terms retained in the final model are highlighted in bold. Values for non-significant interactions represent values just before removal (significance level for interactions <i>P</i> < 0.05). The reference level for the coefficients is the control treatment. CI are 95% lower and upper confidence limits.</p><p>ANOVA table of LMMs and GLMs for incubation behaviour.</p

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ANOVA table of LMMs for nestling and growth.

<p>Terms retained in the final model are highlighted in bold. Values for non-significant interactions represent values just before removal (significance level for interactions <i>P</i> < 0.05). The reference level for the coefficients is a female nestling under control treatment. CI are 95% lower and upper confidence limits.</p><p>ANOVA table of LMMs for nestling and growth.</p