DOI: 10.5061/dryad.31zcrjdx4
title: Data from: Hosts eject conspecific parasitic eggs according to the egg size in a passerine

description: Conspecific brood parasitism (CBP) is an alternative reproductive tactic
 in egg-laying species, where a female lays her eggs in the nest of a
 conspecific. In a population of spotless starlings (Sturnus unicolor),
 some eggs are ejected from the nest during CBP events. It is unclear
 whether this ejection is a parasitic strategy (a host egg is ejected) or
 an anti-parasitism strategy (the parasitic egg is ejected). To clarify
 this, we genotyped the eggs ejected on the ground and found that 100 % of
 them were parasitic. Egg discrimination might be based on tactile or
 visual cues, and we hypothesized that egg size could be used by hosts to
 eject parasitic eggs. We conducted experiments in the field using dummy
 eggs of varying sizes. The results showed that starlings were more likely
 to eject eggs if they were smaller than their own eggs. In contrast, no
 significant pattern of egg ejection was observed for larger eggs. Our
 results suggest that starlings use egg volume recognition as an
 anti-parasitism strategy to avoid the costs of parasitism. Whether this is
 a frequency-dependent strategy is worth further studies.
description: We simulated 63 parasitism events by adding one dummy egg
 (3D-printed plastic or plaster eggs) during first and replacement clutches
 in 2023. To make sure the effect of dummy egg addition, if any, was not
 due to the egg material, we compared the ejection rate between natural
 eggs (N = 74), plaster eggs (N = 14), and plastic eggs (N = 49). For the
 natural type, we used natural parasitism in the population and quantified
 the percentage of egg ejection. As most of the ejected eggs were broken,
 we do not have data on their size. All types of nests (with natural vs.
 artificial parasitism) were checked daily and suffered similar
 experimental disturbances. To reproduce natural
 parasitism, we placed a dummy egg in 54 nests on day three of laying (nine
 nests were used twice: once during the first clutch and once during the
 replacement clutch). We used only nests that were presumed not to have
 been previously parasitized and that presented a regular laying pattern,
 i.e. nests with three natural eggs on day three. We measured the length
 and the width of these three eggs with digital calipers (Mitutoyo, Japan)
 to the nearest 0.01 mm. Egg volume was calculated by the formula: 0.51 ×
 length × width² [39]. According to the mean volume, we chose a dummy egg
 to put in the nest. We had five categories: minus 2
 cm<sup>3</sup>, minus 1 cm<sup>3</sup>, control,
 plus 1 cm<sup>3</sup>, plus 2 cm<sup>3</sup> –
 with respect to the mean volume of eggs already in the nest. Nests were
 randomly assigned to each category (Table 3). To test a nest under a
 specific category, we selected the dummy egg that most closely matched the
 target volume defined by the assigned category (see Table 3 for
 details). We then followed nests daily to detect
 ejection behavior. If the dummy egg was not in the nest box anymore, we
 considered it “ejected”. On the contrary, if on day six of laying, it was
 still inside the nest box, we considered it “accepted”, and we removed it
 from the nest box. 
description: # Data from: Hosts reject conspecific parasitic eggs according to the egg
 size in a passerine. Dryad digital repository
 [https://doi.org/10.5061/dryad.31zcrjdx4](https://doi.org/10.5061/dryad.31zcrjdx4) ## Description of the data and file structure We experimented on egg ejection behavior in a population of spotless starlings (*Sturnus unicolor*). We simulated conspecific brood parasitism events using artificial eggs of different sizes. We analyzed the ejection of these eggs as a function of the relative difference in volume, in length and in width between the natural eggs already present in the nest and the artificial egg added. ### Files and variables #### File: Egg\_size\_reject.csv **Description:** dataset containing the relative size difference between the added egg and the other eggs in the nest. ##### Variables * nest: the identity of the nest in which the egg is added. * laying: the first or replacement clutch wave of the season. * condition: the experimental treatment (- 2 SD, - 1 SD, control, + 1 SD, + 2 SD). For natural parasitism, cells contain "NA" (as they were not included in the experiment). * vol_min [cm3]: volume of the smallest host egg among the three eggs. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * vol_max [cm3]: volume of the biggest host egg among the three eggs. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * vol_rel [cm3]: relative difference in volume between the egg added and the eggs already in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * vol_abs [cm3]: absolute difference in volume between the egg added and the eggs already in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * dummy_vol [cm3]: volume of the dummy egg added in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * length_rel [mm]: relative difference in length between the egg added and the eggs already in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * length_abs [mm]: absolute difference in length between the egg added and the eggs already in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * width_rel [mm]: relative difference in width between the egg added and the eggs already in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * width_abs [mm]: absolute difference in width between the egg added and the eggs already in the nest. For natural parasitism, cells contain "NA" (as they were not included in the experiment). * egg_type: material of the egg (natural, plastic, or plaster). * response: accepting (0) or ejecting (1) the egg. * female: ring of the host female when known. For unknown females, cells contain "NA". * age [years]: age of the host female. For unknown females, cells contain "NA". ## Code/software Statistical models that were used to analyze our data.