DOI: 10.5061/dryad.9zw3r22cp
title: Empirical evidence of different egg morphs that match host eggs in the brush cuckoo (Cacomantis variolosus)

description: One of the most efficient defences against obligate brood parasitism in
 birds is egg ejection, where a host recognises and removes the parasitic
 egg from the nest. This defence often selects for egg mimicry in parasitic
 species to reduce the likelihood of egg ejection. If a parasite uses
 multiple host species with distinctive egg types, this could lead to the
 evolution of egg gentes (host-specific egg types) in the parasite. There
 is observational evidence that the brood parasitic Brush Cuckoo
 (Cacomantis variolosus) might exhibit egg gentes, but there has been no
 objective study conducted to determine how closely eggs of this cuckoo
 species resemble those of its hosts from a bird’s visual perspective.
 Using objective measurements to quantify egg appearance, we found that
 Brush Cuckoos exhibit at least two egg morphs that closely match the eggs
 of two of its primary hosts in colour, luminance and volume. While the
 determination of actual egg gentes in the Brush Cuckoo was beyond the
 scope of our study, our results are a first and necessary step in
 determining whether egg gentes might exist in this species. We suggest at
 least a third egg morph matching another primary host (or at least the
 genus of that host) might exist, but more data would be necessary to
 confirm this. Additionally, we provide a mechanism researchers can use to
 help distinguish between Brush Cuckoo eggs that are closely matched to
 their host eggs for future studies in this system.
description: We are providing three types of data in this dataset: average
 spectral reflectance of eggs (the average for each egg measured), egg
 pattern energy (for each egg measured) and egg volume (for each egg
 meausred). These data could be used in further analyses by other
 researchers to calculate values such as JNDs, photon catch cone values,
 etc. Eggs were measured at the Australian National Wildlife Collection,
 CSIRO, ACT in 2019. Each egg is identified with the clutch ID from the
 museum. Abbreviatons of species names are as follows: BRCU = Brush Cuckoo;
 BRHE = Brown-backed Honeyeater; GRFA = Grey Fantail; LEFL = Leaden
 Flycatcher; RUFT = Rufous Fantail; RBFA = Red-backed Fairy-wren; REFL =
 Restless Flycatcher; BBHE = Bar-breasted Honeyeater; RBHE = Rufous-banded
 Honeyeater. Data of minor host eggs and cuckoo eggs from minor host nests
 were only included for the spectral reflectance dataset. Methods of how
 each dataset was collected are detailed below and can also be found in the
 article associated with this dataset. We measured one
 cuckoo and one host egg from 32 parasitised clutches. We took objective
 measurements of egg colour and luminance by measuring their spectral
 reflectance from 300-700 nm using an Avantes AvaSpec-2048 fiber optic
 spectrometer, an AvaLight-DHc light source, an FCR-7UV200-2-ME reflection
 probe with a 6.35 mm diameter, and the AvaSoft 7.7 software (Avantes, the
 Netherlands). The light probe was held inside an RPH-1 probe holder at a
 90° angle to the egg. Before taking a set of measurements, a WS-2 white
 reflectance tile was used to take a white reference and a dark reference
 was taken after the light source had been switched off. Nine measurements
 of each egg were taken in three regions around the egg (three measurements
 at the cap, three around the middle and three at the blunt end). These
 nine measurements were averaged together to obtain an average spectral
 reflectance measurement of each egg. Due to the small sizes of the eggs
 and their spots, we did not distinguish between spot colour and background
 colour. To reduce noise from ambient light, measurements were taken in a
 darkened room with the lights turned off to obtain the darkest lighting
 possible. What we provide in this dataset is the average spectral
 reflectance of each egg from 300-700 nm averaged to 1-nm intervals using
 the pavo package (Maia et al. 2019) in the R Statistical Package (R Core
 Team 2019). To measure egg pattern, we took a single
 photograph of each clutch (one host egg and one cuckoo egg) on an 18% grey
 standard Mennon card using a Canon EOS M100 camera with a 15-45 mm lens.
 We always had the lens set to 15 mm, as this produced the clearest image
 of the egg. Objective image analysis was performed using the multispectral
 image calibration and analysis toolbox (Troscianko and Stevens 2015) for
 ImageJ (Rasband 1997-2014). The toolbox performs image calibration,
 ensuring linear reflectance images that control for lighting changes are
 used for image processing. Granularity pattern analysis was performed
 using standard bandpass methods on the camera’s green reflectance channel
 as this most closely approximates bird double cone peak sensitivities
 (Spottiswoode and Stevens 2010), which are thought to be important in the
 perception of pattern (Jones and Osorio 2004). We calculated pattern
 energy spectrums for each egg using a size range of 2-512 px with all
 photographs standardised to a 33 px/mm scale (Troscianko and Stevens
 2015), which is the data we have provided in this dataset.
 To determine if Brush Cuckoos match their hosts in egg volume, we
 used the volume of each egg estimated by the multispectral image
 calibration and analysis toolbox (Troscianko and Stevens 2015). This model
 used the same photograph we took for measuring egg pattern. We included
 the same scale bar in each egg photo and used this to standardise the
 measurement to obtain an estimate of volume (ml), which is the data we
 have provided in this dataset.
description: Please see associated README file for information on what this
 data is and how this data was collected and processed.