Egg recognition and nestling discrimination in red-vented bulbul

The field experiment was carried out in the rural area of District Mardan (34°18' N, 72°9' E) about 25 km away from Mardan city. The terrain is a mixture of farmland, open meadows, and encircling forests, with extensive irrigation systems. The area is home to small farming communities mainly involved in agriculture, herding, and commerce.The red-vented bulbul (Pycnonotus cafer) is a passerine bird in the family Pycnonotidae found in the Indian subcontinent. It commonly nests in arboreal, shrubby, and thorny vegetation. Although uncommon in East Asia, and not reported to be brood parasitized there. The red-vented bulbul in the Indian subcontinent breeds from May to August and is parasitized by the Jacobin cuckoo (Clamator jacobinus).Dataset DescriptionThis dataset contains the raw and processed data collected from experimental and observational studies on bulbul eggs, incubation behavior, egg rejection, cross-fostering, and nestling growth. The data are organized in a single Excel file with multiple sub-sheets, each corresponding to a specific experimental component of the study. These datasets support analyses of host responses, egg characteristics, and offspring development.Sub-sheet InformationEgg rejection experimentsContains data on experimental egg rejection trials, including nest identity, treatment type, egg fate (accepted/rejected), observation periods, and relevant behavioral responses.Bulbuls’ egg measurementIncludes morphometric measurements of natural bulbul eggs, such as egg length, width, mass, and associated nest or clutch information.Experimental eggs measurementProvides measurements of artificial or experimentally introduced eggs used in manipulation experiments, including size, mass, and experimental category.Egg incubation experimentsRecords incubation-related data, including incubation duration, nest temperature observations (if applicable), hatching success, and experimental conditions.Cross-fostered experimentsContains data from cross-fostering trials, detailing donor and recipient nests, egg or chick transfer information, and subsequent survival or developmental outcomes.Nestling growthIncludes longitudinal measurements of nestling development, such as body mass, wing length, tarsus length, age (days), and survival status.Simple summary: Most studies on host recognition of brood parasitism have focused on colour and pattern cues, with limited attention being paid to size differences. We examined the role of egg size and nestling discrimination by the red-vented bulbul (Pycnonotus cafer), a common host of the Jacobin cuckoo (Clamator jacobinus). Experimentally, we introduced both small and large foreign eggs and nestlings into bulbul nests. Bulbuls accepted most of the host-sized eggs but consistently rejected large eggs. Nestling discrimination appeared to be influenced by skin colour, with mismatched nestlings being left unfed, although this finding was based on a small sample size and should be interpreted with caution. In a few instances, hosts rejected their own clutch if a foreign egg hatched first, indicating that the hatching order affects the host response. Our results reveal that in the red-vented bulbul, egg recognition is based on size cues, whereas nestling discrimination is based on skin color. Moreover, rejection behavior is significantly affected by the presence of parents in the nest. This study emphasizes context-dependent defense in the red-vented bulbul, whereby visual, temporal, and behavioural cues are processed across breeding stages.