Constraints on host choice: why do parasitic birds rarely exploit some common potential hosts?

Area of Science:

• Behavioral Ecology
• Evolutionary Biology
• Ornithology

Background:

• Obligate brood parasites, like the common cuckoo (Cuculus canorus), present an evolutionary puzzle by not parasitizing all suitable common hosts.
• Previous research often focused on single factors, neglecting the complexity of host-parasite interactions.

Purpose of the Study:

• To investigate why common cuckoos rarely parasitize thrushes (Turdus spp.), despite their common occurrence and conspicuous nests.
• To identify the specific factors contributing to the absence of brood parasitism in thrush species.

Main Methods:

• Comparative analysis of life-history traits between six European thrush species and ten common cuckoo hosts.
• Extensive field experiments (n=1211) involving cross-fostering to assess host defenses during egg-laying, incubation, and nestling stages.

Main Results:

• No single factor explained cuckoo avoidance of thrushes at the egg or incubation stages.
• Cross-fostering experiments revealed that nestling-stage factors are critical: nest cup design impeded cuckoo chick survival, and some thrush species exhibited reduced parental care for cuckoo nestlings.
• Interspecific differences were observed among thrush species regarding the specific reasons for avoiding cuckoo parasitism.

Conclusions:

• Brood parasitism avoidance in common hosts is a multifactorial phenomenon, with nestling-stage interactions playing a crucial role.
• Understanding interspecific ecological relationships requires integrating comparative and experimental approaches to account for multiple interacting factors.