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In mixed-species foraging flocks of forest birds, one or a few nuclear species frequently produce alarm calls and are followed by other species in the flock. We tested the hypothesis that similar asymmetries may exist in a second interspecific social context, multispecies mobbing behaviour. We examined mobbing behaviour evoked by an eastern screech-owl, Megascops asio, model and playback in two nuclear species, Carolina chickadees, Poecile carolinensis, and tufted titmice, Baeolophus bicolor, and in a species that follows them, the white-breasted nuthatch, Sitta carolinensis. Asymmetries in mobbing were not the same as those in mixed-species flocks. Nuthatches and chickadees mobbed with greater frequency and intensity compared to titmice, which remained at greater distances from the owl model and vocalized less frequently. We also tested for the existence and nature of potential interspecific vocal information flow during mobbing. Chickadee and nuthatch calling rates were positively correlated, as were chickadee and titmouse calling rates. Nuthatches and titmice rarely mobbed simultaneously. These results suggest the existence of positive feedback among species'' mobbing intensity during a multispecies mobbing association as opposed to heterospecific vocal interference or a lack of heterospecific influence. However, randomization simulations showed that this positive feedback was not driven by a particular ‘nuclear’ species during mobbing, suggesting that the correlations may result from a mutually interdependent escalation of mobbing intensity. [Copyright &y& Elsevier]

Coevolution between senders and receivers is expected to produce a close match between signal design and sensory biology. We evaluated this hypothesis in songbirds by comparing aspects of acoustic signal space with the frequency range of auditory sensitivity and temporal resolution in tufted titmice, Baeolophus bicolor; house sparrows, Passer domesticus; and white-breasted nuthatches, Sitta carolinensis. Auditory measurements were made electrophysiologically from the scalp using two classes of auditory-evoked potentials: the auditory brain-stem response (ABR) and the envelope-following response (EFR). ABRs to tone-burst stimuli indicated maximum sensitivity from 2.2 to 3.2kHz in all species, but 12–14dB greater sensitivity in titmice than in sparrows and nuthatches at 6.4kHz (the highest frequency tested). Modulation rate transfer functions based on EFRs to amplitude-modulated tones suggested greater temporal resolution in titmice and sparrows than in nuthatches. Conservation of the frequency range of maximum sensitivity across species resulted in a mismatch with the dominant frequency of song in sparrows. The mismatch may reflect auditory constraints coupled with selection for high-frequency song and relaxed selection for a close match between sender and receiver due to small territory size. Consistent with coevolution between senders and receivers, high-frequency sensitivity varied with the maximum frequency of species-specific vocalizations, whereas temporal resolution varied with the maximum rate of envelope periodicity. Enhanced high-frequency sensitivity of the titmouse may reflect a specialization for processing high-frequency communication signals such as alarm calls. [Copyright &y& Elsevier]

Looks at a study which investigated the prediction that birds should decrease their mass in relation to the increased risk of predation, in the Baelophus bicolar, tufted titmouse. Importance of fat reserves for animals in the winter; Details on risk-dependent foraging; Methodology used to conduct the study; Results of the study.