We've all seen nestlings squeaking on the ground, looking lost. Marty L. Leonard and Andrew G. Horn of Dalhousie University in Nova Scotia wanted to discover if failed reception and false alarms operated when nestlings were presented with these "cues." The tree swallow (Tachycineta bicolour), throughout its range, can be subject to human-generated noise sources in cities, but this experiment was conducted in the Gaspereau Valley in Nova Scotia using controllable "white noise," at 65 dB in easily-available nest boxes, used by the birds regularly.
The frequency of begging response failures in the presence of noise was noted when parent contact calls were played with and without the landing sound. Then the sound of a predator landing on the nest box was presented. (This was regarded as a false alarm id the nestling begged in response.)
The results, published in the Royal Society Journal Biology Letters, were impressive in that they produced 96% response to the parent with call stimulus. Obviously the careful experimentation helped, as the 54% response to the parent without its landing sound indicates. Even better, the landing sound of a common grackle (predator) stimulated 35% of the chicks to respond. Perhaps that grackle (Quiscalis versicolor), has a knack for it!
When the noise was added, the significance was that the responses halved (In fact, it reduced from 56% among hungry nestlings to 26% when the noise was played). This means that the babes-in-the-wood had lowered their threshold for response and conversely increased the risk of false alarms. The authors thought that a hair-trigger response is needed by individuals to ensure they are fed first if possible by the parent. Many different stimuli triggered up to a 50% response by the greedy feeders.
The, "susceptibility of nestlings to errors might make them particularly vulnerable to the masking effects of noise," according to Leonard and Horn. This again could be a very good reason for the lack of songbird breeding success in cities. Marty and Andrew also argued that the second experimental response to a simple landing noise would show the lowering of the response threshold, making them vulnerable to predators by responding to their landings more often.
The good news is that the swallows didn't react to noisy situations in these negative ways (ie. responding to landings by either parents or grackles). They still reacted best to the "parent-with-call" stimulus, although less successfully. The cost of missing a feed is obviously smaller than begging from Mr Grackle.
Visual and tactile cues in nature could ensure that every nestling is normally fed a little. Bird parents are often solicitous with calls and other behaviours, while the example of urban foxes and raccoons shows the clear urban animal approach to life that birds can't have failed to miss. Missed feeds and extended feeding visits are suggested in this paper as solutions to unresponsive nestlings. Benefits to smaller birds are also possible if large nestlings miss a cue, due to noise, although overall studies indicate that feeding rates and growth rates are unaffected by noise. The nestling interaction then is one of the next pieces of work for Drs. Andy Horn and Marty Leonard, as they face the music, perhaps, of the big city environment in their future exploits.
Dr. Andy Horn with his nest box; Credit: Dalhousie University, Halifax, Nova Scotia