Our paper on how tree swallow females adjust parental effort and what role glucocorticoid hormones play in this has been published at Royal Society Open Science. The open access paper can be accessed here. We used an automated playback device (see the description here) consisting of an RFID reader and and a Raspberry PI computer to play extra nestling begging calls to the females (but not the males) every time they visited the nest for six hours. The females ended up upping their parental effort for only about the first two hours of the playback duration. We also obtained blood samples to look at corticosterone (cort) levels of females after the playback with the expectation that if the females increase their parental effort and if cort is involved in this adjustment, we should see higher cort levels in females that received the playbacks compared to controls. We did not find such a difference in cort levels, possibly because the effect of playbacks on feeding rates was transient and confined only to the first two hours of playback. It therefore remains an open possibility that cort is involved in some way in the strategic adjustment of parental care.
Our new paper on the fitness consequences of honesty was featured as a cover article in the December 2015 issue of Evolution.The picture of the angry song sparrow that made the cover is below. Incidentally, this issue also became the first one to contain a publication from both Erol and myself. Check out his book review titled “An apology for inclusive fitness”.
Our new paper on aggressive signaling under predation risk has just been published in Behavioral Ecology. In an experiment where we played Cooper’s hawk calls (vs. northern flicker calls in controls) in the middle of a simulated intrusion, we found that that song sparrows cease aggressive signaling , and increase alarm calling during hawk playbacks consistent with an increased perceived risk of predation as a result of the hawk playback. Once the hawk playback was over and the simulated intruder returned however, they rapidly resumed aggressive signaling at the same levels as before, although they kept alarm calling a higher rate after hawk playbacks compared to flicker playbacks.
These results suggest aggressive signaling may indeed carry costs in terms of detection by predators (as suggested by the fact that signaling mostly ceases during hawk playback). This finding might help explain why under-signalers (those who signal at low levels but behave aggressively) have a survival advantage as we reported in our recent paper in Evolution.
The experiment also was designed to test the eavesdropping avoidance hypothesis as an explanation for the low amplitude of soft song: the most reliable signal of aggression in this and several other songbirds. Given that birds did not increase soft song use after hawk playback relative to flicker playback, we found no support for this hypothesis (as in the only other study, by Searcy and Nowicki, 2006, that explicitly tested this hypothesis). It may therefore be time to look at the other hypotheses and test those in future research.
Our new paper on the fitness consequences of honest signaling is online in Evolution. We show that individually consistent variation in honest signaling during aggressive interaction leads to survival differences, with under-signalers (those who do not signal their aggressive intent but nevertheless behave aggressively) being favored for survival compared to reliable signalers. In other words, there is negative correlational selection on aggression and aggressive signaling, even though these two traits are correlated with each other positively across individuals. This result poses a puzzle for how honest signaling is maintained in this reliable signaling system.
You can download the unedited version here, or here for the open access preprint version of the accepted manuscript.
he ain’t no under-signaler.
Our new paper in PLoS One with Adam Lendvai is online here. In it, we ask what the optimal sampling duration is during behavioral observations (e.g. when researchers observe parental feeding rates). Using RFID readers and tags on tree swallows we first validate the feeding rates from the RFID logs and then show that one can calculate an optimal duration for the behavioral sampling that optimizes the trade-off between the variation captured by the behavioral sample (as a total amount of the variation in the behavior if one had sampled it continuously) and the sampling duration. Turns out, 1 hour is the optimal duration for both males and females in this species. This is a good result given that many prior studies on this species (and others) used 1 hour of sampling for estimating parental feeding rates. The paper also illustrates the principle of how one can optimize the sampling duration and as noted above, validates the use of RFID readers as a replacement or augmentation for human observation.
Our critique of a recent paper on chipping sparrow territorial coalitions was published in Biology Letters. The original paper by Sarah Goodwin and Jeff Podos of UMass suggested that the male sparrows form territorial alliances in response to simulated intrusions that depend on the performance levels of the songs sung by the allies and the intruder. We found that the paper had some serious flaws including:
- all subjects and their presumed neighbors were non-banded and most neighbors not even recorded before or after the experiment, complicating the conclusions about their identities;
- the authors did not rule out alternative non-cooperative scenarios that may have led to the presence of extra birds during some of the simulated intrusions;
- analyses were carried out on an inadequate metric of trill performance (namely trill rate) and analyses on better metrics such as vocal deviation were not reported because apparently they yielded no significant effect;
- there were numerous problems with the analyses on coalition forming which led to inflated p-values, at least some of which can be shown to be non-significant once corrected.
You can read our critique and the reply by Goodwin and Podos by clicking the respective links. Based on the above points and others we argued that the conclusions by the authors had no empirical footing. The authors in their reply declare that they stand by their original methods, design and analyses, rather unconvincingly in my opinion. As far as I can tell, their main defense is that they can identify individuals from song (but given that most neighbors are not recorded before or after the experiment, this wouldn’t help them to ascertain that these are neighbors or that these birds are there too “cooperate”), and some rather weak arguments regarding the statistics. In particular, although they grant that all of their original binomial tests gave inflated p-values, they manage squeeze out a p-value smaller than 0.05 for one of them and that seems to be the last thread they hang on to. Ironically, this made-to-order test (see the definition of p-hacking) is also incorrect- the authors use the “observed” chance levels for 3 subjects and 0.74 for the rest of the subjects, but to be correct they actually would have to re-calculate the chance levels for the remaining six (in reality five) subjects. If the 3 subjects that they have trill rate information on presumed neighbors are the ones with the highest trill rate than the chance level for the remaining 6 would be 0.90 and the binomial test would not give a significant effect anymore.
My hope is that our critique will illustrate the pitfalls of studying social behavior in otherwise unmarked individuals as well as establish more rigorous standards for future claims of complex social strategies.
Our review paper “Quiet threats: soft song as an aggressive signal in birds” with Rindy Anderson, Steve Nowicki, Mike Beecher and Bill Searcy was published in the July issue of Animal Behaviour. The paper is part of the special section on Quiet song, that originated from the excellent symposium organized by Rindy Anderson and Dustin Reichard at ABS 2014 meeting in Princeton. Check out the rest of the papers in the special section here.