Common Myna Roosts Are Not Recruitment Centres

<div><p>We studied communal roosting in the Common Myna (<i>Acridotheres tristis</i>) in the light of the recruitment centre hypothesis and predation at the roost. The number and sizes of flocks departing from and arriving at focal roosts were recorded over a two year period. We also recorded the sizes and behaviour of foraging flocks. We found that flock sizes of birds departing from roosts at sunrise were larger than those at the feeding site, suggesting that there was no recruitment from the roosts. Flocks entering the roosts during sunset were larger on average than those leaving the following sunrise, suggesting no consolidation of flocks in the morning. Flocks entering the roosts at sunset were also larger on average than those that had left that sunrise, although there was no recruitment at the feeding site. There was no effect of group size on the proportion of time spent feeding. Contrary to expectation, single birds showed lower apparent vigilance than birds that foraged in pairs or groups, possibly due to scrounging tactics being used in the presence of feeding companions. Thus, the recruitment centre hypothesis did not hold in our study population of mynas. Predation at dawn and dusk were also not important to communal roosting: predators near the roosts did not result in larger flocks, and resulted in larger durations of arrival/departure contrary to expectation. Since flock sizes were smallest at the feeding site and larger in the evening than in the morning, but did not coincide with predator activity, information transfer unrelated to food (such as breeding opportunities) may possibly give rise to the evening aggregations.</p></div

Average flock sizes during sunset and the same or next sunrise across days.

<p>Average flock sizes during sunset and the same or next sunrise across days.</p

Time spent in different activities while foraging in groups of different sizes.

<p>Logit transformed proportions of time spent in feeding, vigilance, and searching by single birds, paired birds, and groups of 3–4 birds during the breeding and post-breeding seasons. Significant pairwise, post-hoc comparisons following a significant effect in the ANOVA are marked with letters (a</p

Flock size distributions while entering/leaving the roost and while foraging.

<p>Proportions of flocks of different sizes during sunrise (departing from the roost), sunset (arriving at the roost), and while foraging, are shown for the A) breeding season, and B) post-breeding season.</p

Flock size distributions at pre-roosts.

<p>Distributions of sizes of flocks entering and leaving pre-roosts during A) the breeding season, and B) the post-breeding season (error bars are 95% CIs).</p

Bird counts across time.

<p>Daily variation in the number of birds present at four roosts (Periphery, Canteen, Gazebo, Main building) during sunset and the following sunrise.</p

Average flock sizes during sunrise and sunset during different seasons.

<p>There was a significant interaction between time of day and breeding season in affecting average flock size (<i>F</i>_2,462 = 3.508, <i>P</i> = 0.031) (error bars are 95% CIs about the individual means). Significant pairwise comparisons based on Tukey’s tests are marked with different letters (c>b>a).</p

Results of the GLM on the duration of arrival/departure of birds at roosts.

<p>Results of the GLM on the duration of arrival/departure of birds at roosts.</p