The ecological significance of extremely large flocks of birds.

Anders Pape Møller,Karsten Laursen

doi:10.1002/ece3.5234

Anders Pape Møller, Karsten Laursen

Open Access

https://doi.org/10.1002/ece3.5234

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Abstract

Population size is generally limited by resource availability during and outside the breeding season. Therefore, maximum size of flocks may provide important information on population regulation and the influence of diet and trophic level on maximal degree of sociality. We hypothesized that (a) flock size should increase with nutrient availability; (b) flock size should decrease with latitude because productivity is higher at lower latitude; (c) aquatic habitats should have larger flocks than terrestrial habitats because the former are less accessible; (d) smaller species should have larger flocks because they require overall less food; (e) human‐impacted species that live in perturbed habitats should have smaller flocks than other species; (f) flock size should decrease with increasing trophic level because there is a reduction in biomass due to conversion at each trophic level; and (g) flocks of species depending on ancestral landscapes should have decreased in size in recent years due to human impact (e.g., land‐use). We obtained 1564 observations of flocks that exceeded 100,000 individuals in order to test the predictions listed above. Most effect sizes were small to medium accounting for 1%–9% of the variance, while large effects accounting for 25% or more were only found for total nitrogen used per km2 and area used for agriculture. Changes in large bird flocks were caused by habitat degradation and persecution, and temporal decline in size of large flocks revealed changes in nutrient use, reductions in nutrient cycling, and changes in flock size linked to trophic level.

Highlights

Population size is commonly limited by resource availability (Newton, 1998)
Flock size was repeatable among species as shown by a generalized linear models (GLM) of log10‐transformed number of species being predicted by species identity (LR = 767.635, df = 158, 1,405, p < 0.0001, repeatability R = 0.320; SE = 0.014)
We have focused on factors that are associated with resource limitation, and factors that reduce access to resources such as aquatic compared to terrestrial habitats, seasonality that reduces access to resources to a few months of peak food availability during the main growing season, habitat degradation and aerial foraging that are linked to superabundant aggregated food

Summary

| INTRODUCTION

Population size is commonly limited by resource availability (Newton, 1998). Resource availability limits population size of consumers with the latter matching the distribution of resources in an ideal free fash‐ ion (Fretwell & Calver, 1969; Fretwell & Lucas, 1970). While resources may limit the abundance of consumers, it is possible to argue the other way around by assuming that a larger aggregation of consumers implies a larger amount of resources We use this latter approach to investigate the factors that contribute to the occurrence of large aggregations of individuals in flocks or colonies. MØLLER and LAURSEN regulation and the influence of ecological factors such as nutrient availability, diet, and trophic level for maximum flock size. This raises the question which are the largest bird flocks and colonies ever observed in the world, and which ecological factors are the main determinants? We collected a total of 1,564 observations of bird flocks or colonies exceeding 100,000 individuals from 154 species in 69 countries in order to test these predictions

| MATERIALS AND METHODS

Findings

| DISCUSSION

| CONCLUSIONS