Broodedness, not latitude, affects the response of reproductive timing of birds to food supplementation

Abstract

IntroductionMultiple food supplementation experiments during the pre-laying period have shown that the availability of additionalfood at this time can advance laying date (laydate) and/orincrease the number and/or quality of the eggs laid (Nageret al. 1997). It would therefore appear that food is a lim-iting factor at this critical time of year, although the extentof this limitation varies between years, locations, andspecies (Nager et al. 1997). Svensson (1995) and Dhondtet al. (2002) showed that the response to supplementaryfeeding depends on the number of clutches a female usu-ally lays in a season: single-brooded bird species onlyrespond weakly to supplementary feeding whereas multi-brooded species respond more strongly. In two recentpapers, Schoech and Hahn (2007, 2008) documented alatitude effect on responses to supplemental feeding andconcluded that birds advance laying in response to feedingmore as the distance from the equator decreased. If theseauthors are correct, this is indeed an important observationas it suggests that birds would be more food limited in thepre-breeding season closer to the equator than furthernorth.In this paper I re-analyze the dataset used by Schoechand Hahn in order to test if latitude, number of broods, orboth influence the effect of supplemental feeding on lay-date. The response to food supplementation has beenmeasured in four species at three or more latitudes, makingit possible to determine if latitudinal variation of theresponse to supplemental food can be confirmed withinsingle species, which is a stronger test than that in which allspecies are combined. If latitude drives the response tosupplemental food, the differences between control birdsand birds provided with additional food should be largest inpopulations furthest south (closest to the equator). If thenumber of clutches a species usually lays in a season drivesthe response, the effect of food supplementation should notvary systematically with latitude.Materials and methodsTo explore to what extent broodedness influences theresponse to supplemental food, I added this information tothe dataset Schoech and Hahn (2008) provided in theirElectronic Supplementary Material (see Electronic Sup-plementary Material of this article). For the within-speciescomparison, I added results on food supplementationexperiments in the Corsican Blue Tit Cyanistes caeruleus(Bourgault et al. 2009). Single-brooded species very rarelylay a second clutch after having fledged a first brood (13species); double-brooded species often lay a second clutchafter having raised a first brood successfully (8 species);triple-brooded species often lay three clutches or more perseason (3 species). I assigned species to a category