VERHULST, S

English

Density dependent processes and the availability of food may play an important role in the determination of population size. To investigate the relationship between density and local survival of juvenile Great Tits Parus m. major Kluyver removed first brood fledglings in two years. An analysis is presented of the effects of this removal experiment, beech crop and additional feeding in winter on local survival. Beech-crop index was positively correlated with local survival of both first- and second-brood fledglings. Additional feeding in winter affected local survival of first, but not of second-brood fledglings. The effect of the removal experiment differed between birds fledging early or late in the season. Removal of first-brood fledglings had a negative effect on local survival of the remaining first-brood fledglings. This effect was in accordance with the (quadratic) relationship between density and survival found in control years, indicating that the relationship is causal. This is a destabilizing density dependent process. Possible underlying mechanisms are discussed. Removal of first-brood fledglings enhanced local survival of second-brood fledglings. The enhancement was in accordance with the relationship found in control years, indicating that this relationship too is causal. Density of second-brood fledglings did not affect local survival of first or second-brood fledglings. This study confirms that food and density-dependent processes play an important role in the determination of population size but the effect of these factors differs between birds fledging early or late in the season.</p

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   University of GroningenEffects of Density, Beech Crop and Winter Feeding on Survival of Juvenile Great TitsVERHULST, SPublished in:ArdeaIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1992Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):VERHULST, S. (1992). Effects of Density, Beech Crop and Winter Feeding on Survival of Juvenile GreatTits: An Analysis of Kluyver's Removal Experiment. Ardea, 80(2), 285-292.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 08-06-2022288 ARDEA 80 (2), 1992DISCUSSIONFig. 2. Local survival of second-brood fledglings.Legend as in Figure 1.©®ooo80.0500.300.25<D§ 0.20Cii 0.15.:':~ 0.10::J<fl0.050.000 10 20 30 40 50 60 70 80 90beech crop index<D 0.05§Cii> 0.00.~::J<fl~ 0.025Cii.:': 0.000 f------"''-----::;_-=----,.,------I~::J<fl-0.025-0.050';-__-:!,---__~---_;;------J"-1 0 1 2 3amount of additional feeding in winter-0.100l;---5!:---1:l::0----:':15=---~2-=-0---::2'=5-------;!30density first brood fledglings~ -0.05'0'(jj~Beech crop and additional foodLocal survival offirst- and second-brood fledg-lings was positively correlated with the beech-cropbirds recaptured as local recruit in the first yearafter fledging as dependent variable (instead of allrecaptures after 31 st of march after the year ofhatching). This proportion was corrected for cap-ture rate of breeding birds (Tinbergen et al. 1985).These analyses yielded the same results.Second-brood fledglingsLocal survival of second-brood fledglings waspositively correlated with beech-crop index (R2 =OAO,P <O.03,Fig. 2a). To test whether the density-manipulation affected local survival rate, the ex-periment was added to this regression model as adummy variable. Local survival after removal offirst brood fledglings was significantly higher thanexpected on the basis of the beech-crop index (R2=0.69, beech crop: P<0.02, experiment: P<0.02).The dummy variable in the model could be re-placed by density of first-brood fledglings (R2 =0.83, beech crop: P < 0.004, density: P < 0.002,Fig. 2b) and subsequent addition of the dummyvariable to this model was no longer significant(experiment: P> 0.3). This implies that the exper-iment raised local survival rate of second-broodfledglings as much as expected on the basis ofnon-experimental data. Density of second-brood fledg-lings (or of late fledglings, hatched after 11 June),density of breeding pairs and additional feeding inwinter did not significantly affect local survivalrate of second-brood fledglings. Analysis of localsurvival of all late fledglings, instead of only sec-ond-brood fledglings, yielded the same result. Alltwo-way interactions between variables in themodel were tested and none was significant.The analysis for both first- and second-broodfledglings was repeated with the proportion ofReplacement ofdensity of first-brood fledglings bythe density of all early fledglings (hatched before12 June) and the survival of first-brood fledglingsby the survival of all early fledglings yielded thesame results. Density of second-brood fledglings(or of late fledglings, hatched after II June) anddensity ofbreeding pairs did not significantly affectlocal survival rate of first-brood fledglings.Additional feeding had apositive effect on localsurvival of first-brood fledglings if the effects ofbeech crop and density of first-brood fledglingswere taken into account (R2 = 0.96, beech crop:P < 0.001, density: P < 0.001, density squared:P < 0.001, additional feeding: P < 0.02, Fig. lc).All two-way interactions between variables inthe model were tested and none was significant.

Effects of Density, Beech Crop and Winter Feeding on Survival of Juvenile Great Tits:An Analysis of Kluyver's Removal Experiment

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Effects of Density, Beech Crop and Winter Feeding on Survival of Juvenile Great Tits:An Analysis of Kluyver's Removal Experiment

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