936
Growth, nutrition, and blow fly parasitism in nestling Pied Flycatchers Santiago Merino and Jaime Potti
Abstract: The nutritional status of the host may play a major role in mediating the detrimental effects of parasites. We performed an experiment with the aim of determining whether increased food availability can compensate for the effects of ectoparasites on growth during the late nestling period, final size, and survival until fledging of Pied Flycatcher ( Ficedula hypoleuca) nestlings. Nests were provided with supplementary food, treated with insecticide, given both treatments, or given neither treatment (control). Differences in the number of blood-sucking, ectoparasitic blow fly larvae (Protocalliphora azurea) occurred between treated nests. Nestlings in the group given supplementary food and with low numbers of parasites grew faster and had a higher haematocrit value than those in groups that were fumigated and given supplementary food, with nestlings from control nests attaining the lowest values. Nestling measurements did not differ between fumigated and foodsupplemented groups. Although the final sizes attained did not differ among nestlings from the different experimental groups, there was a significant difference in the rates of increase in size among groups. Nestlings in nests fumigated and provided with extra food were (nonsignificantly) smaller and leaner than nestlings from the other groups at the beginning of the experiment, but were slightly larger and heavier (again nonsignificantly) at the end of the experiment. Thus, their growth was faster than that of the other groups. The results are discussed, highlighting problems related to the function linking intensity of parasit ism to host fitness and variation in external (climate, food) conditions. Résumé : Le régime alimentaire de l’hôte peut avoir un effet considérable sur l’importance des dommages que causent les parasites. Nous avons procédé à une expérience dans le but d’établir si oui ou non la disponibilité de la nourriture peut contr ebalancer les effets des ectoparasites sur la croissance vers la fin de la période au nid, sur la taille finale et sur la survie des oisillons jusqu’à l’envol, chez le Gobemouche noir (Ficedula hypoleuca). Des nids ont été soumis à divers traitements, addition de nourriture, addition d’insecticide, addition des deux ou aucune addition (témoins). Les traitements expérimentaux ont résulté en des différences de nombres de larves de calliphores Protocalliphora azurea, ectoparasites hématophages, présents dans les nids. Les oisillons du groupe qui ont reçu de la nourriture additionnelle et qui portaient peu de parasites avaient un taux de croissance plus rapide et un hématocrite plus élevé que ceux qui ont été à la fois traités à l’insecticide et approvisionnés en nourriture additionnelle et ce sont les oisillons témoins qui étaient le plus défavorisés. Les valeurs mesurées chez les oisillons fumigés et chez ceux qui ont reçu de la nourriture additionnelle ne différaient pas. Bien que la taille finale des oisillons ait été la même chez tous les groupes, il y avait des différences significatives du taux de croissance entre les groupes. Les oisillons des nids fumigés et approvisionnés en nourriture additionnelle étaient (non significativement) plus petits et moins gras que les oisillons des autres groupes au début de l’expérience, mais étaient légèrement plus gros et plus lourds (encore là non significativement) à la fin de l’expérience. Leur croissance a donc été plus rapide que celle des oisillons des autres groupes. Ces résultats sont examinés en fonction des problèmes associés à la relation entre, d’une part, l’intensité du parasitisme et, d’autre part, le fitness de l’hôte et les fluctuations des conditions externes (climat, nourriture). [Traduit par la Rédaction]
Introduction Parasitism is a common selective pressure faced by animals in the wild (Price 1980; Møller 1994). The effects induced by parasites may be buffered to a considerable extent by other
Received May 5, 1997. Accepted December 12, 1997. S. Merino1 and J. Potti. Departamento de Biología Animal, Universidad Alcalá, E-28871 Alcalá de Henares, Madrid, Spain. 1
Author to whom all correspondence should be sent at the following address: Laboratoire d’Écologie, Universite Pierre-etMarie Curie, Centre National de la Recherche Scientifique, Unité de Recherche Associée 258, Bâtaillon A, 7e étage, 7, quai Saint-Bernard, C.P. 237, F-75252 Paris Cédex 05, France (e-mail:
[email protected]).
Can. J. Zool. 76: 936–941 (1998)
factors, such as parental quality and effort (Møller et al. 1994; Merino et al. 1996), host sex and age (Potti and Merino 1995, 1996), mode of transmission of the parasites (Herre 1993; Yamamura 1993; Ewald 1994), weather conditions (Marshall 1981; Merino and Potti 1996), or host immune responses to the parasites (Olsen 1974), or simply by the chance of encountering parasites (Brown and Brown 1986; Poulin and Vickery 1993). These factors are often interrelated, determining the impact of parasites on their hosts. Host nutrition plays a major role mediating the effects of some of the factors mentioned above (de Lope et al. 1993; Ullrey 1993; Ewald 1994). For example, an adequate food supply may provide sufficient energy for developing an effective immune response against parasites (Lochmiller et al. 1993; Ullrey 1993; Møller and Saino 1994). In addition, as parasites drain resources from hosts, thus increasing their nutritional requirements, food availability may be a crucial © 1998 NRC Canada
Merino and Potti
factor for hosts in the event of parasitism. Although parasite effects on hosts are large under stressful conditions (de Lope et al. 1993), it is not known whether food can compensate for the detrimental effects of parasites on wild hosts. Flies of the genus Protocalliphora (Diptera: Calliphoridae) are common parasites of Holarctic birds (Bennett and Whitworth 1991, 1992). Their effects on hosts, however, are far from clear (Gold and Dahlsten 1983; Price 1991). In some studies, detrimental effects of calliphorids on nestling growth and survival at high parasite intensities have been found (Shields and Crook 1987), but other studies have reported an apparent lack of effects on nestlings infested with as many as 57 parasite larvae per nestling (Rogers et al. 1991). Johnson et al. (1991) and Johnson and Albrecht (1993) found no effect of Protocalliphora spp. on haematocrit in House Wren (Troglodytes aedon) nestlings infested with more than 10 larvae per nestling, but surprisingly, these nestlings had a larger body mass than nestlings suffering from lower levels of parasitism. These authors suggested that the nestlings affected by Protocalliphora spp. had more difficulty in losing water from the tissues and thus attaining the mass loss required before fledging. There are many factors that may explain the differing effects of Protocalliphora spp. on hosts, such as the amount of blood taken from the host, the host’s ability to regenerate blood, and the host’s nutritional status. In a previous study we found that the detrimental effects of the blow fly Protocalliphora azurea Fall, 1817 on the growth of Pied Flycatcher, Ficedula hypoleuca (Pallas) (Aves: Muscicapidae), nestlings might be buffered by the early brood reduction caused by the parasite (Merino and Potti 1995). Here we present the results of a field experiment aimed at elucidating the relative roles of parasites and nutrition on the growth, final size, and survival of Pied Flycatcher nestling parasitized by calliphorid larvae.
Methods General methods The experiment was carried out during the breeding season in 1994, using a Pied Flycatcher population breeding in an old oak (Quercus pyrenaica Willd.) forest in central Spain (Potti and Merino 1994; Merino and Potti 1995). Tarsus length (distance between joints) of all nestlings was measured to the nearest 0.05 mm with a dial caliper, and nestlings were weighed to the nearest 0.1 g using a Pesnet® spring balance at the ages of 8, 10, and 13 days. Within-nest averages of all measurements were used to avoid pseudoreplication. We calculated within-nest increases in average nestling measurements between successive days (i.e., from days 8 to 10, 10 to 13, and 8 to 13). In addition, for each nest we noted the number of hatched nestlings and the number of those that had died before fledging. On day 13 of nestling age (hatching date = day 1) we obtained blood from the brachial vein of all nestlings, which was immediately centrifuged in a capillary tube at 11500 rpm for 8 min in a portable centrifuge (Compur 1101, Bayer Diagnostics Ltd., Germany) to obtain the haematocrit value (percentage of cells in the blood). The laying date, i.e., the date of appearance of the first egg in the clutch, was used as an index of breeding phenology. After the nestlings had fledged, the nests were removed and placed in labelled plastic bags. Nests were defaunated in the laboratory in Berlese funnels for 48 h, and ectoparasitic mites (Dermanyssus gallinoides Moss, 1966) were counted under a binocular microscope. The presence of adults or larvae of the hen flea (Ceratophyllus gallinae Schrank, 1803) was noted. The nest material was subsequently dismantled in order to count P. azurea pupae (Merino and Potti 1995).
937 Experimental treatment When nestlings were 8 days old, we put a feeding tray (10 cm wide × 5 cm long × 3 cm deep) below the outer entrance of the nestbox. Nests were randomly divided into four groups: (1) group FI nests (10) were provided daily with about 25 g of mealworms (Tenebrio mollitor) (about 10 g dry mass of food) until nestling were 13 days of age. This daily ration represents a considerably quantity of food, as a parent Pied Flycatcher carries to the nest about 300 mg of food (dry mass) per hour just before the nestlings fledge (Lundberg and Alatalo 1992, p. 98). Calculating an average of 15 h of work per day, we provided flycatchers with about 667 mg/h dry mass of food. These nests were also fumigated with a commercial insecticide (Dismark, S.A., composed of 0.3% Tetrametrin and 0.1% D-Fenotrin) when the nestlings were 8 and 10 days of age. Nestlings were briefly removed from the nests before fumigation and then returned; (2) group F nests (11) were provided with the same food as group FI, but nests were not fumigated; (3) group I nests (10) were fumigated but not provided with food; and (4) group C (control) nests (11) were neither fumigated nor supplied with food. Nestlings from groups F and C were, however, subjected to the same disturbance as nestlings in group FI. Some nests were predated during the experiment, so sample sizes were reduced to 9, 9, 9, and 8 nests, respectively, when nestlings were 13 days of age. Results did not change when we restricted the analysis to the broods that survived to the age of 13 days, therefore only the results for the larger sample size available in each case are given. We predicted that nestlings in group C would be in poorer condition than nestlings in the remaining groups, as they were exposed to a natural load of parasites and lacked extra food. No differences were expected to occur between nestlings in groups F and I if better nutrition can compensate for the effects of parasites. Finally, we expected that nestlings in group FI would grow better than nestlings in the remaining groups, as they did not support parasites and were provided with extra food. Thus, we predicted that nestling size and body mass in the different groups should be ranked in the order C < F = I < FI. Statistical tests – To test the hypothesis we used isotonic regression (the statistic E 2; Gaines and Rice 1990) because this technique is more powerful than analysis of variance (ANOVA) when one wishes to test ordered predictions. No differences were expected among groups prior to treatment, hence ANOVA was used.
Results Mealworms were readily picked up by flycatchers soon after they were supplied. There were no significant differences between groups in laying date, number of hatchlings, or parental tarsus length and mass (one-way ANOVA, p > 0.05 in all cases). Nestling sizes and masses did not differ significantly among groups on the day when treatment began (see Table 1). The treatment was successful in creating a significant difference between fumigated (groups I and FI; mean = 3.20 (SD = 6.97) pupae per nest; n = 20) and nonfumigated (groups C and F; mean = 9.6 (SD = 10.2) pupae per nest; n = 22; Mann–Whitney U test, Z = 2.45, p = 0.01) nests in the number of calliphorid larvae buried in order to pupate in the nest material. However, differences in numbers of other ectoparasite species (mites and fleas) present in nests were not significant (p = 0.23 for mites and p = 0.18 for fleas). There were no significant differences between groups in mean nestling measurements on day 10 or 13 (isotonic regression, p > 0.3 in all cases; Table 1, Fig.1). The numbers of dead nestlings did not differ among groups – (E 42 = 0.001, p = 0.92). However, our predictions concerning © 1998 NRC Canada
938
Can. J. Zool. Vol. 76, 1998
Table 1. Average measurements of nestlings from different experimental groups at the ages of 8, 10, and 13 days post hatching.
Nestling age (days) 8 10 13
Trait Body mass Tarsus length Body mass Tarsus length Body mass Tarsus length
Treatment groupa ——————————————–——————————————————————————— C n F n I n FI n p 10.48 16.34 12.41 18.20 13.65 19.18
(1.50) (1.27) (1.40) (0.95) (1.43) (0.39)
11 10 8
10.77 16.38 12.64 18.35 14.06 19.07
(0.97) (0.74) (1.14) (0.65) (0.59) (0.61)
11 11 9
10.65 16.39 12.62 18.41 13.87 19.34
(1.15) (0.88) (1.63) (0.96) (1.36) (0.46)
10 10 9
10.45 16.17 13.13 18.44 14.13 19.13
(1.01) (1.01) (0.65) (0.53) (0.53) (0.38)
10 9 9
0.92* 0.96* 0.31 0.60 0.42 0.61
Note: Values in parentheses are standard errors. The p values show differences among groups tested by isotonic regression, except for those marked with an asterisk, which were tested by one-way ANOVA. a C, control nests; F, nests provided with supplementary food; I, fumigated nests; FI, nests fumigated and provided with supplementary food.
Fig. 1. Average body masses of Pied Flycatcher nestlings from different treatment groups (C, control nests; F, nests provided with supplementary food; I, fumigated nests; FI, nests fumigated and provided with supplementary food) at the ages of 8, 10, and 13 days. Numbers beside the data points show the number of nests.
the average increase in mass and tarsus length between days 8 and 10 of nestling age as well as the increase in mass between days 8 and 13 were confirmed (Table 2). The same result was – obtained with haematocrit values (isotonic regression, E 42 = 0.24, p = 0.009; Fig. 2).
Discussion Our results clearly indicate that supplementary food and a low number of parasites had a positive influence on chick growth, although differences were only evident in haematocrit values and some rates of increase in tarsus length and mass between groups. The lower haematocrit value in nestlings from the control group than in the remaining groups may be attributed to differences in the effects of the blow fly larvae, which have been reported to take considerable amounts of blood from
nestlings of other bird species (Johnson et al. 1991; Johnson and Albrecht 1993). Low haematocrit values may therefore be indicative of anaemia, and may thus be related to difficulties in oxygen uptake and transport (Phillips et al. 1985). If the haematocrit values are representative of those of fledglings some days later (i.e., if the haematocrit of anaemic fledglings does not recover), it is conceivable that anaemia impairs flight performance and affects survival during the high-mortality postfledging period (Gold and Dahlsten 1983). However, birds in general may regenerate blood quickly (Hoysak and Weatherhead 1991) once blood-sucking ceases. In this particular system, blood-sucking may greatly diminish after fledging because, with the possible exception of fleas, most ectoparasites take blood in the nest. The good food supply may be the main cause of the higher growth rate among nestlings from nests that were deparasit© 1998 NRC Canada
Merino and Potti
939
Table 2. Within-nest increases in average nestling measurements between successive days (8, 10, and 13 days post hatching).
Nestling age (days) 8–10 8–13 10–13
Trait Body mass Tarsus length Body mass Tarsus length Body mass Tarsus length
Treatment groupa ——————————————————————————————————–——————— C n F n I n FI n p 1.92 1.91 2.84 2.65 0.79 0.70
(1.50) (1.27) (1.40) (0.95) (1.43) (0.39)
10 8 8
1.87 1.97 3.01 2.53 1.07 0.48
(0.97) (0.74) (1.14) (0.65) (0.59) (0.61)
11 9 9
1.96 2.02 2.98 2.79 0.84 0.67
(1.15) (0.88) (1.63) (0.96) (1.36) (0.46)
10 9 9
2.89 2.47 3.89 3.15 1.00 0.69
(1.01) (1.01) (0.65) (0.53) (0.53) (0.38)
9 9 9
0.018 0.046 0.017 0.138 0.432 0.632
Note: Values in parentheses are standard errors. The p values show differences among groups tested by isotonic regression.
Fig. 2. Variation in average haematocrit (SE) of Pied Flycatcher nestlings in different treatment groups (for a description of groups see Fig. 1). Numbers above the data points show the number of nests.
ized and provided with mealworms than in the control group. In this sense, supplementary food is known to increase the nestling growth rate in at least one bird species, the Oystercatcher, Haematopus ostralegus (Ens et al. 1992). Although blow fly larvae have been documented to cause significant nestling mortality in some years, blow flies did not cause reduced growth in surviving Pied Flycatcher nestlings those years (Merino and Potti 1995, 1996). However, nestlings in group I grew at the same rate as nestlings receiving supplementary food, indicating that good nutrition may compensate for the effects of parasites. In other words, the absence of parasites allows nestlings to invest all food in growth, thus equalling the development rate of nestlings with both parasites and extra food. The fact that the final sizes and masses attained by nestlings on day 13 did not differ among groups in our experiment (Table 1) despite their higher growth rate is due to the slight, nonsignificant differences among groups at the beginning of the experiment (Fig. 1). Nestlings in group FI had a
slightly lower mass than those in the remaining groups at 8 days of age, but the contrary was true at 13 days of age, when nestlings in group FI nests were the heaviest. This can only be due to a higher rate of increase in size and mass in FI broods. The lack of differences between groups F and I, as well as their intermediate position between groups C and FI, indicate that the extra food compensated for the effects of parasites in the nestling Pied Flycatchers, at least at the levels of parasitism and food supply in this study. A greater quantity of food or higher parasite load per nest might change this conclusion. Although the qualitative effects of growing in lightly versus heavily parasitized nests have been previously documented (Merino and Potti 1995), uncertainty remains as to the precise form of the function relating intensity of parasitism to offspring fitness. For example, Clayton et al. (1992) noted that the detrimental effect of parasites on host fitness may not necessarily be a linear relationship, so that step (threshold) functions or exponential decreases in host fitness with increasing intensity of parasitism may be frequent in host–parasite sys© 1998 NRC Canada
940
tems. Therefore, when host fitness is not linearly related to the intensity of parasitism, detrimental effects on host fitness will be difficult to demonstrate until the precise form of this function is known. In addition, other parasite species (e.g., mites) may also be important in this respect, and their interactions with host size and nutrition (Merino and Potti 1995) remain to be experimentally tested, as no differences in their abundance were caused by nest fumigation in this study, “natural” prevalences being very low (
