The Auk 118(4):1047-1052, 2001 Nest Defense as Parental Care in the Northern Hobby (Falco subbuteo) FABRIZIO SERGIO'3 AND GIUSEPPE BOGLIANI2 'Raptor Conservation Research Unit, TrentoMuseum of Natural History, via Calepina 14, 38100 Trento,Italy; and 2Dipartimento di Biologia Animale,University of Pavia, Piazza Botta 9, 27100 Pavia, Italy ABSTRACT.-Intensity of nest defense against a hu- man intruder was recorded for 42 male and 43 fe- male Northern Hobbies (Falco subbuteo) from 1992 to 1994. Defense did not vary between repeatedly and rarely visited nests. Except during incubation, inten- sity of nest defense by females was higher than that by males. For both sexes, defense intensity increased from incubation to fledging, within the nestling stage, and from fledging to the first 10 days of the postfledging period. Intensity of nest defense was positively correlated with brood size in females, but not in males. Experiments with dummy nests showed that defense was effective in deterring nest predation, and that its effect was positively related to its intensity. Hobby nest defense was an individ- ually varying "plastic" trait, probably tuned to the reproductive value of the offspring. Parents appar- ently trade off the costs and risks of the behavior against the increasing likelihood of offspring survival. Nest defense is an individually variable compo- nent of parental investment (Montgomerie and Weatherhead 1988). "Parental investment" is defined as any investment by the parent in an individual off- spring that increases the offspring's probability of survival at the cost of the parent's ability to invest in other offspring (Trivers 1972). The cost involved in nest defense includes risk of injury or death of the parent, time and energy lost in defense, and risk of attracting additional predators (McLean et al. 1986). Parental investment theory predicts an increase in intensity of defense during the breeding cycle and with increasing brood size (Winkler 1987, Redondo 1989). Intensity of defense progressively increases during a breeding attempt because of an increase in cumulative past effort (but see Maynard Smith 1977), a progressive increase in expected fitness benefits (Knight and Temple 1986a, Winkler 1987), a decreas- ing ratio between the survival probabilities of the parents and those of their offspring (Andersson et al. 1980), a decrease in parental renesting potential (Biermann and Robertson 1981), or an increase in vulnerability and conspicuousness of the nestlings (Harvey and Greenwood 1978). Knight and Temple (1986b) have argued that the reported increase could 3Present address: Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford OX1 3PS, United Kingdom. E-mail: fabrizio.sergio@zoo.ox.ac.uk be a methodological artifact caused by positive re- inforcement in repeatedly visited parents. Here, we (1) examine sexual differences in the in- tensity of nest defense by a small falcon, the North- ern Hobby (Falco subbuteo); (2) analyze variations in intensity of defense in relation to breeding stage, clutch size, brood size, and renesting potential; (3) test effectiveness of nest defense in reducing nest predation by means of an experimental approach; and (4) test Knight and Temple's (1986b) revisitation hypothesis. Study area and Methods. The study area was along a 45 km stretch of the Po River (45?N, 9?E) in north- ern Italy (see Sergio and Bogliani 1999, 2000 for details). Data on nest defense were collected during 1992- 1994. We censused 16 hobby nests in 1992, 16 in 1993 and 14 in 1994. Intensity of nest defense was record- ed by simulating a predation attempt by a human. During each trial, the predator positioned himself at a point 50 m from the hobby nest. The observer then walked slowly towards the nest. Once under the nest, he recorded the behavior of the adults for 5 min. Intensity of defense was quantified according to the following aggressiveness scores: (1) flies away and disappears; (2) circles silently high above the cano- py; (3) circles high above the canopy and emits a few alarm calls (c4); (4) circles high above the canopy and emits numerous alarm calls (>4); (5) as above, but with some shallow dips at the intruder; (6) flies away (>50 m) from the observer in the woodlot in a zig-zag path, then perches on a tree and gives con- tinuous alarm calls (if the observer approaches, the hobby flies away); (7) as above, but nearer to the ob- server (<50 m); if the observer approaches closely, the hobby remains perched and continues to call; (8) stoops closely at the intruder. Progressive increases in score are associated with increases in energy cost- ly activities, such as alarm calling, flapping flight and stooping, and with a decrease in distance from the potential predator, that is, with higher costs and risks for the defender. During incubation, we also classified incubating females as loose- or tight-sit- ters, depending on whether they fled when ap- proached by the observer, or whether they remained crouched over the clutch in a flattened posture. Tight sitting was considered the more extreme form of de- fense because it allowed closer approach by the pred- ator. Only cases in which both partners were ob- served near the nest since the beginning of the trial were included in the analyses of sexual differences in defense. 1047 This content downloaded from 193.204.40.97 on Mon, 25 Mar 2013 09:37:25 AM All use subject to JSTOR Terms and Conditions